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Absolute Pressure Transducer: A transducer which measures pressure in relation to zero pressure (a vacuum on one side of the diaphragm).

Absolute Pressure: Gage pressure plus atmospheric pressure.

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.

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). Units expressed in "g".

Accelerometer: A device which converts the effects of mechanical motion into
an electrical signal that is proportional to the acceleration value of the motion. A
sensor. A transducer.

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.

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.

Individual ionic activity coefficients, f+ for cation and f- for an anion, cannot be derived thermodynamically. They can be calculated only by using the Debye-Huckel law for low concentration solutions in which the interionic forces depend primarily on charge, radius, and distribution of the ions and on the dielectric constant of the medium rather than on the chemical properties of the ions.

Mean ionic activity coefficient (f±) or the activity of a salt, on the other hand, can be measured by a variety of techniques such as freezing point depression and vapor pressure as well as paired sensing electrodes. It is the geometric mean of the individual ionic activity coefficients: f± = (f+n+f-n-)1/n

Adapter: A mechanism or device for attaching non-mating parts.

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.

ALOMEGA® An aluminum nickel alloy used in the negative leg of a type K thermocouple (registered trademarks of OMEGA ENGINEERING, INC.).

Alphanumeric: A character set that contains both letters and digits.

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 transducer.

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

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 electronics) 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.

Anemometer: An instrument for measuring and/or indicating the velocity of air flow.

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.

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.

AWG: American Wire Gage.

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