Glossary of Terms

For your convenience, we have provided an overview of some of the most common technical terms associated with pressure, temperature and level measurement.  For technical assistance, please contact a member of sales team.


A

Absolute Pressure
The combined local pressure induced by some source and the atmospheric pressure at the measurement location.

Absolute Zero
Temperature at which thermal energy is at a minimum. Defined as Kelvin, calculated to be -273.16° C or -459.69° F.

Acceleration Sensitivity
The electrical output of a pressure transducer at zero applied pressure input, caused by static or dynamic acceleration applied to the transducer case. It is expressed as a percentage of full-range output per “g”.

Accuracy
The ratio of the error to the full-scale output or the ratio of the error output, as specified, expressed in percent. Accuracy may be expressed in terms of units of measurand, or as with ± percent of full-scale output, or span.

Ambient Conditions
The condition(s) around the transducer (pressure, temperature, etc).

Ambient Pressure
The pressure of the medium surrounding the transducer.

Ambient Temperature
The average or mean temperature of the surrounding air, which comes in contact with the equipment and instrument under test.

Amplifier
An electronic device which boots or increases a small signal to a higher level, usually for transmission, scale convenience or noise immunity.

Analog Output
A voltage or current signal that is a continuous function of the measured parameter.

ANSI
Acronym for the American National Standards Institute.


B
Background Noise
The total noise floor from all sources of interference in a measurement system independent of the presence of a data signal.

Boiling Point
The temperature at which a substance in the liquid state transforms to the gaseous state commonly refers to the boiling point of water (+100°C or +212°F at sea level).

Breakdown Voltage Rating
The AC or DC voltage, which can be applied across the insulation portion of a transducer without arcing or conduction above a specific current value.

BTU: British Thermal Unit
The quantity of thermal energy required to rise one pound of water 1°F at or near its maximum density (39.1°F).

Burst Pressure
The maximum pressure applied to a transducer sensing element or case without causing leakage.


C
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.

Calibration Cycle
Pressure calibration in ascending and descending duration.

Cavitation
The boiling of a liquid caused by a decrease in pressure rather than an increase in temperature.

Chatter
The rapid cycling on and off of a control process relay due to insufficient bandwidth in the controller.

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.

Common-Mode Voltage
The AC or DC voltage which is tolerable between signal and ground. One type of CMV is specified between signal low and power ground.

Compensation
The addition of specific material or device(s) to counteract a known error.

Controlled Variable
A process variable which is to be controlled at some desired value by means of error; i.e. cold junction compensation for thermocouples.

Current Loop
A two-wire loop in which the current through the wires is maintained according to a controlling device, usually a two-wire transmitter. The advantages of a current loop are longer distance signal transmission, better noise immunity, and the ability to power the two-wire transmitter throughout the same two wires. The most common current loop is 4-20 mA.

Cycle Time
The time usually expressed in seconds for a controller to complete one on/off cycle.


D

DC Direct Current Damping
The energy-dissipating characteristic, which, together with natural frequency, determines the limit of frequency response and the response time characteristic of a transducer, instrument, or system. In response to a step change of input or measurand, an underdamped system oscillates about its final steady value and an overdamped system come to rest without overshoot, and a critically damped system is at the point between the underdamped and overdamped.

Dead Band
The range through which input can be varied without initiating observable change in output. (There is a separate and distinct input-output relationship for increasing and decreasing signals.)

Dead Volume
The volume of the pressure port of a transducer at room temperature and ambient barometric pressure.

Deviation
The difference between the value of the controlled variable and the value at which it is being controlled.

Differential Pressure
The static pressure difference generated by the primary device when there is no difference in elevation between the upstream and downstream pressure taps.

Digital Output
An output signal, which represents the size of a stimulus or input in the form of a series of discrete quantities.

Disturbance
An undesired change that takes place in a process(es) that tends to affect adversely the value of a controlled variable.

Drift
An undesired change in output over a period of time, of which change is not a function of the measurand.


E
End Point
The output at zero pressure and full-scale pressure.

Environmental Conditions
All conditions to which a transducer may be exposed during shipping, storage, handling, and operation.

Error
The difference between the value indicated by the transducer and the true value of the pressure being sensed.

Error Band
The allowable deviation of output from specific reference norm.

Excitation
The external application of electrical voltage/ current applied to a transducer for normal operation.

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, flashed or the explosion of the container itself, and maintain an external temperature that will not ignite the surrounding gases.


F
Fahrenheit
A temperature scale defined by 32° at the ice point and 212° at the boiling point of water at sea level.

Filter
A device to sort desired result from undesired. Electrically, a selective circuit which passes through certain frequencies, while attenuating or rejecting others.

FM-Approved
An instrument that meets a specific set of specifications established by the Factory Mutual Research Corporation which sets industrial safety standards.

Freezing Point
The temperature at which the substance goes from a liquid phase to a solid phase.

Frequency, Natural
The frequency of free (not forced) oscillation of a transducer, mechanical apparatus, or electrical circuit.

Frequency Output
An output in the form of frequency, which varies as a function of the applied input.

Frequency Response
The change with frequency of the output measurand amplitude ratio (and of the phase difference between output and measurand), for a sinusoidally varying input applied to a transducer, instrument or stem. It is usually specified as “within ± percent (or ±dB) to Hz”.

Full Bridge
A Wheatstone Bridge configuration utilizing active elements or stain gauges.

Full Scale Output
The algebraic difference between the zero pressure output and full scale pressure output.


G
Gauge Factor
A measure of the ratio of the relative change of resistance to the relative change in length of a piezoresistive strain gauge.

Gauge Pressure
The difference between the local absolute pressure of the fluid and the atmospheric pressure at the place of the measurement.

Gain
The ratio of the change in output to the change in input, which caused it.

Ground
The reference point of an electrical system, or alternatively, the local earth potential (earth ground).


H
Half Bridge
2 active elements or stain gauges.

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.

Humidity, Relative
The moisture content of air relative to the maximum that the air can contain at the same pressure and temperature.

Hysteresis
The difference in output when the pressure value is first approached with increasing pressure and then with decreasing pressure.


I
Input Impedance
The resistance measured across the excitation terminals of a transducer.

Insulation Resistance
The resistance measured between two insulated points on a transducer when a specific DC voltage is applied at room temperature.

Intrinsically Safe
An instrument which will not produce any spark or thermal effect, under normal or abnormal conditions, that will ignite a specified gas mixture.

Isolation
Allowing a difference in potential (voltage) between the reference point (ground) of two circuits without any appreciable current flow between them.


K
Kelvin (Symbol K)
The units 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 = 273.16K (there is no degree [°] symbol used with the Kelvin scale).


L
Life Cycle
The minimum number of pressure cycles the transducer can endure and still remain within a specified tolerance.

Linearity
The closeness of a calibration curve to a specific straight line between end points.

Linearity (End Point/or Terminal)
Linearity as referring to a straight line between end points.

Load Impedance
The impedance presented to the output terminals of a transducer or instrument by the associated external circuitry.

Loop Gain
The product of the gains of all the elements in a loop.


M
Measurand
A physical quantity, property or condition which is measured. The term measurand is preferred to “input”, “parameter to be measured”, “physical phenomenon”, “stimulus”, and/or “variable.”

Melting Point
The temperature at which a substance transforms from a solid phase to a liquid phase.

Mounting Error
The error resultant from installing the pressure transducer, both electrical and mechanical.


N

Noise
An unwanted signal which can contribute to errors in measurement. Examples are hum (power lines), radio frequency interference (RFI), electromagnetic interference (EMI), and broadband or white noise.

NEMA-4
A standard from the National Electrical Manufacturers Association, which defines enclosures for indoor or outdoor use primarily to provide a degree of protection against windblown dust, rain, and/or splashing water.


O
Output
The electrical signal, which is produced by a pressure applied to the transducer sensor.

Output Impedance
The impedance across the output terminals of a transducer or instrument presented to the associated external circuitry.


P
PH
The negative base-ten logarithm of the hydrogen-ion activity in a solution.

PID
Proportional, Integral, Derivative. A three-mode control action where the controller has time portioning, integral (auto reset), and derivative action.

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

Positive Feedback
A closed loop in which any change is reinforced until a limit is eventually reached.

Potentiometer
A variable resistor often used to control a circuit.

Power Supply
A separate unit or part of a circuit that supplies power to the rest of the circuit or to a system.

Primary Loop
The outer loop in a cascade system.

Proof Pressure
The maximum amount of pressure that can be applied to a pressure transducer without changing any specification.

PSIA
Pounds per square inch absolute. Pressure referenced to a vacuum.

PSIG
Pounds per square inch gauge. Pressure referenced to ambient air pressure.


R
Range
The upper and lower pressure limits that a transducer is required to measure.

Recovery Time
The length of time that it takes a transducer to return to normal after applying a proof pressure.

Repeatability
The ability of a transducer to repeat its output signal at a given pressure in one direction repeatedly.

Rate Time
The time interval over which the system temperature is sampled for the derivative function.

Resolution
The magnitude of output step changes as pressure is varied continuously over the range and usually expressed as a percentage of range. (Primarily applies to potentiometric type units. Strain gauge units’ resolution is infinite).

Reference Junction
The cold junction in a thermocouple circuit, which is held at a stable known temperature. The standard reference temperature is 0°C (+32°F); however, other temperatures can be used.

Response Time
The length of time required for the output of a transducer to rise to a specified percentage of its final value.

RFI
Radio Frequency Interference.

Room Condition
Ambient conditions used for test purposes.


S
Self-Heating
Internal heating of a transducer as a result of power dissipation.

Self-Regulation
The property of a process or machine which permits attainment of equilibrium, after a disturbance, without the intervention of a controller.

Sensing Element
The part of a transducer, which reacts directly in response to the pressure.

Sensitivity
The ratio in a transducer output change versus a change in applied pressure.

Sensitivity Shift
A change in the calibration slope.

Set Point
An input variable which sets the desired value of a controlled variable.

Shield
A protective enclosure surrounding a circuit or cable which is to protect it from an electrical disturbance such as noise.

Signal Conditioner
A module or device which condition an electrical signal in order to make it more useful. A signal conditioner often contains an amplifier and filter, and may provide isolation, linearization, compensation and other functions.

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, digitizing, and linearizing.

Stability
The ability of a transducer to retain performance throughout its life span.

Strain Gauge
A measuring element for converting force, pressure, tension, etc., into an electrical signal.

Static Calibration
A calibration recording pressure versus output at room temperature.

Static Error Band
The error band applicable at room temperature.

Static Pressure
The pressure of a fluid or gas at rest.


T
Temperature Range, Operable
The range of ambient temperature, given by their extremes, within which the transducer is intended to operate.

Temperature Range, Compensated
The range of ambient temperature for which Thermal Zero Shift is applicable (temperature error). Operation outside this range may require re-calibration.

Thermal Coefficient of Resistance
The changes in resistance of semiconductor per unit change in temperature over a specific range of temperature.

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 Zero Shift
An error due to changes in ambient temperature in which the zero pressure output shifts. Thus, the entire calibration curve moves in parallel displacement.

Transducer
In the broadest sense it is a device (or medium) that converts one energy form to another. Therefore, items such as a windmill, electric light, or an automobile engine could be called a “transducer” – but, in common practice, the term is generally applied to devices that take a physical phenomenon (pressure, temperature, humidity, flow, etc.) and converts it to an electrical output.

Transmitter
A device which translates the low-level output of a sensor or transducer to a higher level signal which is suitable for transmission to a site where it can be processed further.

True RMS
The true Root-Mean-Square of an AC or AC plus DC signal which is 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.

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