Knowledge Base (Key Terms)

Absolute Zero

Theoretically, the lowest possible temperature. A body at absolute zero would have no molecular motion or heat energy. It is the zero point on the Kelvin and Rankine scales, and is estimated to be -273.15 degrees C or -459.67 degrees F.


An abbreviation for alternating current which refers to a current that reverses periodically with time and has alternately positive and negative values.


The maximum deviation to be expected between a meter reading and the actual value being measured, under specified operating conditions. Usually expressed in percent of full scale for analog instruments or percent of reading for digital instruments.

A/D Converter

Analog to Digital converter. A circuit or device for producing a set of digital output signals representing the magnitude of a voltage applied to its input.

Air Damped

A construction utilizing an air vane to achieve movement damping. This vane is usually housed in a closed chamber to increase the damping action.

Alloy, Thermocouple

Any number of metal alloys having standardized and controlled thermoelectric properties. Designated dissimilar pairs of these alloys are used together to realize measuring or extension portions of thermocouple circuits.


The temperature coefficient of resistance of a material, derived from measurements at 0C and at 100C:
a = (R100-R0) / (100 * R0)
It indicates the basic change in resistance in a material for each degree C change in temperature. It is a defining parameter for resistance temperature detectors.

Ambient Temperature

The temperature of the encompassing atmosphere surrounding the equipment and instruments in use.


Unit of electrical current. Amount of electrical current which will flow through a one ohm resistor with one volt applied at a reference time. Also known as an amp.


Electrical quantities having the property of varying in a continuous manner rather than incremental or discrete step manner.

ANSI C39.1 Standard

American National Standards Institute, Inc. Standard C39.1. Applies to analog instruments, defining nomenclature, test conditions and procedures, general construction requirements, mounting dimensions, and performance requirements.


In digital instruments, the ability to measure DC values of either polarity without the need to interchange test lead connections.


In digital instruments, the automatic updating of an error integrating/memory circuit prior to each measurement, eliminating the effects of zero offset and drift, resistance tolerance and drift, and time-constant tolerance and drift in the instrument circuit.

Average Responding

The measurement of an AC voltage or current obtained using a DC instrument with a rectifying input circuit that converts AC energy to DC. The meter scale or readout is usually calibrated in terms of the corresponding RMS values, but it is accurate for pure sine wave inputs only.

BCD, Parallel

A digital data output format where every decimal digit is represented by binary signals on four lines and all digits are presented in parallel. The total number of lines is 4 times the number of decimal digits.

BCD, Serial

A digital data output format where every decimal digit represented by binary signals on four lines and up to five decimal digits are presented sequentially. The total number of lines is four data lines plus one strobe line per digit.

BCD, Three-State

An implementation of parallel BCD which has 0,1 and high impedance output states. The high impedance state is used when the BCD output is not addressed in parallel connect applications.

Bias Current

A very low level DC current generated by digital panel meters and superimposed on the signal. This current may introduce a measurable offset across a very high source impedance.

Bimetal cold junction compensation

Automatic mechanical correction for ambient temperature change at cold junction of thermocouple which would normally cause erroneous readings.


The ability of a panel meter to display both positive and negative readings.


Expressed in Voltanperes (VA) and represents the power consumated by that instrument or in current transformers represents maximum output available at rated primary current.


To determine the indication or output of a device with respect to a standard.

Calendar-Van Dusen Equation

An interpolation equation which provides resistance values as a function of temperature for RTD’s.

Celsius Temperature Scale

A temperature scale with the ice point at 0o and the boiling point of water at 100o

CMV (Common-Mode Voltage)

Common-Mode Voltage specifies the DC voltage which is tolerable between input low and power ground. CMV specifications are typically a few volts at most. However, using batteries or a transformer isolated power supply permits an instrument to be operated at a higher CMV, usually on the order of 100 VDC.

Cold Junction Compensation

In thermocouple instruments, the automatic correction for ambient temperature changes at the cold junction of the thermocouple. Without cold junction compensation, the readings will be erroneous.

Conformity Error

For thermocouples and RTD’s, the difference between the actual output and published standard output for a specified temperature.


The negative leg of thermocouple types E and J. Constantan is 55% copper and 45% nickel. The EMF values can vary significantly for this material depending upon which alloy it is paired with in a thermocouple.

Constactless control

As used to define action of meter ralays, there are no contacting parts for sensing the proper position of control. A light shield connected to the pointer moves between a light source and light receiver. It is the "electric eye" type of operation that control the meter relay.


A measuring device with at least one output which can be correlated to the process variable.

Conversion Rate

The number of analog to digital conversions performed per second by a digital panel meter.


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.

Crest Factor

The ratio of peak voltage to the RMS voltage of a waveform (with the DC component removed).

Criterion Level

Criterion level is a factor used in the dosage equation by the 897 Noise Dosimeter. It equates to the decibel level at which 100% dose will be achieved if exposed to for eight hours. For example: if the criterion level is set to 90 dB, then spending eight hours in an environment at 90 dB will result in a reading of 100%.


The movement of electrons through a conductor. Current is measured in amperes and is symbolically represented with the letter I.


The way the pointer settles at its steady indication after a change in the value of the measured quantity. There are two general classes of damped motion, as follows:
- Periodic, in which the pointer oscillates about the final position before coming to rest
- Aperiodic, in which the pointer comes to rest without overshooting the rest position.
The point of change between periodic and aperiodic damping is called “critical damping.” An instrument is critically damped when overshoot is present but does not exceed an amount equal to one half the rated accuracy of the instrument.

D’Arsonval Movement

A meter movement consisting of a small, lightweight coil of wire supported on jeweled bearings between the poles of a permanent magnet. When the direct current to be measured is sent through the coil, its magnetic field interacts with that of the permanent magnet and causes the coil and attached pointer to rotate.

Decibel (dB)

A logarithmic unit for the expression of the ratios of two amounts of power.
dB = 10 LOG10(P1/P2)


A material with low electrical conductivity. The insulation between two conductors is a dielectric if it can support electrostatic stresses across it.

Dielectric Strength

A measure of the voltage that an insulation material can withstand before an electrical breakdown occurs. It is sometimes referred to as breakdown potential.

DIN (Deutsche Industry 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.


Device that allows current to flow in only one direction.


Donut is the slang term used to describe the class of current transformers which are shaped like a toroid.


The percent dose achieved using an eight-hour time base. Dose is calculated based on OSHA mandated maximum sound exposure. Used by the model 897 Dosimeter.

Dual-Slope Conversion

A digital technique for converting a measured analog quantity to a precise digital equivalent for display as a numerical value. During a fixed interval of time, the output of an integrating circuit rises linearly at a rate proportional to the measured analog input quantity. This output is then switched to a precise reference voltage source of opposite polarity, causing the output to descend at a fixed rate, while a counting circuit clock counts pulses delivered by an internal pulse generator. As the integrator output reaches its base level, the count is terminated; the total count (numerically equivalent to the analog input quantity) is then displayed in a digital readout as a voltage, current, resistance, or other parametric quantity.


Engineering Application Guide; detailed catalog of available Simpson meters and accessories

Electromotive Force (EMF)

An electrical potential difference which produces or tends to produce an electrical current. The unit of measure is the volt.

End Scale Value

The end scale value of an instrument is the value of the actuating electrical quantity that corresponds to end scale indication. When zero is not at the end or at the electrical center of the scale, the higher value is taken.

Exchange Rate

The exchange rate is the number of dBs below the criterion level that results in a 50% reduction in dose. It is used by the model 897 Dosimeter. Example: with the criterion level set at 90 dB and the Exchange Rate set at 5 dB, a reading of 50% dose will occur when the dosimeter is exposed to 85 dB for eight hours.

Expanded Scale

An arrangement that expands a specific position of an overall range to occupy a larger portion of the full-scale length than it normally would

Fahrenheit Temperature Scale

A temperature scale with the ice point at 32o and the boiling point of water at 212o. The formula for conversion to the Celsius scale is:
C = 5/9 (F-32)


An abbreviation for Fluorinated Ethylene Propylene. This component is commonly referred to as “Teflon.”

Fiducial Value

The value to which reference is made to specify the accuracy of an instrument. when the mechanical zero is at one end of the scale or outside the scale. The fiducial value corresponds to the higher end-scale value. When the mechanical zero is displaced within the scale, the fiducial value is the arithmetic sum of the absolute electrical values corresponding to the two limits of the range.


Number of times an electrical signal replicates in one second; usually expressed in Hertz (Hz)

Frequency Response

A measure of how effectively a circuit or device transmits the different frequencies applied to it.


The difference between tapped and untapped meter readings due to the combination of pivot friction and pivot roll. Friction is usually checked by making a substantial change in the applied energy (5-10%) at a sufficiently slow rate so that no overshoot occurs. The meter indication is then noted and, maintaining the same energy level, the meter is tapped. The difference between the two indications is the friction error. It is customary to express the error as a percentage of full scale.

Full-Scale Value

The arithmetic difference of the two end-scale values. When zero is not on the scale, the full-scale value is the higher end-scale value.

Gas Discharge Display

In digital instruments, a numerical readout in which the digit segments are delineated by the glow of ionized gas. Also described as “plasma” or “gas plasma” display.


An electrical connection between an item and the Earth

Grounded Junction

A specific type of thermocouple junction in which the sheath and conductors are welded together forming a completely sealed integral junction. A grounded junction is recommended for use in liquids, gas, and high pressure environments.

Hertz (Hz)

Unit of measure for frequency; represents numbers of wave cycles per second (Hz)


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.


Similar to resistance; represents the amount of AC voltage required to push a specific current through a component.

Input Impedance

The resistance and reactance of a panel meter. In the case of a voltmeter, this impedance must be taken into account when the source impedance is high.

Iron Vane

An instrument comprising a movable piece of ferromagnetic material that is actuated by a fixed coil carrying a current or by a fixed piece of ferromagnetic material magnetized by the current.


Conductor used to connect areas in a circuit; often used to change configuration

Junction (Thermocouple)

The point at which two thermocouple alloys are joined. In a typical thermocouple circuit there is a measuring junction and a reference junction.

Kelvin Temperature Scale

Also known as the thermodynamic temperature scale, the Kelvin scale is an absolute temperature scale in which temperature differences are proportional to the amount of heat energy converted to mechanical work by a Carnot engine. The ice point on the Kelvin scale is 273.15 K. A useful approximation for conversion of the Kelvin scale to the Celsius scale is:
T (K) = T (C) + 273.15


A prefix meaning one thousand or 103; abbreviated as K.

Knife-Edge Pointer

Analog meter pointer with end flattened and turned edgewise so that the thinnest dimension or edge is seen by the observer. Often used with a mirror-backed scale to increase reading accuracy by eliminating parallax error.


Lavg is display of sound pressure level averaged over one minute increments. It is one of the parameters used by the model 897 Dosimeter.

LCD Display

A numerical readout in which the digit segments are delineated by dark areas in a liquid crystal display.

Lead Length Consideration

Connecting leads can affect the accuracy of small current transformers, DC shunts and low voltage analog voltmeters. Use leads supplied with the device or lead size specified.

LED Display

A numerical readout in which the digit segments are delineated by light emitting diodes.


The eight hour time weighted average sound level, expressed in dB. This value takes into account all sound measurement between the threshold level and the Leq upper limit and is one of the parameters on the dosage report generated by the model 897 Dosimeter.

Leq Upper Limit

This is the maximum sound level the model 897 Dosimeter will measure. The dosimeter uses no sound levels above this value in calculating Leq or dose.

Light sensitive switch

Light operared SCR used in the meteer ralay line which turns on with a given amount of light shining upon it and off when it is dark.

Loop Resistance

The total resistance of the thermocouple materials in a thermocouple circuit.

LSD (Least Significant Digit)

The right-most active digit of a digital display

Magnetic Influence

The influence on an analog meter that is caused solely by an external magnetic field which is produced with a current of the same kind and frequency as that which actuates the mechanism.

Magnetically Damped

Meters in which the damping is achieved by moving a metal vane through a magnetic field. The motion induces currents in the vane which creates magnetic fields opposing those of the stationary magnets thus tending to bring the pointer to rest. This type of damping is found in many quality moving iron vane and dynamometer type instruments.

Measuring Junction

The junction of a thermocouple subjected to the temperature to be measured.


A prefix meaning one million or 106; abbreviated as M.


A prefix meaning one millionth or 10-6


A prefix meaning one thousandth or 10-3; abbreviated as m.

Mineral Insulated Thermocouple

A thermocouple probe constructed by loading the metal sheath with thermocouple conductors and a mineral-based dielectric material; and then compacting the entire assembly.

Mirror Scale

Analog meter scale with a mirror arc that enables alignment of the eye’s line of sight perpendicular to the scale of the meter. Eliminates parallax error and helps the operator make a precise reading.


An abbreviation for Measurement Indication Unit. MIU’s are a unit of leakage current specified by IEC. At 60 Hz, one MIU is equal to 1 mA.

MSD (Most Significant Digit)

The left-most digit on a digital display.


A technique which allows different input (or output) signals to use the same lines at different times, controlled by an external signal. Multiplexing is used to save on wiring and I/O ports.


A series resistor that is used to extend the measurable voltage range beyond the particular value which is the physical limit of the meter movement itself. Multiplier can also be used to refer to an external shunt or transformer used in conjunction with limited range meters.

NMR (Normal-Mode Rejection)

The ability of a panel meter to filter out noise superimposed on the signal and applied across the signal HO to LO input terminals. Normally expressed in dB at 50/60 Hz.


Unwanted electrical interference picked up on a signal cable.


Unit of electrical resistance; one Volt can force one Ampere of current through a resistance of one Ohm

Ohms per Volt

A measure of the sensitivity of an analog voltmeter. A 1000 ohms per volt meter has resistance of 150,000 ohms on its 150 volt range, and 300,000 ohms on its 300 volt range. Its basic movement is a 1 milliampere meter (I=V/R = 1/1000)

Open Circuit Voltage

The maximum voltage produced by a power source without a load applied.


An isolation device that provides an electrical barrier between related circuits.


Current is present which exceeds the measuring range


Refers to the percent over full scale value that unit can be subjected to without failure.

Overload Rated

The maximum load over full scale value that an instrument can withstand without damage or failure. Expressed as a percent of a full-scale value.

Overload Capacity

The level of current, voltage, or power beyond which a device will be destroyed. It is usually higher than the rated load capacity.


In digital meters, a reading that exceeds full scale (but is less than an overload) that does not require switching to a higher range, i.e. for 3-1/2 digit DPM 0-999 is full scale reading, 1000-1999 is over range, and over 1999 is overload.


The ratio of the overtravel of the pointer beyond a new steady deflection to the change in steady deflection when a new constant value of the measured quantity is suddenly applied. Overtravel and deflection are determined in angular measure and the overshoot is usually expressed as a percentage of scale length.

Primary Line

The primary line is the line which carries the current to be measured. For normal operation, the primary line will pass through the center of the transformer once.

Primary Wrap

It is possible to manipulate the ratio of a transformer to make it fit into a specific application. One way of doing this is to add primary or secondary wraps. A primary wrap is added whenever the primary line is passed through the center of the current transformer. The process of wrapping a transformer is explained in detail here.

Peak Hold

An instrument function that permits retaining and displaying the peak value momentarily reached by a pulse or other brief signal under measurement.

Peak Voltage

The maximum value present in a varying or altering voltage. This value may be either positive or negative.

Phase Angle

The difference in degrees by which the voltage wave lags or leads the current wave in an AC circuit.


Proportional, Integral, and Differential control algorithm; PID control uses three separate mathematical functions to bring a process variable to a set point and then maintain that set point with minimal error

Pivot and Jewel

Method of suspending a moving coil or moving iron vane in a magnetic field. The moveable element is equipped with two non-magnetic pivots which engage glass or synthetic jewel cup bearings. In most designs, the bearings are spring-backed to prevent damage to the pivots by shock or vibration.


An electrical occurrence which determines the direction in which current tends to flow.


Measure of the amount of work an electrical signal can do; expressed in Watts

Power Consumption

The power necessary to operate the meter.

Power Factor

The cosine of the phase angle between the voltage applied to a load and the current passing through the load.

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 mADC, 10-50 mADC, and 1-5 VDC.


Quadrature signals use two separate channels, A and B, to express speed and direction of motion electronically. Quadrature is primarily used in counters and encoders.

Range (Full Scale)

The range of signal input which can be measured before the instrument goes into overload condition.


The ratio of a current transformer indicates the multiple between the current in the secondary lines and the current in the primary lines. For example: a 50:5 transformer will transmit 5 Amperes through the secondary line when the primary line is carrying 50 Amperes.

Rectifier Type Instrument

A DC instrument equipped with a solid-state rectifier at its input to convert AC energy to DC energy. The instrument provides measurements of the average value of an AC voltage or current, and its scale is usually calibrated in terms of the RMS equivalent.

Reference Junction

The junction in a thermocouple circuit which is maintained at a constant, known temperature. It is also referred to as the cold junction and as a standard it is usually maintained at 0oC, however, any temperature could be used.


The ability of an instrument to repeat its indications when the pointer is deflected upscale, compared to the indications taken when the pointer is deflected down-scale, expressed as a percentage of the fiducial value.


A property of conductors which determines the current produced by a given difference of potential. Dimensions, materials, and temperature all affect resistance.


The degree to which nearly equal values of a quantity can be discriminated. In analog meters, the difference between the values represented by two adjacent scale divisions. In digital meters, the value represented by a one count change in the least significant digit.

Response Time

In analog instruments, refers to the time required after an abrupt change of the measured quantity to a new constant value until the pointer, or indicating means, first comes to apparent rest in its new position. In sensors, it refers to the time required for a sensor to reach 63.2% of the step change in temperature for a particular set of test conditions.

RS232C Output Signal

A serial interface suitable for connection between a digital controller and a personal computer, a host computer, or a printer.

RS422A Output Signal

A serial interface suitable for connection between a digital controller and a personal computer, a host computer, or a printer.


An abbreviation for resistance temperature detector. It is a circuit element whose resistance increases with increasing temperature in a predictable manner. Platinum is the most popular material though copper, nickel, balco, and tungsten are also used in RTD’s. Connected via 2, 3, or 4 wire hookups.


For direct read out in engineering units, the capability of the meter to associate any desired value to the electrical input range.

Secondary Wrap

It is possible to manipulate the ratio of a transformer to make it fit into a specific application. One way of doing this is to add primary or secondary wraps. A secondary wrap is added whenever one of the secondary lines is passed through the center of the current transformer. The process of wrapping a transformer is explained in detail here.

Self-Contained Instrument

A self-contained instrument is one in which no accessory items are required to perform its intended function(s). If not specified, a manufacturer may optionally supply either a self-contained meter or one with external accessories.


Inherent in the construction, unaffected by stray magnetic fields.


A calibrated low resistance connected in parallel with the input terminals of a voltmeter to enable measurements of large currents. It can be internal or external, and typically voltage drops across external shunts are 50 mV or 100 mV.


No moving parts, no warm-up required, transistorized.

Sound Pressure Level (SPL)

The momentary pressure measurement expressed in dB as a ratio to a given power level.


The total measuring capability.
Example: A 110-130 Volt meter has a span of 20 Volts (Mid Span 120 Volts), a 0-150 Volt meter has a span of 150 Volts.

Span Adjustment

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. For instance, a display span of 0-200oF may correspond to the 16 mA span of a 4-20 mA transmitter signal.

Span Range

The span between two points anywhere within a temperature or voltage limit that a recorder can be calibrated to.


Maximum sound level measured.


The ability of an instrument or a sensor to maintain a consistent output or indication with the application of a constant input.


(Applies only to offset zero meters). The measure of a meter’s ability to provide corresponding indication on each side of zero when the polarity of the applied energy is reversed. Symmetry error is customarily expressed as a percentage of actual full scale value.

Symmetry Error

The difference in indications when the polarity of a measured quantity is reversed, expressed as a percentage of fiducail value.


A method of suspension of the moving element in an analog meter movement using metal bands under tension. The bands provide the suspension and the restoring torque during deflection, without the friction inherent in jewel-bearing suspension.

Teflon/Tefzel TM

Materials created by DuPont and used to manufacture thermocouple insulation. Teflon can withstand temperatures up to 500 degree F while Tefzel has some radiation resistance and can withstand temperatures up to 390 degree F.


Abbreviation for “temperature coefficient”; the error introduced by a change in temperature. Normally expressed in percent per degree C or ppm per degree C.


A temperature sensor formed by joining two dissimilar metals and applying a temperature differential between the measuring junction and the reference junction.

Thermocouple Break Protection

A device for sensing malfunction when a thermocouple opens or breaks down during operation, acting to latch the high set point and shut down the system, or display an alarm.

Threshold Level

The lowest sound included in a sound dosage calculation. The model 897 Dosimeter will not include any sound levels under this value in the dose calculation. For example: if the threshold level is set at 80 dB, the dosimeter will not use any sound levels below 80 dB in the calculation of dose.


Toroid is the proper name for the shape of most transformers. A toroid is a solid ring with a hollow center (much like a donut)


A measure of rotational force. At a steady state of deflection, the mechanically applied torque is equal and opposite to the electrically developed torque. Torque is usually expressed in millimeter grams for a given angular deflection.


The ability of an instrument to indicate at the scale mark being checked when energized by the proportional value of actual end-scale excitation.

Tracking Error

The error in indication at a scale mark, expressed in percentage of fiducial value, when the instrument is energized by the proportional value of the actual end-scale excitation.


Electrical device used to convert one signal to another; the transducers carried by Simpson convert high AC current signals to a 4-20 mA DC process signal


Electrical device used to convert one signal to another; the transformers carried by Simpson convert high AC current signals to either a 0-5 AC Amp signal or a 0-10 AC Volt signal


Electrical device used to amplify or convert an electrical signal; the transmitters carried by Simpson convert thermocouple or RTD inputs to a 4-20 mA DC Signal

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 RM value is 1.11072 times the rectified average value, which is utilized for low cost metering. For significantly non-sinusoidal signals, a TRMS converter is required.


An abbreviation for Underwriters Laboratories Inc. UL is an independent organization devoted to testing for public safety.

UL Ratings

The Underwriters Laboratories were instituted to perform tests on materials, components, apparatus, and installations and to establish safe standards based on these tests. They validate components, devices, and their combinations for industrial, commercial, and consumer applications of all kinds by checking them against these standards.

UL Recognized Product

A product which has been produced under UL’s Recognition and Follow-Up Service and which bears the authorized Recognized Markings as the manufacturer’s declaration that the product complies with UL’s requirements in accordance with the terms of the Recognition and Follow-Up Service Agreement.

UL Listed Product

A product which has been produced under UL’s Listing and Follow-Up Service and which bears the authorized Listing Mark of UL as the manufacturer’s declaration that the product complies with UL’s requirements in accordance with the terms of the Listing and Follow-Up Service Agreement.


In digital instruments, a capability for measuring voltage or current of only one polarity


Measurement of electrical potential. One volt will force one ampere of current through a resistance of one ohm.

Volt Ampere(s)

The product of the RMS voltage applied to a circuit and the RMS current, in amperes, flowing through it.

Volume Unit or V.U.

A logarithmic unit for the expression of the ratios of two amounts of power, equal to a decibel when a reference level of one milliwatt at 600 ohms is used.


Volt-Ohm-Milliammeter: This is a meter which is capable of measuring voltage, resistance, and current.


Unit of measurement of electrical power; one Watt is the amount of work that one Ampere at one Volt can do.


It is possible to manipulate the ratio of a transformer to make it fit into a specific application. One way of doing this is to add primary or secondary wraps. The process of wrapping a transformer is explained in detail here.

Zero Adjustment

In analog meters zero adjust refers to the ability of the meter to be precisely calibrated to zero with no signal applied.

Zero Offset

The ability to change the display on a digital process meter to zero when the actual input signal is not zero; for example, displaying 0 % when the actual input signal is 4 mA.


References: Wikipedia,, Simpson