Damping: The suppression of oscillation. The viscosity of a fluid is used in viscous damping, while the induced current in electrical conductors is used to effect magnetic damping.
Deadband: The range through which an input can be changed without causing an observable response.
Dead time: The interval between the initiation of a change in the input and the start of the resulting observable response.
Decibel (dB): Unit for expressing a logarithmic measure of the ratio of two signal levels.
Dielectric: A non-conductor of dc current.
Dielectric constant: A numeral that expresses the degree of non-conductivity of different substances, with full vacuum defined as 1.0.
Distributed control system (DCS): Typically, a large-scale process control system characterized by a distributed network of processors and I/O subsystems that encompass the functions of control, user interface, data collection, and system management.
Dither: A useful oscillation of small magnitude, introduced to overcome the effects of friction, hysteresis, or clogging.
Drift: Undesired change in the input-output relationship over a period of time.
Dynamic range: Ratio of the largest to the smallest signal level a circuit can handle, normally expressed in dB.
Electromotive force: Force that causes the movement of electricity, such as potential difference of voltage. A measure of voltage in an electrical circuit.
Elevation: A range in which the zero value of the measured variable exceeds the lower range value.
Error: The difference between the measured signal value or actual reading and the true (ideal) or desired value.
Error, common mode: Error caused by interference that appears between both measuring terminals and ground.
Error, normal mode: Error caused by interference that appears between the two measuring terminals.
Error, random: The amount of error that remains even after calibrating a sensor. Also called “precision,” while “repeatability” is defined as twice that: the diameter (instead of the radius) of the circle within which the readings fall.
Error, systematic: A repeatable error, which either remains constant or varies according to some law, when the sensor is measuring the same value. This type of error can be eliminated by calibration.
Farad: The unit of capacitance, equivalent to one coulomb of stored charge per volt of applied potential difference. As this is a very large unit, one trillionth of it, the picofarad (pf), is commonly used.
Fieldbus: All-digital communication network used to connect process instrumentation and control systems. Designed to replace systems based on 4-20 mA analog signals with bi-directional, multivariable data communication capability.
Frequency: The number of cycles over a specified time period during which an event occurs. Normally expressed in cycles per second (hertz, Hz).
Frequency response: The frequency-dependent characteristic that determines the phase and amplitude relationship between sinusoidal input and output.