Following is a definition of terms frequently used in the instrumentation, industrial automation and test & measurement communities.

Index: 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 Sensor: A sensor which measures pressure in relation to zero pressure (a vacuum on one side of the diaphragm).

Absolute Pressure: Gauge pressure plus the atmospheric pressure.

Absolute Zero: The temperature at which thermal energy is at a minimum. Defined as 0 Kelvin, calculated to be -273.15 °C or -459.67 °F.

ac: Alternating current; an electric current that reverses its direction at regularly recurring intervals.

Acceleration: The change in the velocity of a body or particle with respect to time. The parameter that an accelerometer measures (dv/dt). Units expressed in “g”.

Accuracy: The closeness of an indication or reading of a measurement device to the actual value of the quantity being measured. Usually expressed as ± percent of full scale output or reading.

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.

ADC: Analog-to-Digital Converter: an electronic device which converts analog signals to an equivalent digital form, in either a binary code or a binary-coded-decimal code. When used for dynamic waveforms, the sampling rate must be high to prevent aliasing errors from occurring.

Address: The label or number identifying the memory location where a unit of information is stored.

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-platinum rhodium 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 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.

ALU: Arithmetic Logic Unit. The part of a CPU where binary data is acted upon with mathematical operations.

Alumel: An aluminum nickel alloy used in the negative leg of a Type K thermocouple (Trade name of Hoskins Manufacturing Company).

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: Pressure of the air surrounding a sensor, transducer or transmitter.

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

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 Span: The Y-axis range of a graphic display of data in either the time or frequency domain. Usually a log display (dB) but can also be linear.

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-to-Digital Converter (A/D or ADC): A device or circuit that outputs a binary number corresponding to an analog signal level at the input.
Angstrom: Ten to the minus tenth meters (10-10) or one millimicron, a unit used to define the wave length of light. Designated by the symbol ‰.

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.)

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 Program: A computer program that accomplishes specific tasks, such as word processing.

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.

ASME: American Society of Mechanical Engineers.

Assembler: A program that translates assembly language instructions into machine language instructions.

Assembly Language: A machine oriented language in which mnemonics are used to represent each machine language instruction. Each CPU has its own specific assembly language.

ASTM: American Society for Testing and Materials.

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.

Auto-Zero: An automatic internal correction for offsets and/or drift at zero voltage input.

AWG: American Wire Gage.

Axis of Rotation (Spin Axis): The axis of rotation (spin axis) is that straight line about which a body rotates.