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.