Calculators
Ohm's Law – Single
Ohm's Law – Triple (Delta)
Ohm's Law – Triple (Wye)
Heat Energy Requirements
Wattage Requirements
Glossary
• # Glossary of Commonly Used Terms

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | R | S | T | U | V | W | Y | Z

Absolute Pressure Transducer
A transducer which measures pressure in relation to zero pressure.
Absolute pressure
Pressure referenced to full vacuum. Pounds per square inch, designated as PIA.
Absolute Zero
Temperature at which thermal energy is at a minimum. Defined as 0 Kelvin, calculated to be –273.15°C or –459.67°F.
Absorptivity
The fraction of incident radiation absorbed by a surface.
AC
Alternating current; an electric current that reverses its direction at regularly recurring intervals.
Acceleration
A change in the velocity of a body or particle with respect to time. The parameter that an accelerometer measures (dv/dt).
Accelerometer
A device which converts the effects of mechanical motion into an electrical signal that is proportional to the acceleration value of the motion.
Accuracy rating
A number that defines a limit that the measurement errors will not exceed under some reference operating conditions. It includes the combined effects of conformity, hysteresis, dead band and repeatability errors.
Accuracy, units
The maximum positive or negative deviation (inaccuracy) observed in testing a device. It can be expressed in terms of the measured variable (plus-minus 1[DEGREE]C), or as a percentage of the actual reading (%AR), of the full scale (%FS), of upper range value (%URL), of the span or of scale length.
Accuracy
Degree of conformity of a measured value to an accepted standard value or closeness of a reading or indication of a sensor to the actual value of the quantity being measured.
Acoustics
The degree of sound. The nature, cause, and phenomena of the vibrations of elastic bodies; which vibrations create compressional waves or wave fronts which are transmitted through various media, such as air, water, wood, steel, etc.
Activity (ai)
A thermodynamic term for the apparent or active concentration of a free ion in solution. It is related to concentration by the activity coefficient.
Activity Coefficient (fi)
A ratio of the activity of species i(ai) to its molality (C). It is a correction factor which makes the thermodynamic calculations correct. This factor is dependent on ionic strength, temperature, and other parameters.
A mechanism or device for attaching non-mating parts.
Admittance of an AC circuit is analogous to the conductivity of a DC circuit, it is the reciprocal of the impedance of an AC circuit.
Air consumption
The maximum rate at which air is consumed by an instrument while operating within its operating range, usually expressed in units of standard cubic feet per minute.
Alias
A false lower frequency component that appears in sampled data acquired at too low a sampling rate.
Aliasing
If the sample rate of a function (fs) is less than two times the highest frequency value of the function, the frequency is ambiguously presented. The frequencies above (fs/2) will be folded back into the lower frequencies producing erroneous data.
Alloy 11
A compensating alloy used in conjunction with pure copper as the negative leg to form extension wire for platinum—platinumrhodium thermocouples Types R and S.
Alloy 200/226
The combination of compensating alloys used with tungsten vs. tungsten/26%-rhenium thermocouples as extension cable for applications under 200°C.
Alloy 203/225
The combination of compensating alloys used with tungsten 3% rhenium vs. tungsten 150 rhenium thermocouples as extension cable for applications under 200°C.
Alloy 203/225
The combination of compensating alloys used with tungsten/3%-rhenium vs. tungsten/25%-rhenium thermocouples as extension cable for applications under 200°C.
Alloy 405/426
The combination of compensating alloys used with tungsten/5%-rhenium vs. tungsten/26%-rhenium thermocouples as extension cable for applications under 870°C.
Alphanumeric
A character set that contains both letters and digits.
Alternating current (AC)
A flow of electric charge (electric current) that undergoes periodic reverses in direction.
Ambient Compensation
The design of an instrument such that changes in ambient temperature do not affect the readings of the instrument.
Ambient Conditions
The conditions around the transducer (pressure, temperature, etc.).
Ambient pressure
The atmospheric pressure of the medium surrounding the particular sensor.
Ambient temperature compensation
An automatic correction which prevents the reading of a sensor or instrument from being affected by variations in ambient temperature. The compensator specifications state the temperature range within which the compensation is effective.
Ambient Temperature
The average or mean temperature of the surrounding air which comes in contact with the equipment and instruments under test.
American National Standards Institute (ANSI)
A professional organization in the United States responsible for accepting and designating the standards developed by other organizations as national standards.
American Standard Code for Information Interchange (ASCII)
Standard seven or eight-bit code used to represent alphanumeric characters for communication among computers.
Ammeter
An instrument used to measure current.
Ampere (amp)
A unit used to define the rate of flow of electricity (current) in a circuit; units are one coulomb (6.28 x 1018 electrons) per second.
Amplifier
A device which draws power from a source other than the input signal and which produces as an output an enlarged reproduction of the essential features of its input.
Amplitude flatness
A measure of gain consistency of a circuit over a range of frequencies.
Amplitude Span
The Y-axis range of a graphic display of data in either the time or frequency domain.
Amplitude
A measurement of the distance from the highest to the lowest excursion of motion, as in the case of mechanical body in oscillation or the peak-to-peak swing of an electrical waveform.
Analog Output
A voltage or current signal that is a continuous function of the measured parameter.
Analog signal
A signal that continuously represents a variable or condition.
Analog trigger
A trigger that occurs at a user-selected point on an incoming analog signal. Can be set to occur at a specific level on either an increasing or a decreasing signal (positive or negative slope).
Anemometer
An instrument for measuring and/or indicating the velocity of air flow.
Angstrom
Ten to the minus tenth (10–10) meters or one millimicron, a unit used to define the wavelength of light.
Angular Frequency
The motion of a body or a point moving circularly, referred to as the circular frequency O which is the frequency in cycles per second (cps) multiplied by the term (2) and expressed in radians per second (2pf).
Anion
A negatively charged ion (Cl-, NO3-, S2- etc.)
Annealing
The process of heating a material just below its heat distrortion point to relieve stresses.
ANSI
American National Standards Institute.
Anti-Reset Windup
This is a feature in a three-mode PID controller which prevents the integral (auto reset) circuit from functioning when the temperature is outside the proportional band.
Application-specific integrated circuit (ASIC)
Custom semiconductor component designed and manufactured to perform a set of specific functions, typically for a single application.
Arithmetic logic unit (ALU)
The part of a digital processing system where binary data is operated upon mathematically.
ASCII
American Standard Code for Information Interchange. A seven or eight bit code used to represent alphanumeric characters. It is the standard code used for communications between data processing systems and associated equipment.
Asymmetry Potential
The potential developed across the glass membrane with identical solutions on both sides. Also a term used when comparing glass electrode potential in pH 7 buffer.
Asynchronous
A communication method where data is sent when it is ready without being referenced to a timing clock, rather than waiting until the receiver signals that it is ready to receive.
ATC
Automatic temperature compensation.
Attenuation
The reciprocal of gain; a dimensionless ratio defining the decrease in signalmagnitude as it passes between two points or two frequencies. Large values of attenuation are expressed in decibels (dB).
Automatic Reset
1. A feature on a limit controller that automatically resets the controller when the controlled temperature returns to within the limit bandwidth set. 2. The integral function on a PID controller which adjusts the proportional bandwidth with respect to the set point to compensate for droop in the circuit, i.e., adjusts the controlled temperature to a set point after the system stabilizes.
Auto-Zero
An automatic internal correction for offsets and/or drift at zero voltage input.
AWG
American Wire Gauge.
Axis of Rotation (Spin Axis)
The axis of rotation (spin axis) is that straight line about which a body rotates.
Backplane
Multi-conductor assembly into which computer-based boards are inserted. Typically supplies power and allows boards to communicate at high speeds.
Beat Frequency
Beat frequencies are periodic vibrations that result from the addition and subtraction of two or more sinusoids.
Beryllia
BeO (Beryllium Oxide), a high-temperature mineral insulation material; toxic when in powder form.
Best Fit Straight Line (BFSL)
A line midway between two parallel straight lines enclosing all output vs. pressure values.
Beta Ratio
The ratio of the diameter of a pipeline constriction to the unconstricted pipe diameter.
BIAS Current
A very low-level DC current generated by a panel meter and superimposed on a signal. This current may introduce a measurable offset across a very high source impedance.
Bipolar
A signal range that includes both positive and negative values (i.e., -10 V to +10 V).
Blackbody
A theoretical object that radiates the maximum amount of energy at a given temperature, and absorbs all the energy incident upon it.
BNC
A quick disconnect electrical connector used to interconnect and/or terminate coaxial cables.
Bode diagram
A plot of log amplitude ratio and phase angle values used in describing transfer functions.
Boiling Point
The temperature at which a substance in the liquid phase transforms to the gaseous phase; commonly refers to the boiling point of water which is 100°C (212°F) at sea level.
Bolometer
Infrared thermometer detector consisting of a resistance thermometer arranged for response to radiation.
Breakdown Voltage Rating
The dc or ac voltage which can be applied across insulation portions of a transducer without arcing or conduction above a specific current value.
BTU
British thermal unit. The quantity of thermal energy required to raise one pound of water at its maximum density, 1 degree F. One BTU is equivalent to .293 watt hours, or 252 calories. One kilowatt hour is equivalent to 3412 BTU.
Buffer Capacity (B)
A measure of the ability of the solution to resist pH change when a strong acid or base is added.
Burn-In
A long term screening test (either vibration, temperature or combined test) that is effective in weeding out premature failures because it simulates actual or worst case operation of the device, accelerated through a time, power, and temperature relationship.
Burst Pressure
The maximum pressure applied to a transducer sensing element or case without causing leakage.
Burst Proportioning
A fast-cycling output form on a time proportioning controller (typically adjustable from 2 to 4 seconds) used in conjunction with a solid state relay to prolong the life of heaters by minimizing thermal stress.
Calender-van Dusen Equation
An equation that defines the resistance-temperature value of any pure metal that takes the form of (RT = RO) (1 + AT + BT2) for values between the ice point (0°C) and the freezing point of antimony (630.7°C) and the form RT = RO [1 + AT + BT2 + C (T–100)T2] between the oxygen point (–183.0°C) and the ice point (0°C).
Calibrate
To ascertain that the output of a device properly corresponds to the information it is measuring, receiving or transmitting. This might involve the location of scale graduations, adjustment to bring the output within specified tolerance or ascertaining the error by comparing the output to a reference standard.
Calibration curve
A graphical representation of the calibration report, which report can be in the form of a table or chart.
Calibration cycle
The application of known values of the measured variable and the recording of the corresponding output readings over the range of the instrument in both ascending and descending directions.
Calibration traceability
The relationship of the calibration process to the calibration steps performed by a national standardizing laboratory.
Calibration
The process of adjusting an instrument or compiling a deviation chart so that its reading can be correlated to the actual value being measured.
Calorie
The quantity of thermal energy required to raise one gram of water 1°C at 15°C.
Capacitance
The capability of a device to store electric charge. Its unit is the farad, which expresses the ratio of stored charge in coulombs to the impressed potential difference in volts.
Capacitor
A device designed to store electric charge. It usually consists of two conductors that are electrically isolated by a nonconductor (dielectric). The plates of a perfect capacitor are isolated by vacuum (dielectric constant of 1.0), in which case no current flows between the plates.
Cation
A positively charged ion (Na+, H+).
Cavitation
The boiling of a liquid caused by a decrease in pressure rather than an increase in temperature.
A temperature scale defined by 0°C at the ice point and 100°C at the boiling point of water at sea level.
Center of Gravity (Mass Center)
The center of gravity of a body is that point in the body through which passes the resultant of weights of its component particles for all orientations of the body with respect to a uniform gravitational field.
Centripetal Force
A force exerted on an object moving in a circular path which is exerted inward toward the center of rotation.
Ceramic Insulation
High-temperature compositions of metal oxides used to insulate a pair of thermocouple wires. The most common are Alumina (Al2O3), Beryllia (BeO), and Magnesia (MgO).
Ceramic
Polycrystalline ferroelectric materials which are used as the sensing units in piezoelectric accelerometers.
CFM
The volumetric flow rate of a liquid or gas in cubic feet per minute.
Character
A letter, digit or other symbol that is used as the representation of data. A connected sequence of characters is called a character string.
Charge Sensitivity
For accelerometers that are rated in terms of charge sensitivity, the output voltage (V)is proportional to the charge (Q) divided by the shunt capacitance (C). This type of accelerometer is characterized by a high output impedance. The sensitivity is given in terms of charge; picocoulombs per unit of acceleration (g).
Chatter
The rapid cycling on and off of a relay in a control process due to insufficient bandwidth in the controller.
Chemical seal
A diaphragm assembly which detects the pressure of the process and transmits it to a (usually stable and inert) filling fluid, which then transmits that pressure to an instrument.
Clear
To restore a device to a prescribed initial state, usually the zero state.
Clipping
The term applied to the phenomenon which occurs when an output signal is limited in some way by the full range of an amplifier, ADC or other device. When this occurs, the signal is flattened at the peak values, the signal approaches the shape of a square wave, and high frequency components are introduced. Clipping may be hard, as is the case when the signal is strictly limited at some level, or it may be soft, in which case the clipping signal continues to follow the input at some reduced gain.
Clock
The device that generates periodic signals for synchronization.
Closeness of Control
Total temperature variation from a desired set point of system. Expressed as “closeness of control” is ±2°C or a system bandwidth with 4°C, also referred to as “amplitude of deviation.”
CMR (Common-Mode Rejection)
The ability of a panel meter to eliminate the effect of AC or DC noise between signal and ground. Normally expressed in dB at dc to 60 Hz.
CMV (Common-Mode Voltage)
The AC or DC voltage which is tolerable between signal and ground.
Code width
Smallest voltage an A/D converter can detect; a function of resolution, gain, and range.
Coherence Function.
A frequency domain function computed to show the degree of a linear, noise-free relationship between a system's input and output. The value of the coherence function ranges between zero and one, where a value of zero indicates there is no causal relationship between the input and the output. A value of one indicates the existence of linear noise-free frequency response between the input and the output.
Cold junction compensation (CJC)
The referencing of thermocouple voltage outputs to ambient temperature in a thermocouple measurement circuit.
Color Code
The ANSI established color code for thermocouple wires in the negative lead is always red. Color Code for base metal thermocouples is yellow for Type K, black for Type J, purple for Type E and blue for Type T.
Colour Code
The ANSI established colour code for thermocouple wires in the negative lead is always red. Colour Code for base metal thermocouples is yellow for Type K, black for Type J, purple for Type E and blue for Type T.
Common Mode Rejection Ratio
The ability of an instrument to reject interference from a common voltage at its input terminals with relation to ground. Usually expressed in db (decibels).
Common mode rejection
The ability of a circuit to discriminate against a common mode voltage.
Common mode voltage
A voltage of the same polarity on both sides of a differential input relative to ground.
Common Mode
The output form or type of control action used by a temperature controller to control temperature, i.e. on/off, time proportioning, PID.
Common-mode range
Input range over which a circuit can handle a common-mode signal.
Common-mode signal
Mathematical average voltage, relative to a computer's ground, of the signals from a differential input.
Compensated Connector
A connector made of thermocouple alloys used to connect thermocouple probes and wires.
Compensating Alloys
Alloys used to connect thermocouples to instrumentation. These alloys are selected to have similar thermal electric properties as the thermocouple alloys (however, only over a very limited temperature range).
Compensating Loop
Lead wire resistance compensation for RTD elements where an extra length of wire is run from the instrument to the RTD and back to the instrument, with no connection to the RTD.
Compensation
An addition of specific materials or devices to counteract a known error.
Compensator
A device which eliminates the effect of an unmeasured variable or condition on the measurement of interest.
Compound detector
A detector whose measurement range extends both above and below zero.
Conductance, conductivity
The reciprocal of resistance in a DC circuit is conductance. Its unit is the mho. The unit of conductivity is cm-mho or cm/ohm.
Conductance
The measure of the ability of a solution to carry an electrical current.
Conduction
The conveying of electrical energy or heat through or by means of a conductor.
Confidence Level
The range (with a specified value of uncertainty, usually expressed in percent) within which the true value of a measured quantity exists.
Conformity Error
For thermocouples and RTD’s, the difference between the actual reading and the temperature shown in published tables for a specific voltage input.
An enclosure attached to the end of a thermocouple which can be cast iron, aluminum or plastic within which the electrical connections are made.
Constantan
A copper-nickel alloy used as the negative lead in Type E, Type J, and Type T thermocouples.
Continuous Spectrum
A frequency spectrum that is characterized by non-periodic data. The spectrum is continuous in the frequency domain and is characterized by an infinite number of frequency components.
Control Character
A character whose occurrence in a particular context starts, modifies or stops an operation that affects the recording, processing, transmission or interpretation of data.
Control Mode
The output form or type of control action used by a temperature controller to control temperature, i.e., on/off, time proportioning, PID.
Control Point
The temperature at which a system is to be maintained.
Controller
A device that operates automatically to regulate a controlled variable.
Conversion time
Time required to convert an analog or digital signal into its converse.
Coriolis Force
A result of centripetal force on a mass moving with a velocity radially outward in a rotating plane.
Correction (Balancing) Plane
A plane perpendicular to the shaft axis of a rotor in which correction for unbalance is made.
Coulomb Sensitivity
Charge/unit acceleration, expressed in Pc/g (charge sensitivity).
Coulomb
A measurement of the quantity of electrical charge, usually expressed as pico coulomb (10-12 coulombs).
Counter Weight
A weight added to a body so as to reduce a calculated unbalance at a desired place.
Counts
The number of time intervals counted by the dual-slope A/D converter and displayed as the reading of the panel meter, before addition of the decimal point.
CPS
Cycles per second; the rate or number of periodic events in one second, expressed in Hertz (Hz).
Critical Damping
Critical damping is the smallest amount of damping at which a given system is able to respond to a step function without overshoot.
Critical Speed
The rotational speed of the rotor or rotating element at which resonance occurs in the system. The shaft speed at which at least one of the "critical" or natural frequencies of a shaft is excited.
Cryogenics
Measurement of temperature at extremely low values, i.e., below –200°C.
CSA
Cure Point
The temperature at which a normally magnetic material goes through a magnetic transformation and becomes non-magnetic.
Current drive capability
Amount of current a digital or analog output channel is capable of sourcing or sinking while still operating within voltage range specifications.
Current Proportioning
An output form of a temperature controller which provides a current proportional to the amount of control required.
Current sink capability
Ability of a data acquisition board to dissipate current for analog or digital output signals.
Current source capability
Ability of a data acquisition board to supply current for analog or digital output signals.
Current
The rate of flow of electricity. The unit is the ampere (a) defined as 1 ampere = 1 coulomb per second.
Curve Fitting
Curve fitting is the process of computing the coefficients of a function to approximate the values of a given data set within that function. The approximation is called a “fit”. A mathematical function, such as a least squares regression, is used to judge the accuracy of the fit.
Cycle Time
The time, usually expressed in seconds, for a controller to complete one on/off cycle.
Damping
The reduction of vibratory movement through dissipation of energy. Types include viscous, coulomb, and solid.
Data acquisition (DAQ)
The activity of measuring, transmitting, and recording electrical signals from sensors, switches, and transducers. Often implies the conversion of these signals into computer-compatible digital information.
Data Base
A large amount of data stored in a well-organized manner. A data base management system (DBMS) is a program that allows access to the information.
dB (Decibel)
20 times the log to the base 10 of the ratio of two voltages. Every 20 dB’s correspond to a voltage ratio of 10, every 10 dB’s to a voltage ratio of 3.162. For instance, a CMR of 120 dB provides voltage noise rejection of 1,000,000/1. An NMR of 70 dB provides voltage noise rejection of 3,162/1.
DC
Direct current; an electric current flowing in one direction only and substantially constant in value.
The range through which an input can be changed without causing an observable response.
The interval between the initiation of a change in the input and the start of theresulting observable response.
The volume of the pressure port of a transducer at room temperature and ambient barometric pressure.
Decibel (dB)
Unit for expressing a logarithmic measure of the ratio of two signal levels.
Decimal
Refers to a base ten number system using the characters 0 through 9 to represent values.
Default
The value(s) or option(s) that are assumed during operation when not specified.
Degree
An incremental value in the temperature scale, i.e., there are 100 degrees between the ice point and the boiling point of water in the Celsius scale and 180°F between the same two points in the Fahrenheit scale.
Delta
An electrical network where loads are connected directly between the three phases.
Density
Mass per unit of volume of a substance, i.e.
Derivative control
Control action in which output correction is proportional to the rate of change of the error signal. Derivative control anticipates the magnitude difference between the process variable and the setpoint.
Derivative
The derivative function senses the rate of rise or fall of the system temperature and automatically adjusts the cycle time of the controller to minimize overshoot or undershoot.
Deviation
The difference between the value of the controlled variable and the value at which it is being controlled.
Diaphragm
The sensing element consisting of a membrane which is deformed by the pressure differential applied across it.
Dielectric
A material with low electrical conductivity, commonly called an electrical insulator
Dielectric Constant
Related to the force of attraction between two opposite charges separated by a distance in a uniform medium.
Differential Input
A signal-input circuit where SIG LO and SIG HI are electrically floating with respect to ANALOG GND (METER GND, which is normally tied to DIG GND). This allows the measurement of the voltage difference between two signals tied to the same ground and provides superior common-mode noise rejection.
Differential Pressure
The difference in static pressure between two identical pressure taps at the same elevation located in two different locations in a primary device.
Differential
For an on/off controller, it refers to the temperature difference between the temperature at which the controller turns heat off and the temperature at which the heat is turned back on. It is expressed in degrees.
Diffuse emitter
A surface that emits radiation equally in all directions.
Digit
A measure of the display span of a panel meter. By convention, a full digit can assume any value from 0 through 9, a 1/2-digit will display a 1 and overload at 2, a 3/4-digit will display digits up to 3 and overload at 4, etc. For example, a meter with a display span of ±3999 counts is said to be a 3 3/4 digit meter.
Digital input/output (DIO)
Input or output points allowed only two discrete states, typically on or off, 1 or 0.
Digital Output
An output signal which represents the size of an input in the form of a series of discrete quantities.
Digital signal processing (DSP)
The manipulation of signal information while it exists in digital rather than analog form.
Digital-to-analog converter (D/A)
Electronic device, often an integrated circuit, that converts a digital number into a corresponding analog voltage or current.
DIN (Deutsche Industrial Norm)
A set of German standards recognized throughout the world. The 1/8 DIN standard for panel meters specifies an outer bezel dimension of 96 x 48 mm and a panel cutout of 92 x 45 mm.
DIN 43760
The standard that defines the characteristics of a 100 ohm platinum RTD having a resistance vs. temperature curve specified by a = 0.00385 ohms per degree.
Discharge Time Constant
The time required for the output-voltage from a sensor or system to discharge 37% of its original value in response to a zero rise time step function input. This parameter determines a low frequency response.
Displacement
The measured distance traveled by a point from its position at rest. Peak to peak displacement is the total measured movement of a vibrating point between its positive and negative extremes. Measurement units expressed as inches or milli-inches.
Dissipation Constant
The ratio for a thermistor which relates a change in internal power dissipation to a resultant change of body temperature.
Dissociation Constant (K)
A value which quantitatively expresses the extent to which a substance dissociates in solution.
Distributed control system (DCS)
Typically a large-scale process control system characterized by a distributed network of processors and I/O subsystems that encompass the functions of control, user interface, data collection, and system management. DCSs are commonly used in large industrial facilities, such as chemical plants, petroleum refineries, and paper mills.
Dither
A useful oscillation of small magnitude, introduced to overcome the effects of friction, hysteresis, or clogging.
Drift
A change of a reading or a set point value over long periods due to several factors including change in ambient temperature, time, and line voltage.
Droop
A common occurrence in time-proportional controllers. It refers to the difference in temperature between the set point and where the system temperature actually stabilizes due to the timeproportioning action of the controller.
Dual Element Sensor
A sensor assembly with two independent sensing elements.
Dual-Slope A/D Converter
An analog-to-digital converter which integrates the signal for a specific time, then counts time intervals for a reference voltage to bring the integrated signal back to zero. Such converters provide high resolution at low cost, excellent normal-mode noise rejection, and minimal dependence on circuit elements.
Duplex Wire
A pair of wires insulated from each other and with an outer jacket of insulation around the inner insulated pair.
Duplex
Pertaining to simultaneous two-way independent data communication transmission in both directions. Same as “full duplex”.
Duty Cycle
The total time to one on/off cycle. Usually refers to the on/off cycle time of a temperature controller.
Dynamic (Two-Plane) Balancing Machine
A dynamic balancing machine is a centrifugal balancing machine that furnishes information for performing two-plane balancing.
Dynamic Calibration
Calibration in which the input varies over a specific length of time and the output is recorded vs. time.
Dynamic Pressure
The difference in pressure levels from static pressure to stagnation pressure caused by an increase in velocity. Dynamic pressure increases by the square of the velocity.
Dynamic range
Ratio of the largest to smallest signal level a circuit can handle, normally expressed in dB.
Dynamic Unbalance
Dynamic unbalance is that condition in which the central principal axis is not coincident with the shaft axis.
Electrical Interference
Electrical noise induced upon the signal wires that obscures the wanted information signal.
Electrode Potential (E)
The difference in potential established between an electrode and a solution when the electrode is immersed in the solution.
Electrolyte
Any substance which, when in solution will conduct an electric current. Acids, bases, and salts are common electrolytes.
Electromagnetic interference (EMI)
electrical noise induced upon signal wires with the possible effect of obscuring the instrument signal.
Electromotive force (EMF)
A measure of voltage in an electrical circuit. .
Electronic Industries Association (EIA)
A standards organization specializing in the electrical and functional characteristics of interface equipment.
Elevation
A range in which the zero value of the measured variable exceeds the lower range value.
EMF
Electromotive force. A rise in (electrical) potential energy. The principal unit is the volt.
EMI
Electromagnetic interference.
Emissive power
Rate at which radiation is emitted from a surface, per unit surface area per unit wavelength.
Emissivity/emittivity
The ratio of energy emitted by a surface to the energy emitted by a blackbody at the same temperature, symbolized by e. Emissivity refers to an overall property of a substance, whereas emittvity refers to a particular surface's characteristics.
Encoder
Device that converts linear or rotary displacement into digital or pulse signals.
End Point (Potentiometric)
The apparent equivalence point of a titration at which a relatively large potential change is observed.
End Points
The end points of a full scale calibration curve.
Endothermic
A process is said to be endothermic when it absorbs heat.
Enthalpy
The sum of the internal energy of a body and the product of its volume multiplied by the pressure.
Environmental Conditions
All conditions to which a transducer may be exposed during shipping, storage, handling, and operation.
Equilibrium Constant
The product of the concentrations (or activities) of the substances produced at equilibrium in a chemical reaction divided by the product of concentrations of the reacting substances, each concentration raised to that power which is the coefficient of the substance in the chemical equation.
Equitransference
Equal diffusion rates of the positively and negatively charged ions of an electrolyte across a liquid junction without charge separation.
Equivalent Conductance (l)
Equivalent conductance of an electrolyte is defined as the conductance of a volume of solution containing one equivalent weight of dissolved substances when placed between two parallel electrodes 1 cm apart, and large enough to contain between them all of the solution. l is never determined directly, but is calculated from the specific conductance (Ls). If C is the concentration of a solution in gram equivalents per liter, then the concentration of a solution in gram equivalents per liter, then the concentration per cubic centimeter is C/1000, and the volume containing one equivalent of the solute, is, therefore, 1000/C.
Error, common mode
Error caused by an interference that appears between both measuring terminals and ground.
Error, normal mode
Error caused by an interference that appears between the two measuring terminals.
Error, random
The amount of error that remains even after calibrating a sensor. It is also called precision, while repeatability is defined as twice that, the diameter insteadof the radius of the circle within which the readings fall.
Error, systematic
A repeatable error, which either remains constant or varies according to some law, when the sensor is measuring the same value. This error can be eliminated by calibration.
Error
The difference between the value indicated by the transducer and the true value of the measured value being sensed. Usually expressed in percent of full scale output.
Eutectic Temperature
The lowest possible melting point of a mixture of alloys.
Excitation
The external application of electrical voltage current applied to a transducer for normal operation.
Exothermic
A process is said to be exothermic when it releases heat.
Expansion Factor
Correction factor for the change in density between two pressure measurement areas in a constricted flow.
Explosion-Proof Enclosure
An enclosure that can withstand an explosion of gases within it and prevent the explosion of gases surrounding it due to sparks, flashes or the explosion of the container itself, and maintain an external temperature which will not ignite the surrounding gases.
Exposed Junction
A form of construction of a thermocouple probe where the hot or measuring junction protrudes beyond the sheath material so as to be fully exposed to the medium being measured. This form of construction usually gives the fastest response time.
External trigger
Voltage pulse from an external source that triggers an event such as A/D conversion.
Fahrenheit
A temperature scale defined by 32° at the ice point and 212° at the boiling point of water at sea level.
The unit of capacitance, equivalent to one coulomb of stored charge per volt of applied potential difference. As this is a very large unit, one trillionth of it, the picofarad (pf) is commonly used.
Ferrule
A compressible tubular fitting that is compressed onto a probe inside a compression fitting to form a gas-tight seal.
Radiation thermometer that uses a fiber optic probe to separate the detector, housing, and electronics from the radiation gathering point itself. Used to measure temperature in hard-to-reach places or in hostile conditions.
Field Balancing Equipment
An assembly of measuring instruments for performing balancing operations on assembled machinery which is not mounted in a balancing machine.
Field of View
A volume in space defined by an angular cone extending from the focal plane of an instrument.
Fieldbus
All-digital communication network used to connect process instrumentation and control systems. Designed to replace systems based on 4-20 mA analog signals with bidirectional, multivariable data communication capability.
Filling Solution
A solution of defined composition to make contact between an internal element and a membrane or sample.
Flag
Any of various types of indicators used for identification of a condition or event, for example, a character that signals the termination of a transmission.
High-speed ADC whose output code is determined in a single step by a bank of comparators and encoding logic.
Flow Rate
Actual speed or velocity of fluid movement .
Flow
Travel of liquids or gases in response to a force (i.e. pressure or gravity).
Flowmeter
A device used for measuring the flow or quantity of a moving fluid.
FM Approved
An instrument that meets a specific set of specifications established by Factory Mutual Research Corporation.
FM
Factory Mutual Research Corporation. An organization which sets industrial safety standards.
Force balance
Instruments which operate by force-balance between the detected variable and the generated output, require no motion and therefore tend to be more maintenance free than are motion-balance devices.
Forced Vibration
Vibration of a system caused by an imposed force. Steady-state vibration is an unchanging condition of periodic or random motion.
FPM
Flow velocity in feet per minute.
FPS
Flow velocity in feet per second.
Freezing Point
The temperature at which the substance goes from the liquid phase to the solid phase.
Frequency Modulated Output
A transducer output which is obtained in the form of a deviation from a center frequency, where the deviation is proportional to the applied stimulus.
Frequency of Vibration
The number of cycles occurring in a given unit of time. RPM - revolutions per minute. CPM- cycles per minute.
Frequency Output
An output in the form of frequency which varies as a function of the applied input.
Frequency response
The frequency-dependent characteristics that determines the phase and amplitude relationship between sinusoidal input and output.
Frequency, Natural
The frequency of free (not forced) oscillations of the sensing element of a fully assembled transducer.
Frequency
The number of cycles over a specified time period over which an event occurs. The reciprocal is called the period.
Full Scale Output
The algebraic difference between the minimum output and maximum output.
Functions
Three mode PID controller. A timeproportioning controller with integral and derivative functions. The integral function automatically adjusts the system temperature to the set point temperature to eliminate droop due to the time proportioning function.
g
The force of acceleration due to gravity equal to 32.1739 ft/sec2 or 386 in./sec2.
Gage Factor
A measure of the ratio of the relative change of resistance to the relative change in length of a piezoresistive strain gage.
Gage Length
The distance between two points where the measurement of strain occurs.
Gage Pressure Transducer
A transducer which measures pressure in relation to the ambient pressure.
Gage pressure
Absolute pressure minus local atmospheric pressure.
Gain (magnitude ratio)
For a linear system or element, the ratio of the magnitude (amplitude) of a steady-state sinusoidal output relative to a causal input. In an electrical circuit, it is the amount of amplification used and is sometime expressed in decibels, dB.
Gain accuracy
Measure of deviation of the gain (of an amplifier or other device) from the ideal gain.
Gain, dynamic
For a sinusoidal signal, the magnitude ratio of the steady-state amplitude of the output signal to the amplitude of the input.
Gain, static
The ratio of change of steady state value to a step change in input, provided that the output does not saturate.
Gain
The amount of amplification used in an electrical circuit.
Galvanometer
An instrument that measures small electrical currents by means of deflecting magnetic coils.
GPH
Volumetric flow rate in gallons per hour.
GPM
Volumetric flow rate in gallons per minute.
Ground
1. The electrical neutral line having the same potential as the surrounding earth. 2. The negative side of DC power supply. 3. Reference point for an electrical system.
Grounded Junction
A form of construction of a thermocouple probe where the hot or measuring junction is in electrical contact with the sheath material so that the sheath and thermocouple will have the same electrical potential.
Half Bridge
Two active elements or strain gages.
The loss of pressure in a flow system measured using a length parameter (i.e., inches of water, inches of mercury).
Pressure in terms of the height of fluid, P = yrg, where r = fluid density and y = the fluid column heights. Expression of a pressure in terms of the height of fluid, r = yrg, where r is fluid density and y = the fluid column height. g = the acceleration of gravity.
Heat Sink
1. Thermodynamic. A body which can absorb thermal energy. 2. Practical. A finned piece of metal used to dissipate the heat of solid state components mounted on it.
Heat Transfer
The process of thermal energy flowing from a body of high energy to a body of low energy. Means of transfer are
Heat Treating
A process for treating metals where heating to a specific temperature and cooling at a specific rate changes the properties of the metal.
Heat
Thermal energy. Heat is expressed in units of calories or BTU’s.
Hertz (Hz)
Units in which frequency is expressed. Synonymous with cycles per second.
Hi-Pot Test
A test which applies a high voltage to a conductor to assure the integrity of the sorrounding insulation
Hold
Meter HOLD is an external input which is used to stop the A/D process and freeze the display. BCD HOLD is an external input used to freeze the BCD output while allowing the A/D process to continue operation.
Hooke's Law
Defines the basis for the measurement of mechanical stresses via the strain measurement. The gradient of Hooke's line is defined by the ratio of which is equivalent to the Modulus of Elasticity E (Young's Modulus).
Hunting
An undesirable oscillation which continues for some time after the external stimuli has disappeared.
Hydrogen Ion Activity (aH+)
Activity of the hydrogen ion in solution. Related to hydrogen ion concentration (CH+) by the activity coefficient for hydrogen (f H+).
Hysteresis (Electrode Memory)
When an electrode system is returned to a solution, equilibrium is usually not immediate. This phenomenon is often observed in electrodes that have been exposed to the other influences such as temperature, light, or polarization.
Hysteresis
The difference in output when the measurand value is first approached with increasing and then with decreasing values. Expressed in percent of full scale during any one calibration cycle. See Deadband
Ice point
The temperature at which pure water freezes, 0°C, 32°F, 273.15°K.
ICP
Integrated Circuit Piezoelectric; term sometimes used to describe an accelerometer with built-in electronics.
Impedance
The total opposition to electrical flow (resistive plus reactive).
Inductance
The property by which an electromotive force (emf) is induced in a conductor when the magnetic field is changing about it. This is usually caused by changes in the current flow in the circuit or in a neighboring circuit.
Infrared (IR)
A range of the electromagnetic spectrum extending beyond red visible light from 760 nanometers to 1000 microns.
Infrared thermocouple
Radiation thermometer whose output simulates that of a standard type thermocouple, typically over a more limited temperature range. Interchangeability error
Infrared
an area in the electromagnetic spectrum extending beyond red light from 760 nanometers to 1000 microns (106 nm). It is the form of radiation used for making non-contact temperature measurements.
Initial Unbalance
Initial unbalance is that unbalance of any kind that exists in the rotor before balancing.
Input Impedance
The resistance of a panel meter as seen from the source. In the case of a voltmeter, this resistance has to be taken into account when the source impedance is high; in the case of an ammeter, when the source impedance is low.
Input offset current
Difference in the input bias currents of the two inputs of an instrumentation amplifier.
Input Resistance (Impedance)
The input resistance of a pH meter is the resistance between the glass electrode terminal and the reference electrode terminal. The potential of a pH-measuring electrode chain is always subject to a voltage division between the total electrode resistance and the input resistance.
Insulation Resistance
The resistance measured between two insulated points on a transducer when a specific dc voltage is applied at room temperature.
Integral control
A control mode which generates a corrective output signal in proportion to the time integral of the past error. It eliminates the offset inherent inproportional control.
Integral nonlinearity (INL)
A measure in LSB of the worst-case deviation from the ideal A/D or D/A transfer characteristic of analog I/O circuitry.
Integral
A form of temperature control.
ADC that works by integrating an unknown voltage over time. Time required is compared to the time required to integrate a known reference voltage.
Interchangeability Error
A measurement error that can occur if two or more probes are used to make the same measurement. It is caused by a slight variation in characteristics of different probes.
Interface
The means by which two systems or devices are connected and interact with each other.
Internal Reference electrode (Element)
The reference electrode placed internally in a glass electrode.
Interrupt
To stop a process in such a way that it can be resumed.
Intrinsically Safe
An instrument which will not produce any spark or thermal effects under normal or abnormal conditions that will ignite a specified gas mixture.
Ionic Mobility
Defined similarly to the mobility of nonelectrolytic particles, viz., as the speed that the ion obtains in a given solvent when influenced by unit power.
Ionic Strength
The weight concentration of ions in solution, computed by multiplying the concentration of each ion in solution (C) by the corresponding square of the charge on the ion (Z) summing this product for all ions in solution and dividing by 2
IPTS-48
International Practical Temperature Scale of 1948. Fixed points in thermometry as specified by the Ninth General Conference of Weights and Measures which was held in 1948.
IPTS-68
International Practical Temperature Scale of 1968. Fixed points in thermometry set by the 1968 General Conference of Weights and Measures.
ISA
Formerly the Instrument Society of America, now referred to as the International Society for Measurement & Control.
Isolation
The reduction of the capacity of a system to respond to an external force by use of resilient isolating materials.
Isopotential Point
A potential which is not affected by temperature changes. It is the pH value at which dE/dt for a given electrode pair is zero. Normally, for a glass electrode and SCE reference, this potential is obtained approximately when immersed in pH 7 buffer.
Isothermal
A process or area that is a constant temperature. Joule
Joint time-frequency analysis (JTFA)
Technique for spectral analysis of rapidly changing waveforms.
Joule
The basic unit of thermal energy.
Journal
A journal is that part of a rotor that is in contact with or supported by a bearing in which it revolves.
Junction
The point in a thermocouple where the two dissimilar metals are joined.
Kelvin
Symbol K. The unit of absolute or thermodynamic temperature scale based upon the Celsius scale with 100 units between the ice point and boiling point of water. 0°C = 273.15K (there is no degree (°) symbol used with the Kelvin scale).
Kilovolt amperes (kva)
1000 volt amps.
Kilowatt (kw)
Equivalent to 1000 watts.
Kilowatt Hour (kwh)
1000 watthours.
Kinetic Energy
Energy associated with mass in motion, i.e., 1/2 rV2 where r is the density of the moving mass and V is its velocity.
KVA
Kilovolt amperes (1000 volt amps).
Lag
1. A time delay between the output of a signal and the response of the instrument to which the signal is sent. 2. A time relationship between two waveforms where a fixed reference point on one wave occurs after the same point of the reference wave.
Laminar Flow
Streamlined flow of a fluid where viscous forces are more significant than inertial forces, generally below a Reynolds number of 2000.
Large Scale Integration (LSI)
The combining of about 1,000 to 10,000 circuits on a single chip. Typical examples of LSI circuits are memory chips and microprocessor.
Laser
Narrow, intense beam of coherent light.
Latent Heat
Expressed in BTU per pound. The amount of heat needed (absorbed) to convert a pound of boiling water to a pound of steam.
Leakage Rate
The maximum rate at which a fluid is permitted or determined to leak through a seal.
Least significant bit (LSB)
Refers to the smallest increment of resolution in an A/D or D/A conversion.
Least-squares Line
The straight line for which the sum of the squares of the residuals (deviations) is minimized.
Life Cycle
The minimum number of pressure cycles the transducer can endure and still remain within a specified tolerance.
Limits of Error
A tolerance band for the thermal electric response of thermocouple wire expressed in degrees or percentage defined by ANSI specification MC-96.1 (1975).
Linear stroke
For a transducer, it is its calibrated mechanical movement over which itselectrical output linearity meets the specifications.
Linearity
The closeness of a calibration curve to a specified straight line. Linearity is expressed as the maximum deviation of any calibration point on a specified straight line during any one calibration cycle.
Linescanner
Device that uses a series of moving mirrors to measure temperature or other properties at various points across a moving web or surface.
Liquid Junction Potential
The potential difference existing between a liquid-liquid boundary. The sign and size of this potential depends on the composition of the liquids and the type of junction used.
The impedance presented to the output terminals of a transducer by the associated external circuitry.
The electrical demand of a process expressed as power (watts), current (amps) or resistance (ohms).
Logarithmic Scale
A method of displaying data (in powers of ten) to yield maximum range while keeping resolution at the low end of the scale.
Loop gain characteristics
Of a closed loop, the characteristic curve of the ratio of the change in the return signal to the change in the error signal for all real frequencies.
Loop Resistance
The total resistance of a thermocouple circuit caused by the resistance of the thermocouple wire. Usually used in reference to analog pyrometers which have typical loop resistance requirements of 10 ohms.
Loop transfer function
Of a closed loop, the transfer function obtained by taking the ratio of the Laplace transform of the return signal to the Laplace transform of its corresponding error signal.
Lower range limit (LRL)
The lowest value of the measured variable that the device can be adjusted to measure.
Lower range value (LRV)
The lowest value of the measured variable that a device is adjusted to measure.
Mainframe
Chassis that mechanically contains boards or modules inserted into a backplane. Provides environment conditioning and vibration and shock-resistant connections.
Mandrel (Balancing Arbor)
An accurately machined shaft on which work is mounted for balancing.
Manipulated variable
A quantity of condition which is varied as a function of an actuating error signal so as to change the value of the directly controlled variable.
The adjustment on a proportioning controller which shifts the proportioning band in relationship to the set point to eliminate droop or offset errors.
Manual Reset (Switch)
The switch in a limit controller that manually resets the controller after the limit has been exceeded.
Mass Flow Rate
Volumetric flowrate times density, i.e. pounds per hour or kilograms per minute.
Maximum Elongation
The strain value where a deviation of more than ±5% occurs with respect to the mean characteristic (diagram of resistance change vs strain).
Maximum Excitation
The maximum value of excitation voltage or current that can be applied to the transducer at room conditions without causing damage or performance degradation beyond specified tolerances.
Maximum Operating Temperature
The maximum temperature at which an instrument or sensor can be safely operated.
Maximum Power Rating
The maximum power in watts that a device can safely handle.
Mean Temperature
The average of the maximum and minimum temperature of a process equilibrium.
Measurand
A physical quantity, property, or condition which is measured.
Measurement signal
The electrical, mechanical, pneumatic, digital or other variable applied to the input of a device. It is the analog of the measured variable produced by the transducer.
Measurement variable
A quantity, property or condition which is being measured. It is sometimes referred to as the measurand.
Measuring Junction
The thermocouple junction referred to as the hot junction that is used to measure an unknown temperature.
Mechanical Hysteresis
The difference of the indication with increasing and decreasing strain loading, at identical strain values of the specimen.
Medium Effect (f m)
For solvents other than water the medium effect is the activity coefficient related to the standard state in water at zero concentration. It reflects differences in the electrostatic and chemical interactions of the ions with the molecules of various solvents. Solvation is the most significant interaction.
Melting Point
The temperature at which a substance transforms from a solid phase to a liquid phase.
Membrane
The pH-sensitive glass bulb is the membrane across which the potential difference due to the formation of double layers with ion-exchange properties on the two swollen glass surfaces is developed. The membrane makes contact with and separates the internal element and filling solution from the sample solution.
Method of Correction
A procedure whereby the mass distribution of a rotor is adjusted to reduce unbalance, or vibration due to unbalance, to an acceptable value. Corrections are usually made by adding material to, or removing it from, the rotor.
MgO
The chemical symbol for magnesium oxide which is a good conductor of heat and a good electrical insulator
Mica
A transparent mineral used as window material in high temperature ovens.
Microamp
One millionth of an ampere, 10¨C6 amps.
Microcomputer
A computer which is physically small. It can fit on top of or under a desk; based on LSI circuitry, computers of this type are now available with much of the power currently associated with minicomputer systems.
Micron
One millionth of a meter, 10¨C6 meters.
Microvolt
One millionth of a volt, 10¨C6 volts.
Mil
One thousandth of an inch (0.001¡Ü).
Milliamp (mA)
One thousandth of an ampere.
Millimeter
One thousandth of a meter, symbol mm.
Millivolt
Unit of electromotive force. It is the difference in potential required to make a current of 1 millampere flow through a resistance of 1 ohm; one thousandth of a volt, symbol mV.
Mineral-insulated Thermocouple
A type of thermocouple cable which has an outer metal sheath and mineral (magnesium oxide) insulation inside separating a pair of thermocouple wires from themselves and from the outer sheath. This cable is usually drawn down to compact the mineral insulation and is available in diameters from 0.375 to 0.010 inches. It is ideally suited for high temperature and severe-duty applications.
Minor Scale Division
On an analog scale, the smallest indicated division of units on the scale.
Molality
A measure of concentration expressed in mols per kilogram of solvent.
Monovalent Ion
An ion with a single positive or negative charge (H+, C1-).
Mounting Error
The error resultant from installing the transducer, both electrical and mechanical.
MSD (Most-Significant Digit)
The leftmost digit of the display.
Mueller Bridge
A high-accuracy bridge configuration used to measure three-wire RTD thermometers.
Radiation thermometer that measures radiation in a tightly controlled range of wavelengths, typically determined by the optical filter used.
NEC
National Electric Codes.
Negative Temperature Coefficient
A decrease in resistance with an increase in temperature.
NEMA-12
A standard from the National Electrical Manufacturers Association, which defines enclosures with protection against dirt, dust, splashes by non-corrosive liquids, and salt spray.
NEMA-4
A standard from the National Electrical Manufacturers Association, which defines enclosures intended for indoor or outdoor use primarily to provide a degree of protection against windblown dust and rain, splashing water, and hose-directed water.
NEMA-7
A standard from the National Electrical Manufacturers Association, which defines explosion-proof enclosures for use in locations classified as Class I, Groups A, B, C or D, as specified in the National Electrical Code.
NEMA-Size Case
An older US case standard for panel meters, which requires a panel cutout of 3.93 x 1.69 inches.
Nernst Equation
A mathematical description of electrode behavior
Nicrosil/Nisil
A nickel-chrome/nickel-silicone thermal alloy used to measure high temperatures. Inconsistencies in thermoelectric voltages exist in these alloys with respect to the wire gauge.
NMR (Normal-Mode Rejection)
The ability of a panel meter to filter out noise superimposed on the signal and applied across the SIG HI to SIG LO input terminals. Normally expressed in dB at 50/60 Hz.
Normal (axial) Stress
The force per unit area on a given plane within a body a = F/A
Normal Hydrogen Electrode
A reversible hydrogen electrode (Pt) in contact with hydrogen gas at 1 atmosphere partial pressure and immersed in a solution containing hydrogen ions at unit activity.
Normal-Mode Rejection Ratio
The ability of an instrument to reject interference usually of line frequency (50–60 Hz) across its input terminals.
NPT
Null
A condition, such as balance, which results in a minimum absolute value of output.
O.D.
Outside diameter.
Octal
Pertaining to a base 8 number system.
Offset
The difference in temperature between the set point and the actual process temperature. Also referred to as droop.
ofhc
Oxygen-free high-conductivity copper. The industrial designation of the pure copper used in a Type T thermocouple.
Ohmmeter
An instrument used to measure electrical resistance.
On/off Controller
A controller whose action is fully on or fully off.
Open Circuit
The lack of electrical contact in any part of the measuring circuit. An open circuit is usually characterized by rapid large jumps in displayed potential, followed by an off-scale reading.
Operational pH
The determination of sample pH by relating to pH measurements in a primary standard solution. This relationship assumes that electrode errors such as sensitivity and changes in asymmetry potential can be disregarded or compensated for, provided the liquid junction potential remains constant between standard and sample.
Optical Isolation
Two networks which are connected only through an LED transmitter and photoelectric receiver with no electrical continuity between the two networks.
Optical pyrometer
Infrared thermometer that measures the temperature of very hot objects by the visible wavelength radiation given off.
Output Impedance
The resistance as measured on the output terminals of a pressure transducer.
Output Noise
The RMS, peak-to-peak (as specified) ac component of a transducer’s dc output in the absence of a measurand variation.
Output settling time
Time required for the analog output voltage to reach its final value within specified limits.
Output signal
A signal delivered by a device, element or system.
Output slew rate
Maximum rate of change of analog output voltage from one level to another.
Output
The electrical signal which is produced by an applied input to the transducer.
Overshoot
The number of degrees by which a process exceeds the set point temperature when coming up to the set point temperature.
Overtravel
That part of the stroke which falls between the end of the calibrated range and the travel stop.
Parallax
An optical illusion which occurs in analog meters and causes reading errors. It occurs when the viewing eye is not in the same plane, perpendicular to the meter face, as the indicating needle.
Peltier Effect
When a current flows through a thermocouple junction, heat will either be absorbed or evolved depending on the direction of current flow. This effect is independent of joule I2 R heating.
Perfectly Balanced Rotor
A rotor is perfectly balanced when its mass distribution is such that it transmits no vibratory force or motion to its bearings as a result of centrifugal forces.
PFA
A fluorocarbon polymer used for insulation of electrical wires (trademark of DuPont).
pH Junctions
The Junction of a reference electrode or combination electrode is a permeable membrane through which the fill solution escapes (called the liquid junction).
pH(S) (Standard pH Scale)
The conventional standard pH scale established on the basis that an individual ionic activity coefficient can be calculated from the Debye-H¸ckel law for primary buffers.
Phase Difference
The time expressed in degrees between the same reference point on two periodic waveforms.
Phase Proportioning
A form of temperature control where the power supplied to the process is controlled by limiting the phase angle of the line voltage.
Phase shift
The angle in degrees between an energizing voltage waveform and an output signal waveform.
Phase
A time-based relationship between a periodic function and a reference. In electricity, it is expressed in angular degrees to describe the voltage or current relationship of two alternating waveforms.
Photon detector
Radiation thermometer detector that releases electric charges in response to incident radiation.
PID
Proportional, integral, derivative. A three-mode control action where the controller has time proportioning, integral (auto reset) and derivative rate action.
Piezoelectric Accelerometer
A transducer that produces an electrical charge in direct proportion to the vibratory acceleration.
Piezoresistance
Resistance that changes with stress.
Plane Separation
Of a balancing machine, is the operation of reducing the correction plane interference ratio for a particular rotor.
Platinum 10% Rhodium
The platinum-rhodium alloy used as the positive wire in conjunction with pure platinum to form a Type S thermocouple.
Platinum 13% Rhodium
The platinum-rhodium alloy used as the positive wire in conjunction with pure platinum to form a Type R thermocouple.
Platinum 30% Rhodium
The platinum-rhodium alloy used as the positive wire in conjunction with platinum 6% rhodium to form a Type B thermocouple.
Platinum 6% Rhodium
The platinum-rhodium alloy used as the negative wire in conjunction with platinum-30% rhodium to form a Type B thermocouple.
Platinum 67
To develop thermal emf tables for thermocouples, the National Bureau of Standards paired each thermocouple alloy against a pure platinum wire (designated Platinum 2 prior to 1973, and currently Platinum 67). The thermal emf’s of any alloy combination can be determined by summing the “vs. Pt-67” emf’s of the alloys, i.e., the emf table for a Type K thermocouple is derived from the Chromel vs. Pt-67 and the Alumel vs .Pt-67 values.
Platinum
A noble metal which in its pure form is the negative wire of Type R and Type S thermocouples.
Poisson Ratio
The ratio between the strain of expansion in the direction of force and the strain of contraction perpendicular to that force v = -Et/E1.
Polarity
In electricity, the quality of having two oppositely charged poles, one positive, one negative.
Polarization
The inability of an electrode to reproduce a reading after a small electrical current has been passed through the membrane. Glass pH electrodes are especially prone to polarization errors caused by small currents flowing from the pH meter input circuit and from static electrical charges built up as the electrodes are removed from the sample solution, or when the electrodes are wiped.
Positive Temperature Coefficient
An increase in resistance due to an increase in temperature.
Potential Energy
Energy related to the position or height above a place to which fluid could possibly flow.
Potentiometer
1. A variable resistor often used to control a circuit. 2. A balancing bridge used to measure voltage.
Power Supply
A separate unit or part of a circuit that supplies power to the rest of the circuit or to a system.
PPM
Abbreviation for “parts per million,” sometimes used to express temperature coefficients. For instance, 100 ppm is identical to 0.01%.
Pressure
Force per unit area, usually expressed in pounds per square inch (PSI).
Pressure, ambient
The pressure of the medium surrounding a device.
Pressure, design
The pressure used in the design of a vessel or other equipment for the purpose of determining the minimum permissible wall thickness or size of parts for a given maximum working pressure (MWP) at a given temperature.
Pressure Drop
The difference in pressure between any two points of a system or component
Pressure, maximum working
The maximum permissible operating pressure at a specified temperature.
Pressure, operating
The actual (positive or negative) pressure at which a device operates under normal conditions.
Pressure, rupture
The burst pressure of a device (determined by testing).
Pressure, static
The steady-state pressure applied to a device.
Pressure, supply
The pressure at which a utility (such as air) is supplied to the device.
Pressure, surge
Operating pressure plus the increment to which a device can be subjected for a very short time, during temporary pressure surges caused by such phenomena as pump start-up or valve shut-off.
Pretravel
That part of the stroke which falls below the calibrated range, between zero and the travel stop.
Primary Device
Part of a flowmeter which is mounted internally or externally to the fluid conduit and produces a signal corresponding to the flowrate and from which the flow may be determined.
Primary element
The element which converts the measured variable into a force, motion or other form suitable for measurement.
Primary Standard (NBS)
The standard reference units and physical constants maintained by the National Bureau of Standards upon which all measurement units in the United States are based.
Primary Standards
Aqueous pH buffer solutions established by the National Bureau of Standards within the 2.5 to 11.5 pH range of ionic strength less than 0.1 and which provide stable liquid junction potential and uniformity of electrode sensitivity.
Principal Axes
The axes of maximum and minimum normal stress.
Probe
A generic term that is used to describe many types of temperature sensors.
Process measurement
The acquisition of information that establishes the magnitude of process quantities.
Process Meter
A panel meter with sizeable zero and span adjustment capabilities, which can be scaled for readout in engineering units for signals such as 4–20 mA, 10–50 mA and 1–5 V.
Process
Physical or chemical change of matter or conversion of energy.
Programmable gain amplifier (PGA)
Signal amplifier that can be programmed to apply a different signal gain depending on the input voltage. Effectively increases dynamic range and sensitivity of A/D converter.
Programmable logic controller (PLC)
Computer based industrial monitoring and controlpackage with applications mostly in the areas of safety, sequential or logical operations, where control actions are based on equipment and alarm status.
Prom
Programmable read-only memory. A semiconductor memory whose contents cannot be changed by the computer after it has been programmed.
Proof Pressure
The specified pressure which may be applied to the sensing element of a transducer without causing a permanent change in the output characteristics.
Propagation delay
Amount of time required for a signal to pass through a circuit.
Proportional control
Control action in which output corrections are directly proportional to the process variable's deviation from setpoint.
Proportional-integral-derivative (PID) control
Three-term control algorithm combining proportional, integral, and derivative control actions.
Proportioning Band
A temperature band expressed in degrees within which a temperature controller’s time proportioning function is active.
Proportioning Control Mode
A time proportioning controller where the amount of time that the relay is energized is dependent upon the system’s temperature.
Proportioning Control plus Derivative Function
A time proportioning controller with a derivative function. The derivative function senses the rate at which a system’s temperature is either increasing or decreasing and adjusts the cycle time of the controller to minimize overshoot or undershoot.
Proportioning Control plus Integral
A two-mode controller with time proportioning and integral (auto reset) action. The integral function automatically adjusts the temperature at which a system has stabilized back to the set point temperature, thereby eliminating droop in the system.
Proportioning Control with Integral and Derivative Functions
Three mode PID controller. A time-proportioning controller with integral and derivative functions. The integral function automatically adjusts the system temperature to the set point temperature to eliminate droop due to the time proportioning function. The derivative function senses the rate of rise or fall of the system temperature and automatically adjusts the cycle time of the controller to minimize overshoot or undershoot.
An enclosure usually made out of metal at the end of a heater or probe where connections are made.
Protection Tube
A metal or ceramic tube, closed at one end, into which a temperature sensor is inserted. The tube protects the sensor from the medium into which it is inserted.
PSIA
Pounds per square inch absolute, the pressure unit used when the zero reference is full vacuum.
PSID
Pounds per square inch differential. Pressure difference between two points.
PSIG
Pound per square inch gauge. Pressure referenced to ambient air pressure.
PSIS
Pounds per square inch standard. Pressure referenced to a standard atmosphere.
Pulse Width Modulation
An output in the form of duty cycle which varies as a function of the applied measurand.
Pyroelectric detector
Pyrometer
Device used to measure the infrared radiation (hence temperature) given off by a body or surface.
The movement of energy in the form of electromagnetic waves.
Noise induced upon signal wires by ambient radio-frequency electromagnetic radiation with the effect of obscuring the instrument signal.
The frequency range between ultrasonic and infrared. AM broadcastfrequencies range from 540 to 1,800 kHz, while FM broadcasts from 88 to 108 MHz.
Ramp
The total (transient plus steady state) time response resulting from a sudden increase in the rate of change from zero to some finite value of the input stimulus.
Range
Those values which a transducer is intended to measure, specified by upper and lower limits.
Rangeability
The ratio of the maximum flowrate to the minimum flowrate of a meter.
Rankine (°R)
An absolute temperature scale based upon the Fahrenheit scale with 180° between the ice point and boiling point of water. 459.67°R = 0°F.
Rate Action
The derivative function of a temperature controller.
Rate Time
The time interval over which the system temperature is sampled for the derivative function.
Ratiometric Measurement
A measurement technique where an external signal is used to provide the voltage reference for the dualslope A/D converter. The external signal can be derived from the voltage excitation applied to a bridge circuit or pick-off supply, thereby eliminating errors due to power supply fluctuations.
Reactance
The opposition to the flow of AC current, which is created by either inductance or capacitance. In such a circuit the total impedance is therefore the sum of reactance and resistance. Its units are ohms.
Real Time
The time interval over which the system temperature is sampled for the derivative function.
Recovery Time
The length of time which it takes a transducer to return to normal after applying a proof pressure.
Redox Potential
The potential developed by a metallic electrode when placed in a solution containing a species in two different oxidation states.
Reference input
An external signal serving as a setpoint or as a standard of comparison for a controlled variable.
Reference Junction
The cold junction in a thermocouple circuit which is held at a stable, known temperature.
Reference Mark
Any diagnostic point or mark which can be used to relate a position during rotation of a part to its location when stopped.
Reference Plane
Any plane perpendicular to the shaft axis to which an amount of unbalance is referred.
Reflectivity/reflectance
The fraction of incident radiation reflected by an object or surface.
Refractory Metal Thermocouple
A class of thermocouples with melting points above 3600°F. The most common are made from tungsten and tungsten/rhenium alloys, Types G and C. They can be used for measuring high temperatures up to 2200°C (4000°F) in non-oxidizing, inert, or vacuum environments.
Relative accuracy
Measure in LSB of the accuracy of an ADC. It includes all non-linearity and quantization errors.
Relay (Mechanical)
An electromechanical device that completes or interrupts a circuit by physically moving electrical contacts into contact with each other.
Relay (Solid State)
A solid state switching device which completes or interrupts a circuit electrically with no moving parts.
Reliability
The probability that a device will perform its objective adequately for the period of time specified, under the operating conditions specified.
Remote terminal unit (RTU)
Industrial control and data collection device similar to a PLC but designed for remote data collection, transfer and communication via wire-based or radio telemetry links to DCS or computer systems.
Repeatability
The ability of a transducer to reproduce output readings when the same measurand value is applied to it consecutively, under the same conditions, and in the same direction. Repeatability is expressed as the maximum difference between output readings.
Reproducibility
The closeness of agreement among repeated measurements of the output for the same value of the input made under the same operating conditions over a period of time, approaching from both directions. It includes hysteresis, dead band, drift and repeatability.
Residual (Final) Unbalance
Residual unbalance is that unbalance of any kind that remains after balancing.
Resistance Ratio Characteristic
For thermistors, the ratio of the resistance of the thermistor at 25°C to the resistance at 125°C.
Resistance Temperature Characteristic
A relationship between a thermistor’s resistance and the temperature.
Resistance temperature detector (RTD)
A metallic probe that measures temperature based upon its coefficient of resistivity.
Resistance, resistivity
Resistance is the opposition to the flow of current in a DC circuit. Its unit is the ohm, which is defined as a resistance, that will give a one ampere current flow, if a one volt potential difference is applied in the circuit Resistivity is the reciprocal of conductivity, its unit is ohm/cm.
Resistance
The resistance to the flow of electric current measured in ohms (.). For a conductor, resistance is a function of diameter, resistivity (an intrinsic property of the material) and length.
Resonance
A condition of oscillation caused by a small amplitude of periodic input has afrequency approaching one of the natural frequencies of the driven system.
Resonant Frequency
The measurand frequency at which a transducer responds with maximum amplitude.
Response Time (time constant)
The time required by a sensor to reach 63.2% of a step change in temperature under a specified set of conditions. Five time constants are required for the sensor to stabilize at 100% of the step change value.
Response Time
The length of time required for the output of a transducer to rise to a specified percentage of its final value as a result of a step change of input.
Reynolds Number
The ratio of inertial and viscous forces in a fluid defined by the formula Re = rVD/µ, where
RFI
Rheostat
A variable resistor.
Ribbon cable
Flat cable in which the conductors are side by side.
Rise Time
The time required for a sensor or system to respond to an instantaneous step function, measured from the 10% to 90% points on the response waveforms.
RMS value
The square root of the average of the squares (root-mean-square) of theinstantaneous values. It is the square root of the arithmetical mean of the squares.
Room Conditions
Ambient environmental conditions under which transducers must commonly operate.
Root Mean Square (RMS)
Square root of the mean of the square of the signal taken during one full cycle.
Rotor
A rotor is a rotating body whose journals are supported by bearings.
RTD
Resistance temperature detector.
Salt Effect (fx)
The effect on the activity coefficient due to salts in the solution.
SAMA
Scientific Apparatus Makers Association. An association that has issued standards covering platinum, nickel, and copper resistance elements (RTD’s).
Sample-and-hold (S/H)
Circuit that acquires and stores an analog voltage on a capacitor for subsequent conversion.
Sampling period
The time interval between observations.
Scale factor
The factor by which the number of scale divisions indicated or recorded by an instrument should be multiplied to compute the value of the measured variable.
SCE
Saturated calomel electrode.
SCFM
Standard cubic foot per minute, where the term "standard" usually refers to 14.7 PSIA pressure and 68[DEGREE]F temperature.
SCR
Silicon controlled rectifier.
Secondary Device
A part of the flowmeter which receives a signal proportional to the flowrate, from the primary device, and displays, records and/or transmits the signal.
Secondary standard
A standard of unit measurement derived from a primary standard.
Seebeck Coefficient
The derivative (rate of change) of thermal EMF with respect to temperature, normally expressed as millivolts per degree.
Seebeck Effect
When a circuit is formed by a junction of two dissimilar metals and the junctions are held at different temperatures, a current will flow in the circuit caused by the difference in temperature between the two junctions.
Seebeck EMF
The open circuit voltage caused by the difference in temperature between the hot and cold junctions of a circuit made from two dissimilar metals.
Self-calibrating
Data acquisition board that calibrates its own A/D and D/A circuits with reference to a stable onboard reference.
Self-Heating
Internal heating of a transducer as a result of power dissipation.
Sensing Element
That part of a transducer which reacts directly in response to input.
Sensitivity Shift
A change in slope of the calibration curve due to a change in sensitivity.
Sensitivity
The minimum change in input signal to which an instrument can respond.
Sensor
An element or device which detects a variable by receiving information in the form of one quantity and converts it to information in the form of that or an other quantity.
Servomechanism
An automatic feedback device in which the controlled variable is mechanical position or any of its time derivatives.
Set Point
The temperature at which a controller is set to control a system.
Settling time
The time required after a stimulus for the output to center and remain within a specified narrow band centered on its steady-state value.
Shear Modulus
The ratio of the shear stress and the angular shear distortion.
Shear Stress
Where normal stress is perpendicular to the designated plane, shear stress is parallel to the plane.
Shearing Strain
A measure of angular distortion also directly measurable, but not as easily as axial strain.
Sheath Thermocouple
A thermocouple made out of mineral-insulated thermocouple cable which has an outer metal sheath.
Shielded twisted pair (STP)
Cable construction that includes an external grounded shield as well as twisting on a regular basis to help minimize noise interferences.
SI
System Internationale. The name given to the standard metric system of units.
Signal Conditioner
A circuit module which offsets, attenuates, amplifies, linearizes and/or filters the signal for input to the A/D converter. The typical output signal conditioner is +2 V dc.
Signal Conditioning
To process the form or mode of a signal so as to make it intelligible to, or compatible with, a given device, including such manipulation as pulse shaping, pulse clipping, compensating, digitizing, and linearizing.
Signal
A variable that carries information about another variable that it represents.Signal-to-noise ratio
Signal-to-noise ratio (SNR)
The ratio of the overall rms signal level to the rms noise level, expressed in dB.
Simultaneous sampling (SS)
System in which each input or output channel is digitized or updated at the same time.
Single-ended (SE)
An analog input that is measured with respect to a common ground.
Single-Ended Input
A signal-input circuit where SIG LO (or sometimes SIG HI) is tied to METER GND. Ground loops are normally not a problem in AC-powered meters, since METER GND is transformer-isolated from AC GND.
Single-Plane (Static) Balancing Machine
A single plane balancing machine is a gravitational or centrifugal balancing machine that provides information for accomplishing single plane balancing.
The smallest radius that a strain gage can withstand in one direction, without special treatment, without suffering visible damage.
Soft Start
A method of using phase angle control to gradually increase the output powerover a period of several seconds. Used for heaters with a low electrical resistance when cfold or for limiting in-rush current to inductive loads.
Solvation
Ions in solution are normally combined with at least one molecule of solvent. This phenomenon is termed solvation.
The ability to adjust the gain of a process or strain meter so that a specified display span in engineering units corresponds to a specified signal span.
Span shift
Any change in slope of the input-output curve.
Span
The difference between the upper and lower limits of a range expressed in the same units as the range.
Spare
A connector point reserved for options, specials, or other configurations. The point is identified by an (E#) for location on the electrical schematic.
Specific Gravity
The ratio of mass of any material to the mass of the same volume of pure water at 4°C.
Specific Heat
The ratio of thermal energy required to raise the temperature of a body 1° to the thermal energy required to raise an equal mass of water 1°.
Spectral Filter
A filter which allows only a specific band width of the electromagnetic spectrum to pass, i.e., 4 to 8 micron infrared radiation.
Spectrum Analysis
Utilizing frequency components of a vibration signal to determine the source and cause of vibration.
Spectrum
The resolving of overall vibration into amplitude components as a function of frequency.
Spot Size
The diameter of the circle formed by the cross section of the field of view of an optical instrument at a given distance.
Spurious Error
Random or erratic malfunction.
SSR
Solid state relay.
Stability
The ability of an instrument or sensor to maintain a consistent output when a constant input is applied.
Stagnation Pressure
The sum of the static and dynamic pressure.
Standard Electrode Potential (E0)
The standard potential E0 of an electrode is the reversible emf between the normal hydrogen electrode and the electrode with all components at unit activity.
Standardization
a process of equalizing electrode potentials in one standardizing solution (buffer) so that potentials developed in unknown solutions can be converted to pH values.
Static Calibration
A calibration recording pressure versus output at fixed points at room temperature.
Static Error Band
The error band applicable at room temperature.
Static Pressure
Pressure of a fluid whether in motion or at rest.
Static Unbalance
Static unbalance is that condition of unbalance for which the central principal axis is displayed only parallel to the shaft axis
Statistical process control (SPC)
Analysis methodology in which characteristics of a process are measured or counted and tracked. Statistical rules are used to determined whether variations are random or need correction.
A flow rate in the measuring section of a flow line that does not vary significantly with time.
That condition of vibration induced by an unchanging continuing periodic force.
Sterling cycle
Thermodynamic cycle commonly used to cool thermographic detectors.
Stiffness
The ratio of change of force (or torque) to the resulting change in deflection of a spring-like element. It is the opposite of compliance.
Strain gage
Sensor whose resistance varies with applied force. A measuring element forconverting force, pressure, tension, weight, etc., into a change in electrical resistance.
Strain
The ratio of the change in length to the initial unstressed reference length of an element under stress.
Strouhal Number
A nondimensional parameter important in vortex meter design defined as
Subsidence
The progressive reduction or suppression of oscillation in a device or system.
Super Cooling
The cooling of a liquid below its freezing temperature without the formation of the solid phase.
Super Heating
1. The heating of a liquid above its boiling temperature without the formation of the gaseous phase. 2. The heating of the gaseous phase considerably above the boiling-point temperature to improve the thermodynamic efficiency of a system.
Suppressed range
A range in which the zero value of the measured variable is greater than the lower-range value (LRV). The terms elevated zero, suppression or suppressed span are also used to express the condition that the zero of the measured variable is greater than LRV.
Suppressed span
The span in which the zero of the measured variable is greater than the LRV.
Suppressed zero
The range in which the zero value of the measured variable is less than the lower range value. The terms
Suppression ratio
The ration of the lower-range value to the span. If range is 20-100 andtherefore span is 80 and LRV is 20, the suppression ratio is 20/80 = 0.25 or 25%.
Surge Current
A current of short duration that occurs when power is first applied to capacitive loads or temperature dependent resistive loads such as tungsten or molybdenum heaters—usually lasting not more than several cycles.
Suspension Effect
The source of error due to varied reference liquid junction potential depending upon whether the electrodes are immersed in the supernatant fluid or deeper in the sediment. Normally encountered with solutions containing resins or charged colloids.
Swaging
A sheathed electrical element manufacturing process when the element sheath is hammered in a die to reduce its diameter and compact its insulation
Synchronous
An event or action that is synchronized to a reference clock.
System noise
Measure of the amount of noise seen by an analog circuit or an ADC when the analog inputs are grounded.
Temperature coefficient
The amount of drift, in percent of full scale output, that might result from a 1[DEGREE]C change in ambient temperature.
Temperature Error
The maximum change in output, at any measurand value within a specified range, when the transducer temperature is changed from room temperature to specified temperature extremes.
Temperature Range, Compensated
The range of ambient temperatures within which all tolerances specified for Thermal Zero Shift and Thermal Sensitivity Shift are applicable (temperature error).
Temperature Range, Operable
The range of ambient temperatures, given by their extremes, within which a transducer may be operated. Exceeding compensated range may require recalibration.
Thermal Coefficient of Resistance
The change in resistance of a semiconductor per unit change in temperature over a specific range of temperature.
Thermal Conductivity
The ability of a material to conduct heat in the form of thermal energy.
Thermal detector
Radiation thermometer detector that generates a signal based on the heat energy absorbed.
Thermal Expansion
An increase in size due to an increase in temperature expressed in units of an increase in length or increase in size per degree, i.e. inches/inch/degree C.
The distribution of a differential temperature through a body or across a surface.
Thermal Sensitivity Shift
The sensitivity shift due to changes of the ambient temperature from room temperature to the specified limits of the compensated temperature range.
Thermal shock
An abrupt temperature change applied to the device.
Thermal Zero Shift
An error due to changes in ambient temperature in which the zero pressure output shifts. Thus, the entire calibration curve moves in a parallel displacement.
Thermistor
A temperature-sensing element composed of sintered semiconductor material which exhibits a large change in resistance proportional to a small change in temperature. Thermistors usually have negative temperature coefficients.
Thermocouple
The junction of two dissimilar metals which has a voltage output proportional to the difference in temperature between the hot junction and the lead wires (cold junction) (refer to Seebeck emf).
Thermography
The presentation and interpretation of two-dimensional temperature pictures.
Thermometry
The science of temperature measurement.
Thermopile
An arrangement of thermocouples in series such that alternate junctions are at the measuring temperature and the reference temperature. This arrangement amplifies the thermoelectric voltage. Thermopiles are usually used as infrared detectors in radiation pyrometry.
Thermostat
An electro-mechanical device which opens or closes a contact at a specified temperature. The most common forms of thermostat are bulb and capillary and bi-metal strip
Thermowell
A closed-end tube designed to protect temperature sensors from harsh environments, high pressure, and flows. They can be installed into a system by pipe thread or welded flange and are usually made of corrosion-resistant metal or ceramic material, depending upon the application.
Thomson Effect
When current flows through a conductor within a thermal gradient, a reversible absorption or evolution of heat will occur in the conductor at the gradient boundaries.
Time constant
The value T in an exponential term A(-t/T). For the output of a first-order system forced by a step or an impulse, T is the time required to complete 63.2% of the total rise or decay. For higher order systems, there is a time constant for each of the first-order components of the process.
Torque tube
A torsion spring used to measure force or pressure.
Transducer Vibration
Generally, any device which converts movement, either shock or steady state vibration, into an electrical signal proportional to the movement; a sensor.
Transducer
A device (or medium) that converts energy from one form to another. The term is generally applied to devices that take physical phenomena (pressure, temperature, humidity, flow, etc.) and convert them to electrical signals.
Transient Vibration
A temporary vibration or movement of a mechanical system.
Transient
The behavior of a variable during transition between two steady-states.
Transistor-to-transistor logic (TTL)
Voltage-level changes readily communicated and interpreted by microprocessors, typically in the 0-5 V range.
Transmittance/transmissivity
The fraction of incident radiation passed through an object.
Transmitter (Two-Wire)
A device which is used to transmit temperature data from either a thermocouple or RTD via a two-wire current loop. The loop has an external power supply and the transmitter acts as a variable resistor with respect to its input signal.
Transmitter
A transducer which responds to a measured variable by means of a sensing element, and converts it to a standardized transmission signal which is a function only of the values of themeasured variable.
Triac
A solid state switching device used to switch alternating current wave forms.
Triboelectric Noise
The generation of electrical charges caused by layers of cable insulation. This is especially troublesome in high impedance accelerometers.
Triple Point (Water)
The thermodynamic state where all three phases, solid, liquid, and gas, may all be present in equilibrium. The triple point of water is .01°C.
Triple Point
The temperature and pressure at which solid, liquid, and gas phases of a given substance are all present simultaneously in varying amounts.
True RMS
The true root-mean-square value of an AC or AC-plus-DC signal, often used to determine power of a signal. For a perfect sine wave, the RMS value is 1.11072 times the rectified average value, which is utilized for low-cost metering. For significantly nonsinusoidal signals, a true RMS converter is required.
A load with TTL voltage levels, which will draw 40 µA for a logic 1 and –1.6 mA for a logic 0.
TTL
Transistor-to-transistor logic. A form of solid state logic which uses only transistors to form the logic gates.
TTL-Compatible
For digital input circuits, a logic 1 is obtained for inputs of 2.0 to 5.5 V which can source 40 µA, and a logic 0 is obtained for inputs of 0 to 0.8 V which can sink 1.6 mA. For digital output signals, a logic 1 is represented by 2.4 to 5.5 V with a current source capability of at least 400 µA, and a logic 0 is represented by 0 to 0.6 V with a current sink capability of at least 16 mA.
Turbulent Flow
When forces due to inertia are more significant than forces due to viscosity.
Two-color pyrometer
A radiation thermometer that measures the radiation output of a surface at two wavelengths, thus reducing any effects of emissivity variation with wavelength.
Typical
Error within plus or minus one standard deviation (±1%) of the nominal specified value, as computed from the total population.
UL
Underwriters Laboratories, Inc. An independent laboratory that establishes standards for commercial and industrial products.
Ultraviolet
That portion of the electromagnetic spectrum below blue light (380 nanometers).
Unbalance Tolerance
The unbalance tolerance with respect to a radial plane (measuring plane or correction plane) is that amount of unbalance which is specified as the maximum below which the state of unbalance is considered acceptable.
Unbalance
That condition which exists in a rotor when vibratory force or motion is imparted to its bearings as a result of centrifugal forces.
Undershoot
The difference in temperature between the temperature a process goes to, below the set point, after the cooling cycle is turned off and the set point temperature.
Ungrounded Junction
A form of construction of a thermocouple probe where the hot or measuring junction is fully enclosed by and insulated from the sheath material.
Union
A form of pipe fitting where two extension pipes are joined at a separable coupling.
Unipolar
A signal range that is always positive (for example, 1 to 5 V).
Unshielded twisted pair (UTP)
Cable construction consisting of pairs of wires twisted at regular intervals (called the pitch) in order to reduce electrical noise interferences.
Upper range limit (URL)
The highest value of the measured variable that a device can beadjusted to measure. (This value corresponds to the top of the range.)
Upper range value (URV)
The highest value of the measured variable that a device is adjusted to measure. (This value corresponds to the top of the span.)
Vacuum
A pressure less than atmospheric pressure.
Vapor pressure
The pressure exerted by a vapor which is in equilibrium with its own liquid.
Variable
Any condition which is measured, controlled (directly or indirectly) or manipulated.
Velocity limit
A limit on the rate of change, which a particular variable may not exceed.
Velocity
The time rate of change of displacement; dx/dt.
Vibration Error Band
The error recorded in output of a transducer when subjected to a given set of amplitudes and frequencies.
Vibration Error
The maximum change in output of a transducer when a specific amplitude and range of frequencies are applied to a specific axis at room temperature.
Vibration Transducer
Generally, any device which converts movement, either shock or steady state vibration, into an electrical signal proportional to the movement; a sensor.
Vibration
A periodic motion or oscillation of an element, device, or system.
Viscosity
The inherent resistance of a substance to flow.
Volt
The (electrical) potential difference between two points in a circuit. The fundamental unit is derived as work per unit charge— (V = W/Q). One volt is the potential difference required to move one coulomb of charge between two points in a circuit using one joule of energy.
Voltage
An electrical potential which can be measured in volts.
Voltmeter
An instrument used to measure voltage.
Volume Flow Rate
Calculated using the area of the full closed conduit and the average fluid velocity in the form, Q = V x A, to arrive at the total volume quantity of flow. Q = volumetric flowrate, V = average fluid velocity, and A = cross sectional area of the pipe.
Warm Up Period
The time required after energizing a device before its rated performance characteristics start to apply.
Watt Density
The watts emanating from each square inch of heated surface area of a heater. Expressed in units of watts per square inch.
Wavelength
Distance, from peak to peak, of any waveform. For electromagnetic radiation in the infrared region, typically measured in microns and symbolized by l.
Wet leg
When the low pressure side of a d/p cell is connected to the vapor space of the tank and high pressure side is filled with a stable, noncorrosive, known density liquid, the installation is called a "wet leg" arrangement.
Working Standard
A standard of unit measurement calibrated from either a primary or secondary standard which is used to calibrate other devices or make comparison measurements.
Wye
An electrical connection when one end of three loads is connected together and the other end to one each of the three phases of a power supply
Young's Modulus
Young's Modulus (the Modulus of Elasticity) is equivalent to the ratio of normal stress to strain.
The ability to adjust the display of a process or strain meter so that zero on the display corresponds to a non-zero signal, such as 4 mA, 10 mA, or 1 V dc. The adjustment range is normally expressed in counts.
Zero Offset
The difference expressed in degrees between true zero and an indication given by a measuring instrument.
Zero Point
The electrical zero point where zero millivolts would be displayed. Used in conjunction with the slope control to provide a narrower range calibration.
Zero Power Resistance
The resistance of a thermistor or RTD element with no power being dissipated.
Zero Suppression
The span of an indicator or chart recorder may be offset from zero (zero suppressed) such that neither limit of the span will be zero.
Zero Voltage Switching
The making or breaking of circuit timed such that the transition occurs when the voltage wave form crosses zero voltage; typically only found in solid state switching devices.
Zone, neutral
A predetermined range of input values which do not result in a change of the previously existing output value.

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