Enginneering dictionary

ENGINEERING DEFINITIONS

DEFINITIONS
absolute encoder An electronic or electromechanical device which produces a unique digital output (in coded form) for each value of an analogue or digital input; in an absolute position encoder, for instance, the position following any incremental movement can be determined directly, without reference to the starting position. absolute humidity The weightof water vapour in a gas-water vapour mixture per unit volume of space occupied. absolute measurement A measured value expressed in terms of fundamental standards of distance, mass and time. absolute pressure The pressure measured relative to zero pressure (vacuum). absolute stability A linear system is absolutely stable if there exists a limiting value of the Open loop gain such that the systemis stable for all lower values of that gain and unstable for all higher values. absolute value error The magnitude of the error disregarding the algebraic sign or, if a vector error, disregarding its direction. absolute viscosity A measure of the internal shear properties of fluids expressed as the tangential force per unit area at either of two horizontal planes separated by one unit thickness ofa given fluid, one of the planes being fixed and the other moving with unit velocity. absorbance An optical property expressed as log (SIT), where T is the transmittance. absorptance The fraction of the incident light absorbed. absorption-emission pyrometer An instrument for determining gas temperature by measuring the radiation emitted by a calibrated reference source both before and after theradiation passes through the gas, where it is partly absorbed. absolute pressure The pressure measured relative to zero pressure (vacuum). absorption tower A vertical tube in which a gas rising through a falling stream of liquid droplets is partially absorbed by the liquid. accelerometer A transducer used to measure linear or angular acceleration. access time The interval between a request forstored information and the delivery of the information; often used as a reference to the speed of memory. accuracy The ratio of the error to the full-scale output or the ratio of the error to the output, as specified, expressed in percent. acidity Represents the amount of free carbon dioxide mineral acids and salts which hydrolise to give hydrogen ions in water. pH is the measure of hydrogen ionsconcentration. ACK Transmission control character transmitted by a receiving device as an affirmative response to a sending device. acoustical ohm The unit of measure for acoustic resistance, reactance or impedance; it equals unity when a sound pressure of one microbar produces a volume velocity of one cubic centimetre per second. acoustic compliance The reciprocal of acoustic stiffness.

acousticdispersion Separation of a complex sound wave into its various frequency components, usually due to variation of wave velocity in the medium with sound frequency; usually expressed in terms of the rate of change of velocity with frequency. acoustic impedance The complex quotient obtained by dividing sound pressure on a surface by the flux through the surface. acoustic inertance A property relatedto the kinetic energy of a sound medium which equals Za/2 7rf, where Za is the acoustic reactance and f is sound frequency; the usual units of measure are g/cm4. Also known as "acoustic mass." acoustic radiometer An instrument that measures sound intensity by determining unidirectional steady state pressure when the sound wave is reflected or absorbed at a boundary. acoustic sensitivity The outputof a transducer (not due to rigid body motion) in response to a specified acoustical environment. This is sometimes expressed as the acceleration in g rms sufficient to produce the same output as induced by a specified sound pressure level spectrum having an overall value of 140 dB referred to 0.0002 dyne per sq cm rms. acoustic stiffness A property related to the potential energy of a medium or...