Glossary H

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

Half Bridge: Two active elements or strain gages.

Half-Duplex: One way at a time data communication; both devices can transmit and receive data, but only one at a time.

Handshake: An interface procedure that is based on status/data signals that assure orderly data transfer as opposed to asynchronous exchange.

Hardcopy: Output in a permanent form (usually a printout) rather than in temporary form, as on disk or display terminal.

Hardware: The electrical, mechanical and electromechanical equipment and parts associated with a computing system, as opposed to its firmware or software.

Head Loss: The loss of pressure in a flow system measured using a length parameter (i.e., inches of water, inches of mercury).

Head Pressure: Pressure in terms of the height of fluid, P = yrg, where r = fluid density and y = the fluid column heights. Expression of a pressure in terms of the height of fluid, r = yrg, where r is fluid density and y = the fluid column height. g = the acceleration of gravity.

Heat Sink: (1) Thermodynamic. A body which can absorb thermal energy. (2) Practical. A finned piece of metal used to dissipate the heat of solid state components mounted on it.

Heat Transfer: The process of thermal energy flowing from a body of high energy to a body of low energy. Means of transfer are: conduction; the two bodies contact. Convection; a form of conduction where the two bodies in contact are of different phases, i.e. solid and gas. Radiation: all bodies emit infrared radiation.

Heat Treating: A process for treating metals where heating to a specific temperature and cooling at a specific rate changes the properties of the metal.

Heat: Thermal energy. Heat is expressed in units of calories or BTU’s.

Hertz (Hz): Units in which frequency is expressed. Synonymous with cycles per second.

Hexadecimal: Refers to a base sixteen number system using the characters 0 through 9 and A through F to represent the values. Machine language programs are often written in hexadecimal notation.

Hold: Meter HOLD is an external input which is used to stop the A/D process and freeze the display. BCD HOLD is an external input used to freeze the BCD output while allowing the A/D process to continue operation.

Hooke’s Law: Defines the basis for the measurement of mechanical stresses via the strain measurement. The gradient of Hooke’s line is defined by the ratio of which is equivalent to the Modulus of Elasticity E (Young’s Modulus).

Host: The primary or controlling computer in a multiple part system.

Hydrogen Ion Activity (aH+): Activity of the hydrogen ion in solution. Related to hydrogen ion concentration (CH+) by the activity coefficient for hydrogen (f H+).

Hysteresis (Electrode Memory): When an electrode system is returned to a solution, equilibrium is usually not immediate. This phenomenon is often observed in electrodes that have been exposed to the other influences such as temperature, light, or polarization.

Hysteresis: The difference in output when the measurand value is first approached with increasing and then with decreasing values. Expressed in percent of full scale during any one calibration cycle. See also Deadband.