A Flow Measurement Orientation
  The Flow Pioneers
  Flow Sensor Selection
  Accuracy vs. Repeatability

Differential Pressure Flowmeters
  Primary Element Options
  Pitot Tubes
  Variable Area Flowmeters

Mechanical Flowmeters
  Positive Displacement   Flowmeters
  Turbine Flowmeters
  Other Rotary Flowmeters

Electronic Flowmeters
  Magnetic Flowmeters
  Vortex Flowmeters
  Ultrasonic Flowmeters

Mass Flowmeters
  Coriolis Mass Flowmeters
  Thermal Mass Flowmeters
  Hot-Wire Anemometers

A Level Measurement Orientation
  Level Sensor Selection
  Boiling & Cryogenic Fluids
  Sludge, Foam, & Molten   Metals

Pressure/Density Level Instrumentation
  Dry & Wet Leg Designs
  Bubbler Tubes
  Floats & Displacers

RF/Capacitance Level Instrumentation
  Theory of Operation
  Probe Designs
  Installation Considerations

Radiation-Based Level Instrumentation
  Radar & Microwave
  Ultrasonic Level Gages
  Nuclear Level Gages

Specialty Level Switches
  Thermal Switches
  Vibrating Switches
  Optical Switches

  About OMEGA
  Information Resources


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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.

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, deadband 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 (± 1°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.

Admittance: Admittance of an ac circuit is analogous to 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.

Alphanumeric: A character set containing both letters and numbers.

Alternating current (ac): A flow of electric charge (electric current) that undergoes periodic reverses in direction. In North America, household current alternates at a frequency of 60 times per second.

Ambient pressure: The atmospheric pressure of the medium surrounding a particular sensor. When no specific information is available, it is assumed to be 14.7 psia.

Ambient temperature: The average or mean temperature of the atmospheric air which is surrounding a sensor or instrument. If the sensor is a heat generator, this term refers to the temperature of the surroundings when the sensor is in operation. The ambient temperature is usually stated under the assumption that the sensor is not exposed to the sun or other radiant energy sources.

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.

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.

Ampere (A or amp): The unit of electric current flow, defined as the rate at which one coulomb of electric charge (6.25 x 1018 electrons) is transferred in a second.

Amplifier: A device that generates an output which is stronger than and bears some predetermined relationship (often linear) to its input. It generates the amplified output signal while drawing power from a source other than the signal itself.

Analog signal: A signal that continuously represents a variable or condition.

Analog-to-digital (A/D) conversion: A generic term referring to the conversion of an analog signal into a digital form.

Analog-to-digital converter (ADC): An electronic device that converts analog signals to an equivalent digital form.

Attenuation: The reciprocal of gain; a dimensionless ratio defining the decrease in magnitude of a signal as it passes between two points or two frequencies. Large values of attenuation are expressed in decibels (dB).


Backlash: The relative movement of interlocked mechanical parts that occurs when motion is reversed.

Baud rate: Serial communications data transmission rate expressed in bits per second (bps).

Bipolar: A signal range that includes both positive and negative values (i.e., -10 V to +10 V).

Bode diagram: A plot of log amplitude ratio and phase angle values used in describing transfer functions.

Breakdown voltage: Threshold voltage at which circuit components begin to be damaged.

Byte (B): Eight related bits of data or an eight-bit binary number. Also denotes the amount of memory required to store one byte of data.


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: The process of adjusting an instrument or compiling a deviation chart so that its reading can be correlated to the actual values being measured.

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 a 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.

Capacitance: The capability of a device to store electric charge. The 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.

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.

Compensator: A device that 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. The unit is the mho. The unit of conductivity is cm-mho or cm/ohm.

Controller: A device that operates automatically to regulate a controlled variable.

Coulomb: The amount of electric charge transferred in one second by a current flow of one ampere.


Damping: The suppression of oscillation. The viscosity of a fluid is used in viscous damping, while the induced current in electrical conductors is used to effect magnetic damping.

Deadband: The range through which an input can be changed without causing an observable response.

Dead time: The interval between the initiation of a change in the input and the start of the resulting observable response.

Decibel (dB): Unit for expressing a logarithmic measure of the ratio of two signal levels.

Dielectric: A non-conductor of dc current.

Dielectric constant: A numeral that expresses the degree of non-conductivity of different substances, with full vacuum defined as 1.0.

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.

Dither: A useful oscillation of small magnitude, introduced to overcome the effects of friction, hysteresis, or clogging.

Drift: Undesired change in the input-output relationship over a period of time.

Dynamic range: Ratio of the largest to the smallest signal level a circuit can handle, normally expressed in dB.


Electromotive force: Force that causes the movement of electricity, such as potential difference of voltage. A measure of voltage in an electrical circuit.

Elevation: A range in which the zero value of the measured variable exceeds the lower range value.

Error: The difference between the measured signal value or actual reading and the true (ideal) or desired value.

Error, common mode: Error caused by interference that appears between both measuring terminals and ground.

Error, normal mode: Error caused by interference that appears between the two measuring terminals.

Error, random: The amount of error that remains even after calibrating a sensor. Also called "precision," while "repeatability" is defined as twice that: the diameter (instead of 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 type of error can be eliminated by calibration.


Farad: 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.

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 bi-directional, multivariable data communication capability.

Frequency: The number of cycles over a specified time period during which an event occurs. Normally expressed in cycles per second (hertz, Hz).

Frequency response: The frequency-dependent characteristic that determines the phase and amplitude relationship between sinusoidal input and output.


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, the amount of amplification used, 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 an 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.

Ground: The electrical neutral line having the same potential as the surrounding earth; the negative side of a direct current power system; the reference point for an electrical system.


Hertz (Hz): The unit of frequency, defined as one cycle per second.

Hunting: An undesirable oscillation which continues for some time after an external stimulus has disappeared.

Hysteresis: The property of an element or sensor, whereby output is dependent not only on the value of the input, but on the direction of the current traverse. (That is, the reading of the same value differs as a function of whether the measurement is rising or falling.)


Impedance: Opposition to the flow of ac current; the equivalent of resistance in dc circuits. The unit is the ohm. The impedance of an ac circuit is one ohm if a potential difference of one volt creates a current flow of one ampere within it.

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.

Input/output (I/O): The analog or digital signals entering or leaving a DCS or other central control or computer system involving communications channels, operator interface devices, and/or data acquisition and control interfaces.

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 in proportional control.

Intrinsically safe: Equipment or wiring which is incapable of releasing sufficient electrical or thermal energy to ignite a hazardous mixture of hydrocarbon vapors and air. In such equipment, the electrical energy is limited so that it cannot generate a spark or otherwise ignite a flammable mixture.

ISA: Formerly, The Instrument Society of America; now referred to as the International Society for Measurement & Control.


Laser: Narrow, intense beam of coherent light.

Linearity: The closeness to which a curve approximates a straight line, or the deviation of an instrument's response from a straight line.

Linear stroke: For a transducer, the calibrated mechanical movement over which its electrical output linearity meets its specifications.

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 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 a device can be adjusted to measure.

Lower range value (LRV): The lowest value of the measured variable that a device is adjusted to measure.


Manipulated variable: A quantity or condition which is varied as a function of an actuating error signal so as to change the value of the directly controlled variable.

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, sometimes referred to as the measurand.

Milliamp (mA): One thousandth of an ampere.

Millivolt (mV): One thousandth of a volt.

Multiplexer (Mux): A switching device that sequentially connects multiple inputs or outputs in order to process several signal channels with a single A/D or D/A converter.


Noise: Any undesirable electrical signal, from external sources such as ac power lines, motors, electrical storms, radio transmitters, as well as internal sources such as electrical components.

Non-linearity: The deviation from the best fit straight line that passes through zero.

Normal-mode rejection ratio: The ability of an instrument to reject electrical interference across its input terminals, normally of line frequency (50-60 Hz).

Nyquist theorem: The law that is the basis for sampling continuous information. It states that the frequency of data sampling should be at least twice the maximum frequency at which the information might vary. This theorem should be observed in order to preserve patterns in the information or data, without introducing artificial, lower frequency patterns.


Ohmmeter: A device used to measure electrical resistance.

One-to-one repeater: A diaphragm-operated device which detects process pressure and generates an air (or nitrogen) output signal of equal pressure.

Optical isolation: Two networks or circuits in which an LED transmitter and receiver are used to maintain electrical discontinuity between the circuits.

Output settling time: The time required for an 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: The maximum rate of change of analog output voltage from one level to another.

Overtravel: That part of a stroke which falls between the end of the calibrated range and the travel stop.


Phase: A time-based relationship between a periodic function and a reference.

Phase shift: The angle in degrees between an energizing voltage waveform and an output signal waveform.

Polarity: In electricity, the quality of having two charged poles, one positive and one negative.

Port: A communications connection on an electronic or computer-based device.

Power supply: A separate unit or part of a system that provides power (pneumatic, electric, etc.) to the rest of a system.

Pressure, ambient: The pressure of the medium surrounding a device.

Pressure, design: The pressure used in the design of a vessel or other item of 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, maximum working: The maximum permissible operating pressure at a specified temperature. This is the highest pressure to which the device will be subjected during regular use.

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 a 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 a stroke which falls below the calibrated range, between zero and the travel stop.

Primary element: An element that converts a measured variable into a force, motion or other form suitable for measurement.

Process: Physical or chemical change of matter or conversion of energy.

Process measurement: The acquisition of information that establishes the magnitude of process quantities.

Programmable logic controller (PLC): Computer-based industrial monitoring and control package with applications mostly in the areas of safety, sequential or logical operations, where control actions are based on equipment and alarm status.

Proportional control: A control mode which generates an output correction in proportion to error (the process variable's deviation from setpoint).

Proportional-integral-derivative (PID): Also referred to as a 3-mode controller, combining proportional, integral, and derivative control actions.

psia: Pounds per square inch absolute; the unit of pressure used when the zero reference is full vacuum.

psig: Pounds per square inch gauge; the unit of pressure used when the zero reference is the barometric pressure of the atmosphere.


Radio frequency: The frequency range between ultrasonic and infrared. AM broadcast frequencies range from 540 to 1,800 kHz, while FM broadcasts from 88 to 108 MHz.

Radio frequency interference (RFI): Noise induced upon signal wires by ambient radio-frequency electromagnetic radiation with the effect of obscuring an instrument signal.

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 input stimulus.

Range: The region between the limits within which a quantity is measured, received or transmitted, expressed by stating lower and upper range values.

Reactance: The opposition to the flow of ac current, which is created by either inductance or capacitance. In such a circuit, total impedance is therefore the sum of reactance and resistance. The unit is the ohm.

Reference input: An external signal serving as a setpoint or as a standard of comparison for a controlled variable.

Reliability: The probability that a device will perform its objective adequately for the period of time specified, under the operating conditions specified.

Repeatability: The maximum difference between output readings when the same input is applied consecutively; the closeness of agreement among consecutive measurements of an output for the same value of input under the same operating conditions, approaching from the same direction, usually measured as non-repeatability and expressed as percent of span.

Reproducibility: The closeness of agreement among repeated measurements of an output for the same value of input made under the same operating conditions over a period of time, approaching from both directions. It includes hysteresis, deadband, drift and repeatability.

Resistance; Resistivity: The opposition to the flow of current in a dc circuit. The unit is the ohm, which is defined as the resistance that will give a one-ampere current flow if a one-volt potential difference is applied in a circuit Resistivity is the reciprocal of conductivity; its unit is the ohm/cm.

Resolution: The smallest change in input which produces a detectable change in output. This is the smallest increment of change that can be detected by a measurement system. Resolution can be expressed in bits, in proportions, in percent of actual reading or in percent of full scale. For example, a 12-bit system has a resolution of one part in 4,096 or 0.0244% of full scale.

Resonance: A condition of oscillation caused when a small amplitude of periodic input has a frequency approaching one of the natural frequencies of the driven system.

Response time: An output expressed as a function of time, resulting from the application of a specified input under specified operating conditions.


Sampling period: The time interval between observations.

Scale factor: The factor by which the number of scale divisions indicated or recorded by an instrument must be multiplied to compute the value of a measured variable.

Sensing element: The element that is directly responsive to the value of a measured variable.

Sensitivity: The minimum change in a physical variable to which an instrument can respond; the ratio of the change in output magnitude to the change of the input which causes it after the steady-state has been reached.

Sensor: An element or device that detects a variable by receiving information in the form of one quantity and converting it to information in the form of that or another quantity.

Setpoint: A variable, expressed in the same units as the measurement, which sets either the desired target for a controller, or the condition at which alarms or safety interlocks are to be energized.

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.

Signal: A variable that carries information about another variable that it represents.

Signal-to-noise ratio: Ratio of signal amplitude to noise amplitude. The ratio of overall rms signal level to rms noise level, expressed in dB. For sinusoidal signals, amplitude may be peak or rms.

Span: The algebraic difference between the upper and lower range values expressed in the same units as the range.

Span shift: Any change in slope of the input-output curve.

Stability: The ability of an instrument or sensor to maintain a consistent output when a constant input is applied.

Steady-state: A characteristic of a condition, such as value, rate, periodicity, or amplitude, exhibiting only negligible change over an arbitrary, long period of time.

Stiffness: The ratio of change of force (or torque) to the resulting change in deflection of a spring-like element, the opposite of compliance.

Strain: The ratio of the change in length to the initial unstressed reference length of an element under stress.

Subsidence: The progressive reduction or suppression of oscillation in a device or system.

Suppressed range: A range in which the zero value of a 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 when the zero of the measured variable is greater than the 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 a measured variable is less than the lower range value. The terms "elevation," "elevated range" and "elevated span" are frequently used to express the condition in which the zero of the measured variable is less than the lower range value.

Suppression ratio: The ration of the lower-range value to the span. If range is 20-100 and, therefore, span is 80 and LRV is 20, the suppression ratio is 20/80 = 0.25 or 25%.

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°C change in ambient temperature.

Thermal shock: An abrupt temperature change applied to a device.

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.

Transducer: An element or device that receives information in the form of one quantity and converts it to information in the same or another quantity or form. Primary elements and transmitters are also referred to as transducers.

Transfer function: Mathematical, graphical, or tabular statement of the influence which a system or element has on a signal or action compared at input and at output terminals.

Transient: The behavior of a variable during transition between two steady-states.

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 the measured variable.


Upper range limit (URL): The highest value of a measured variable that a device can be adjusted to measure. (This value corresponds to the top of the range.)

Upper range value (URV): The highest value of a measured variable that a device is adjusted to measure. (This value corresponds to the top of the span.)


Vapor pressure: The pressure exerted by a vapor which is in equilibrium with its own liquid.

Variable: Any condition that is measured, controlled (directly or indirectly) or manipulated.

Velocity limit: A limit on the rate of change which a particular variable may not exceed.

Vibration: The periodic motion or oscillation of an element, device, or system.

Volt (V): The electrical potential difference between two points in a circuit. One volt is the potential needed to move one coulomb of charge between two points while using one joule of energy.


Warm-up period: The time required after energizing a device before its rated performance characteristics start to apply.


Zero offset: The non-zero output of an instrument, expressed in units of measure, under conditions of true zero.

Zero suppression: For a suppressed-zero range, the amount by which a measured variable's zero is less than the lower-range value; can be expressed as a percentage of either the measured variable or of the span.

Zone, neutral: A predetermined range of input values that do not result in a change in the previously existing output value.