E - H

EAROM See "electrically alterable read only memory."

ebullition The act of boiling or bubbling.

eccentric orifice An orifice whose centre does not coincide with with the centreline of the pipe or tube; usually, the eccentricity is toward the bottom of a pipe carrying flowing gas and toward the top of a pipe carrying liquid, which tends to promote the passage of entrained water or air rather than allowing entrained water or gas to build up in front of the orifice.

economiser Heat exchanger used to recover excess thermal energy from process streams.

eddy current A circulating current induced in a conductive material by a changing electromagnetic field.

edge filter An interference filter which abruptly shifts from transmitting to reflecting over a narrow range of wavelengths.

eductor A device that withdraws a fluid by aspiration and mixes it with another fluid. See "injector."

EEPROM See "electrically erasable and programmable read only memory."

effective bandwidth An operating characteristic of a specific transmission system equal to the bandwidth of an ideal system whose uniform pass band transmission equals maximum transmission of the real system and whose transmitted power is the same as the real system for equal input signals having a uniform distribution of energy at all frequencies.

effective value The root mean square value of a cyclically varying quantity; it is determined by finding the average of the squares of the values throughout one cycle and taking the square root of the average.

efficiency 1. The efficiency of a boiler is the ratio of heat absorbed by water and steam to the heat equivalent of the fuel fired. 2. In manufacturing, the average output of a process expressed as percent of its expected output under ideal conditions. 3. The ratio of useful energy supplied by a dynamic system to the energy supplied to it over a given period of time.

EIA Electronics Industry Association who provide standards for such things as interchangeability between manufacturers.

ejector A device which utilises the kinetic energy in a jet of water or other fluid to remove a fluid or fluent material from tanks or hoppers.

elastomer A material that can be stretched to approximately twice its original length with relatively low stress at room temperature, and which returns forcibly to about its original size and shape when the stretching force is released.

elastomeric energised liner A resilient elastomeric ring under the main liner in a butterfly valve body is compressed by the disk acting through the main liner, thus generating a resilient sealing action between the disk and the main liner.

elbow 1. A fitting that connects two pipes at an angle, usually 90° but may be any other angle less than 100°. 2. A sharp bend in a pipe.

elbow meter A pipe elbow that is used as a flow measurement device by placing a pressure tap at both the inner and outer radius and measuring the pressure differential caused by differences in flow velocity between the two flow paths.

electrical apparatus category 'ia' An electrical apparatus that is incapable of causing ignition in normal operation, with a single fault and with any combination of two faults applied, with safety factors of 1.5 in normal operation and with one fault and 1.0 with two faults.

electrical apparatus category 'ib' An electrical apparatus that is incapable of causing ignition in normal operation and with a single fault applied, with safety factors of 1.5 in normal operation and with one fault and 1.0 with one fault, if the apparatus contains no unprotected switch contacts in parts likely to be exposed to a potentially explosive atmosphere and the fault is self revealing.

electrically alterable read only memory (EAROM) A type of computer memory for which its contents can be changed only under special conditions.

electrically erasable and programmable read only memory (EEPROM) Nonvolatile memory that may be electrically erased and reprogrammed.

electrical resistivity A material characteristic indicative of its relative resistance to the flow of electrons. Usual units are ohm m (SI) or ohms per circular mil foot (U. S.); it is the reciprocal of electrical conductivity.

electromagnetic interference (EMI)See "interference, electromagnetic."

electromagnetic radiation Any wave having both an electric and a magnetic component. Electromagnetic waves include in order of increasing photon energy, increasing frequency and decreasing wavelength: radio waves, infrared, visible light, ultraviolet, x rays, gamma rays and cosmic rays.

electromagnetic wave A wave in which both the electric and magnetic fields vary periodically, usually at the same frequency.

electrometer An instrument for measuring electric charge, usually by means of the forces exerted on one or more charged electrodes in an electric field.

element, primary The system element that quantitatively converts the measured variable energy into a form suitable for measurement. For transmitters not used with external primary elements, the sensing portion of the transmitter is the primary element.

elevated range See "range, suppressed-zero."

elevated span See "range, suppressed zero."

elevated zero range See "range, elevated zero."

elevation error A type of error in temperature measuring or pressure measuring systems that incorporate capillary tubes partly filled with liquid; the error is introduced when the liquid filled portion of the system is at a different level than the instrument case, the amount of error varying with distance of elevation or depression.

elute To wash out or remove by dissolving.

elutriation Separation of fine, light particles from coarser, heavier particles by passing a slow stream of fluid upward through a mixture so that the finer particles are carried along with it.

embrittlement cracking A form of metal failure that occurs in steam boilers at riveted joints and at tube ends, the cracking being predominantly intercrystalline.

EMIElectromagnetic interference. See "interference, electromagnetic."

emissivity 1. A material characteristic determined as the ratio of radiant energy emission rate due solely to temperature for an opaque, polished surface of a material divided by the emission rate for an equal area of a blackbody at the same temperature. 2. The rate at which electrons are emitted from a solid or liquid surface when additional energy is imparted to the system by radiant energy such as heat or light or by energetic particles such as a beam of electrons.

emulsifier A substance that can be mixed with two immiscible liquids to form an emulsion. Also known as "disperser"; "dispersing agent."

encapsulated body liner In a butterfly valve body, all surfaces of the body are covered by a continuous surface layer of a different material, usually an elastomeric or plastic material. A soft elastomer behind a harder encapsulating material may be used to provide interference for disk and stem sealing areas.

encapsulation An international term describing a type of protection in which the parts that could ignite an explosive atmosphere by either sparking or heating are enclosed in an encapsulant in such a way that this explosive atmosphere cannot be ignited. This type of protection is referred to by CENELEC as "Ex m" in draft standard EN50028. NOTE: Encapsulation is the potting or casting of electrical components with epoxy, elastomer, silicone, asphaltic, or similar compounds for the purpose of excluding moisture or vapours.

end connections, flanged End connections incorporating flanges that mate with corresponding flanges on the piping. end connections, split clamp End connections of various proprietary designs using split clamps to apply gasket or mating surface loading.

end connections, threaded End connections incorporating threads, either male or female.

end connections, welded End connections that have been prepared for welding to the line pipe or other fittings. May be butt weld (BW), or socket weld (SW).

endothermic reaction A reaction which occurs with the absorption of heat.

end-point In titration, an experimentally determined point close to the equivalence point, which is used as the signal to terminate titration; it is used instead of equivalence point in most calculations, and corrections for the error between end point and equivalence point usually are not applied.

end to end dimension See "face to face dimension" and "centre to end dimension".

entrainment The conveying of particles of water or solids from the boiler water by the steam.

entropy Function of the state of a thermodynamic system whose change in any differential reversible process is equal to the heat absorbed by the absolute temperature of the system. Also known as "thermal charge. "

erasable programmable read-only memory (EPROM) A read-only memory in which stored data can be erased (typically by ultraviolet light) and reprogrammed bit by bit with appropriate voltage pulses.

erg The unit of energy in the CGS system; it is the amount of energy consumed (work) when a force of one dyne is applied through a distance of one centimeter.

error squared Control technique of introducing the square of the error in the error term of a linear algorithm so as to produce a nonlinear correction.

error, systematic An error which, in the course of a number of measurements made under the same conditions of the same value of a given quantity, either remains constant in absolute value and sign or varies according to a definite law when the conditions change.

error, zero In process instrumentation, error of a device operating under specified conditions of use, when the input is at the lower range value. It is usually expressed as percent of ideal span. Zero error, when displayed on an input/output calibration graph, is represented by an as found line that is parallel to the ideal line. The zero error may produce a constant offset.

evaporator A device where liquid undergoes a change of state from liquid to gas under relatively low temperature and low pressure.

exception reporting An information system which reports on situations only when actual results differ from plarmed results. When results occur within a normal range they are not reported.

excess air Air supplied for combustion in excess of theoretical combustion air.

expert system Software system that provides a knowledge base (the symbolic representation of the knowledge and reasoning of experienced human experts in a particular field) and a rule interpreter, or inference engine, to analyse problems and recommend solutions.

explosion Combustion which proceeds so rapidly that a high pressure is generated suddenly.

explosion door A door in a furnace or boiler setting designed to be opened by a predetermined gas pressure.

explosionproof enclosure An enclosure that is capable of withstanding an explosion of a gas or vapour within it and of preventing the ignition of an explosive gas or vapour that may surround it and that operates at such an external temperature that a surrounding explosive gas or vapour will not be ignited thereby. Refer also to flameproof enclosure.

exponent In floating point representation, one of a pair of numerals representing a number that indicates the power to which the base is raised. Synonymous with "characteristic."

exponential notation A way to express very large or small numbers in data processing.

exponentiation A mathematical operation that denotes increases in the base number by a previously selected factor.

Extended Binary Coded Decimal Interchange Code (EBCDIC) An 8 bit code containing alpha, numeric, and control characters.

extension bonnet A bonnet with a packing box that is extended above the bonnet joint of the valve body so as to minimise the effect on the temperature of the packing above or below the temperature by the process fluid. The length of the extension bonnet is dependent upon the difference between the fluid temperature and the packing design temperature limit as well as upon the valve body design.

extractive distillation A distillation technique (employing the addition of a solvent) used when the boiling points of the components being separated are very close [within 3°C (5°F)] or the components are constant boiling mixtures. In extractive distillation, which is a combination of fractionation and solvent extraction, the solvent is generally added to the top of the column and recovered from the bottom product by means of subsequent distillation. The chemical added is a solvent only to the less volatile components. See "azeotrope."

face to face dimension The dimensions from the face of the inlet opening to the face of the outlet opening of a valve or fitting.

facing, flange The finish on the end connection gasket surfaces of flanged or flangeless valves

Fahrenheit A temperature scale where the freezing point of pure water occurs at 32°F and the span between freezing point and boiling point of pure water at standard pressure is defined to be 180 scale divisions (180 degrees).

fail close A condition wherein the valve closure member moves to a closed position when the actuating energy source fails.

fail in place A condition wherein the valve closure component stays in its last position when the actuating energy source fails

fail open A condition wherein the valve closure member moves to an open position when the actuating energy source fails.

fail safe 1. A characteristic of a particular valve and its actuator, which upon loss of actuating energy supply, will cause a valve closure member to fully close, fully open or remain in fixed position. 2. Protection against effects of failure of equipment, for example, burner fuel shut off on flame failure.

fail safe device A component, system or control device so designed that it places the controlled parameter in a safe condition in case of a power interruption, controller malfunction or failure of a load carrying member.

failure mode The position to which the valve closure component moves when the actuating energy source fails.

failure rate A measure of component reliability usually expressed as the probability of failure after a specified length of time in service.

fan performance A measure of fan operation in terms of volume, total pressures, static pressures, speed, power input, mechanical and static efficiency, at a stated air density.

fan performance curves The graphical presentation of total pressure, static pressure, power input, mechanical and static efficiency as ordinates and the range of volumes as abscissa, all at constant speed and air density.

farad Metric unit of electrical capacitance.

fast-Fourier transform (FFT) A computer technique to calculate the frequency components of a time waveform from the digitised voltage measurements. The result is a display of amplitude versus frequency, and phase versus frequency.

feedback 1. Process signal used in control as a measure of response to control action. 2. The part of a closed loop system which automatically brings back information about the condition under control. 3. Part of a closed-loop system which provides information about a given condition for comparison with the desired condition.

feedback control An error-driven control system in which the control signal to the actuators is proportional to the difference between a command signal and a feedback signal from the process variable being controlled. See "control, feedback." feedback control signal The output signal which is returned to the input in order to achieve a desired effect, such as fast response.

feedforward control A method of control that compensates for a disturbance before its effect is felt in the output. It is based on a model that relates the output to the input where the disturbance occurs. In distillation the disturbances are usually feed rate and feed compositions. Steady state feedforward models are usually combined with dynamic compensation functions to set the manipulative variables and combined with feedback adjustment (trim) to correct for control model accuracy constraints.

feedstock Material delivered to a process or processing unit, especially raw material delivered to a chemical process or reaction vessel.

feedwater Process water supplied to a vessel such as a boiler or still, as opposed to circulating water or cooling water.

feedwater treatment The treatment of boiler feedwater by the addition of chemicals to prevent the formation of scale or eliminate other objectionable characteristics.

ferrography Wear analysis conducted by withdrawing lubricating oil from an oil reservoir and using a ferrograph analyzer to determine the size distribution of wear particles picked up as the oil circulates between moving mechanical parts; the technique also may be used to assess deterioration of human joints

fibres and flyings Materials not normally in suspension in air; such materials are of larger particles size than dusts. Fibres and flyings include materials such as cotton lint, sawdust, textile fibres, and other large particles that are typically a fire hazard rather than an explosion hazard.

filled system thermometer A device consisting of a temperature sensitive element (bulb), an element sensitive to changes in pressure or volume (Bourdon tube, bellows or diaphragm), capillary tubing and an indicating or recording device; the bulb, capillary tube and pressure or volume sensitive element are partly or completely filled with a fluid that changes its volume or pressure in a predictable manner with changes in temperature.

final control element Component of a control loop (such as a valve) which directly changes the value of the manipulated variable.

first order system A system definable by a first order differential equation.

fixed point arithmetic A method of calculation in which operations take place in an invariant manner, and in which the computer does not consider the location of the radix point. Contrasted with "floating point arithmetic. ".

flame propagation rate Speed of travel of ignition through a combustible mixture.

flameproof enclosure A term describing an enclosure that can withstand the pressure developed during an internal explosion of an explosive mixture and that prevents the transmission of the explosion to the explosive atmosphere surrounding the enclosure and that operates at such an external temperature that a surrounding explosive gas or vapour will not be ignited thereby. Refer also to explosionproof enclosure. This type of protection is referred to by IEC as "Ex d"].

flammability Susceptibility to combustion. flammable (explosive) limits The flammable (explosive) limits of a gas or vapour are the lower (LFL or LEL) and the upper (UFL or UEL) percentages by volume of concentration of gas in a gas-air mixture that will form an ignitable mixture.

flanged body Valve body with full flanged end connections.

flangeless control valve A valve without integral line flanges, which is installed by bolting between companion flanges,with a set of bolts, or studs, generally extending through the companion flanges. Also called a "wafer style" or "wafer body control valve.

flange taps Refer to "orifice flange taps."

flashing Vapour produced by discharging liquid at saturation temperature into a region of lower pressure.

flash point The minimum temperature at which a liquid emits vapour in a concentration sufficient to form an ignitable mixture with air near the surface of the liquid but not sufficient to sustain combustion.

flip flop A bistable device, i.e., a device capable of assuming two stable states. A bistable device is capable of storing one bit of information.

floating ball A full ball positioned within the ball valve that contacts either of two seat rings and is free to move toward the seat ring opposite the pressure source when in the closed position to effect tight shutoff

floating control action A type of control system action in which a fixed relationship exists between a measured deviation and the rate of motion of the final control element. Floating control reduces the tendency to overshoot the setpoint by reducing power input to the system as the controlled variable approaches the setpoint value.

floating control mode A controller mode in which an error in the controlled variable causes the output of the controller to change at a constant rate. The error must exceed preset limits before controller change starts.

flooding The consequence of excessive column liquid loading where, in effect, the liquid on trays becomes too deep for the vapour to pass through or where the vapour flow rate is too high, creating an excessive differential pressure or a decrease in the differential temperature across the column.

flotation A process for separating particulate matter in which differences in surface chemical properties are used to make one group of particles float on water while other particles do not; it is used primarily to separate minerals from gangue but is also used in some chemical and biological processes.

flow nozzle A differential pressure producing element having a contoured entrance. Can be mounted between flanges and has a lower permanent pressure loss than an orifice plate.

flow rate instability (bistable flow) An abrupt change in the control valve flow rate that occurs independent of changes in valve position. It can be caused by variable wall attachment of the fluid stream at the valve orifice, by flashing, or by cavitation.

flow straightener A supplementary length of straight pipe or tube, containing straightening vanes or the equivalent, which is installed directly upstream of a flowmeter for the purpose of eliminating swirl from the fluid entering the flowmeter

fluidised bed A dynamic mixture of a gas and/or vapour and minute solid particles of such a size that the mixture resembles a fluid in motion.

flume An adaptation of the venturi concept of flow constriction for open channel flow measurement.

fluted rotor flowmeter A type of flow measurement device in which fluid is trapped between two fluted rotors which are dynamically balanced but hydraulically unbalanced so that they turn at a rate proportional to the volume rate of fluid flow.

foot pound A force of one pound applied to a lever one foot long.

force balance transmitter A transmitter design technique utilising feedback of the output signal to balance the primary input signal from the measuring element. The balanced output signal is proportional to the measured variable.

forced draft Air under positive pressure produced by fans at the point where air or gases enter a unit, such as a combustion furnace or boiler.

forced oscillation Oscillation of a system parameter where the period of oscillation is determined by an external periodic driving force.

forced vibration Oscillation occurring at the frequency of a driving force input.

forcing Applying control signals greater than those warranted by a given deviation in the controlled variable in order to induce a more rapid rate of adjustment in the controlled variable.

four wire What is normally required for a full duplex circuit. Two wires are used for the transmission and two are used for the reception.

four-wire transmitter Electronic transmitter that has separate pairs of wires for signal and power.

fraction 1. In classification of powdered or granular solids, the proportion of the sample (by weight) that lies between two stated particle sizes. 2. In chemical distillation, the proportion of a solution of two liquids consisting of a specific chemical substance.

fractional distillation A thermal process whereby a mixture of liquids that boil at different temperatures is heated at a series of increasing temperatures, and the distillates boiled off at each temperature are collected separately.

fractionating column An apparatus for fractional distillation in which rising vapour and falling liquid are brought into intimate contact.

fraction defective In quality control, the average number of units of product containing one or more defects for each 100 units of product in a given lot.

free vibration Oscillation occurring at a natural frequency, after an initial force input.

free water The amount of water released when a wet solid is dried to its equilibrium moisture content.

frequency, damped The apparent frequency of a damped oscillatory time response of a system resulting from a nonoscillatory stimulus

frequency division multiplex (FDM) A system for the transmission of information about two or more quantities (measurands) over a common channel, by dividing the available frequency bands; amplitude, frequency, or phase modulation of the subcarriers may be employed.

frequency domain Vibration represented as a graph of amplitude versus frequency.

frequency, gain crossover On a Bode diagram of the transfer function of an element or system, the frequency at which the gain becomes unity (and its decibel value zero).

frequency, natural The frequency that a part, or system, will oscillate at if excited with an impulse.

frequency, phase crossover Of a loop transfer function, the frequency at which the phase angle reaches + 180°.

frequency, resonant The measurand frequency at which a transducer responds with maximum output amplitude. When major amplitude peaks occur at more than one frequency, the lowest of these frequencies is the resonant frequency.

frequency response A measure of the effectiveness with which a circuit or device transmits signals of different frequencies, usually expressed as a graph of magnitude or phase of an output signal as a function of frequency. Also known as "amplitude frequency response"; "sine wave response.".

frequency response characteristic In process instrumentation, the frequency dependent relation, in both gain and phase, between steady state sinusoidal inputs and the resulting fundamental steady state sinusoidal outputs. Frequency response is commonly plotted on a Bode diagram.

frequency response method A method of tuning a process control loop for optimum operation by proper selection of controller settings. This method is based on a study of the frequency response of the open process control loop.

frequency, ringing The frequency of the oscillatory transient occurring in the transducer output as a result of a step change in measurand.

frequency shift keying (FSK) Modulation accomplished by switching from one discrete frequency to another discrete frequency.

frequency, undamped (frequency, natural) 1 Of a closed loop control system or controlled system, a frequency at which continuous oscillation (hunting) can occur without periodic stimuli. In linear systems, the undamped frequency is the phase crossover frequency. With proportional control action only, the undamped frequency of a linear system may be obtained in most cases by raising the proportional gain until continuous oscillation occurs.

frost plug A device for determining liquid level when the contents of a tank are at a temperature below 0°C; a side tube resembling a sight glass but having a series of closed tubes (plugs) at different levels instead of the glass; the tubes below liquid level are cooled so that moisture from the atmosphere forms frost on them, while the tubes above liquid level remain frost free.

full ball A valve closure member that is a complete spherical surface with a flow passage through it. The flow passage may be round, contoured or otherwise modified to yield a desired flow characteristic.

full duplex Communications that appear to have information transfer in both directions (transmit and receive) at the same time.

full-face gasket A flat gasket which contacts the entire flat contact surface of two mating flanges, extending past the bolt holes. This term applies to flat face flanges only.

fusible alloy An alloy with a very low melting point, in some instances approaching 150°F (65°C), usually based on Bi, Cd, Sn or Pb; the fusible alloys have varied uses, the most widely known being solders and fusible links for automatic sprinklers and fire alarms.

fusible plug A hollowed threaded plug having the hollowed portion filled with a low melting point material.

g Acceleration due to gravity; equal to 980 cm/sec2 (32.2 ft/sec2) on the surface of the earth.

gauge cock A valve attached to a water column or drum for checking water level.

gauge pressure Pressure measured relative to ambient pressure.

gain 1. Ratio of output signal magnitude to input signal magnitude; when less than one this is usually called attenuation. 2. The relative degree of amplification in an electronic circuit. 3. The ratio of the change in output to the change in input which caused the change. 4. In a controller, the reciprocal of proportional band Proportional band can be expressed as a dimensionless number (gain) or as a percent.

gain, closed-loop In process instrumentation, the gain of a closed-loop system, expressed as the ratio of the output change to the input change at a specified frequency.

gain, derivative action (rate gain) The ratio of maximum gain resulting from proportional-plus- derivative control action to the gain due to proportional control action alone.

gain, dynamic The magnitude ratio of the steady state amplitude of the output signal from an element or system to the amplitude of the input signal to that element or system, for a sinusoidal signal.

gain, loop In process instrumentation, the ratio of the absolute magnitude of the change in the feedback signal to the change in its corresponding error signal at a specified frequency. NOTE: The gain of the loop elements may be measured by opening the loop, with appropriate termination. The gain so measured is often called the open-loop gain

gain (magnitude ratio) For a linear system or element, the ratio of the magnitude (amplitude) of a steady state sinusoidal output relative to the causal input; the length of a phasor from the origin to a point of the transfer locus in a complex plane. NOTE: The quantity may be separated into two factors: 1. a proportional amplification often denoted as K, which is frequency-independent, and associated with a dimensioned scale factor relating to the units of input and output; 2. A dimensionless factor often denoted as G which is frequency dependent. Frequency, conditions of operation, and conditions of measurement must be specified. A loop gain characteristic is a plot of log gain vs. log frequency. In nonlinear systems, gains are often amplitude dependent.

gain margin The reciprocal of the open loop gain for a stable feedback system at the frequency at which the phase angle reaches -180°.

gain, proportional The ratio of the change in output due to proportional control action to the change in input.

gain, static (zero frequency gain) Of gain of an element, or loop gain of a system, the value approached as a limit as frequency approaches zero. Its value is the ratio of change of steady state output to a step change in input provided the output does not saturate.

gal A unit of acceleration equal to 1 cm/s2. The milligal is frequently used because it is about 0.001 times the earth's gravity.

galling Localised adhesive welding with subsequent spalling and roughening of rubbing metal surfaces as a result of excessive friction and metal to metal contact at high spots.

gallon A unit of capacity (volume) usually referring to liquid measure in the British or U. S. customary system of units. The capacity defined by the British (Imperial) gallon equals 1.20095 U.S. gallons; one U.S. gallon equals four quarts or 3.785 10 3 m3.

galvanic corrosion Electrochemical corrosion associated with current in a galvanic cell, which is set up when two dissimilar metals (or the same metal in two different metallurgical conditions) are in electrical contact and are immersed in an electrolytic solution.

game theory A mathematical process of selecting an optimum strategy in the face of an opponent who has a strategy of his own.

gamma ray 1. Electromagnetic radiation emitted by the nucleus of an atom, each photon resulting from the quantum transition between two energy levels of the nucleus. 2. A term sometimes used to describe any high energy electromagnetic radiation, such as rays exceeding about 1 MeV or photons of annihilation radiation.

gas chromatography A separation technique involving passage of a gaseous moving phase through a column containing a fixed adsorbent phase; it is used principally as a quantitative analytical technique for volatile compounds.

gasket A sealing member, usually made by stamping from a sheet of cork, rubber, metal or impregnated synthetic material and clamped between two essentially flat surfaces to prevent pressurised fluid from leaking through the crevice; typical applications include flanged joints in piping, head seals in a reciprocating engine or compressor, casing seals in a pump, or virtually anywhere a pressure tight joint is needed between stationary members. Also known as "static seal."

gas lift The technique of raising a liquid in a vertical flow line by injecting a gas below a portion of the liquid column causing upward flow.

gate array A circuit consisting of an array of logic gates aligned on a substrate in a regular pattern.

gate valve 1. A valve with a linear motion closure member that is a flat or wedge shaped gate which may be moved in or out of the flow stream. It has a straight through flow path. 2. A type of valve whose flow control element is a disc or plate that undergoes translational motion in a plane transverse to the flow passage through the valve body.

gauss The CGS unit of magnetic flux density or magnetic induction; the SI unit, the tesla, is preferred.

Geiger Muller counter A radiation measuring instrument whose active element is a gas filled chamber usually consisting of a hollow cathode with a fine wire anode along its axis; in operation, the voltage between anode and cathode is high enough that the discharge caused by a primary ionising event spreads over the entire anode until stopped when the space charge reduces the electric field magnitude. Also known as "Geiger counter."

General-Purpose Interface Bus (GPIB) IEEE-488 standard bus. Used for computer interfacing to electronic instruments.

gilbert The CGS unit of magnetomotive force; the SI unit, the ampere (or ampere turn) is preferred.

gland 1. A device for preventing a pressurised fluid from leaking out of a casing at a machine joint, such as at a shaft penetration. Also known as "gland seal. " 2. A movable part that compresses the packing in a stuffing box.

globe valve 1. A valve with a linear motion closure member, one or more ports and a body distinguished by a globular shaped cavity around the port region. 2. A type of flow regulating valve consisting of a movable disc and a stationary ring seat in a generally spherical body. In the general design, the fluid enters below the valve seat and leaves from the cavity above the seat.

go/no go test A test in which one or more parameters are determined, but which can result

only in acceptance or rejection of the test object, depending on the value(s) measured.

governor A device for automatically regulating the speed or power of a prime mover

grab sampling A method of sampling bulk materials for analysis, which consists of taking one or more small portions (usually only imprecisely measured) at random from a pile, tank, hopper, railcar, truck or other point of accumulation.

graceful degradation A system attribute wherein when a piece of equipment fails, the system falls back to a degraded mode of operation rather than failing catastrophically and giving no response to its users.

graded index fiber (GRIN) An optical fiber in which the refractive index changes gradually between the core and cladding, in a way designed to refract light so it stays in the fiber core. Such fibers have lower dispersion and broader bandwidth than step index fibers.

gravitational constant A dimensionless conversion factor in English units which arises from Newton's second law (F= ma) when mass is expressed in pounds mass (lbm). gravitometer See "densimeter."

gravity Weight index of fuels; liquid, petroleum products expressed either as specific, Baume or API (American Petroleum Institute) gravity; weight index of gaseous fuels as specific gravity related to air under specified conditions; or weight index of solid fuels as specific gravity related to water under specified conditions.

gray Metric unit for absorbed dose.

graybody An object having the same spectral emissivity at every wavelength, or one whose spectral emissivity equals its total emissivity.

Gray code A generic name for a family of binary codes which have the property that a change from one number to the next sequential number can be accomplished by changing only one bit in a code for the original number. This type of code is commonly used in rotary shaft encoders to avoid ambiguous readings when moving from one position to the next.

grounding The act of establishing a conductive connection, whether intentional or accidental, between an electrical circuit or electrical equipment and either the earth or some other conducting body that serves in place of the earth.

ground loop Circulating current between two or more connections to electrical ground. This signal can be detected and displayed by electronic instruments. These signals are generally not associated with the variable to be measured and represent noise in the measuring system.

group Lists of materials of similar explosion hazard.

group velocity The velocity corresponding to the rate of change of average position of a wave packet as it travels through a medium.

half duplex Communications in both directions (transmit and receive), but in only one direction at a given instant in time.

half life The time span necessary for the atoms of a nuclide to disintegrate by one half.

Hall effect An electromotive force developed as a result of interaction when a steady state current flows in a steady state magnetic field; the direction of the emf is at right angles to both the direction of the current and the magnetic field vector, and the magnitude of the emf is proportional to the product of current intensity, magnetic force, and sine of the angle between current direction and magnetic field vector.

halogen Any one of the four chemical elements; chlorine, fluorine, bromine or iodine.

Hamming distance A characteristic of any given data code that indicates the ability to detect single bit errors; it equals the number of bits in any given character that must be changed to produce another legitimate character.

harmonic content The distortion in a transducer's sinusoidal output, in the form of harmonics other than the fundamental component. NOTE: (S) It is usually expressed as a percentage of rms output.

harmonic distortion 1. Distortion characterised by the appearance in the output of harmonics other than the fundamental component when the input wave is sinusoidal. 2. Distortion caused by the presence of harmonics of a desired signal. 3. Generation of unwanted harmonics by a device as expressed by the amplitude of the harmonics as a percentage or ratio (in dB) of the primary signal.

Hastelloy B An International Nickel co. alloy having a nominal composition of nickel (Ni) 66.7%; iron (Fe) 5%; molybdenum (Mo) 28%; vanadium (V) 0.3%.

Hastelloy C An International Nickel co. alloy having a nominal composition of nickel (Ni) 59%; iron (Fe) 5%; molybdenum (Mo) 16%; tungsten (W) 4%; chromium (Cr) 16%.

hazardous area An area in which explosive gas/air mixtures are, or may be expected to be, present in quantities such as to require special precautions for the construction and use of electrical apparatus.

hazardous area classifications

1. Division 1 (hazardous). Where concentrations of flammable gases or vapours exist a) continuously or periodically during normal operations; b) frequently during repair or maintenance or because of leakage; or c) due to equipment breakdown or faulty operation which could cause simultaneous failure of electrical equipment. (See "National Electrical Code, Paragraph 500 4(a)" for detailed definition.)

2. Division 2 (normally nonhazardous). Locations in which the atmosphere is normally nonhazardous and may become hazardous only through the failure of the ventilating system, opening of pipe lines, or other unusual situations. (See "National Electrical Code, Paragraph 500 4(b)" for detailed definition.)

3. Nonhazardous. Areas not classified as Division 1 or Division 2 are considerednonhazardous. NOTE: It is safe to have open flames or other continuous sources of ignition in nonhazardous areas [S12.4].

hazardous atmosphere 1. A combustible mixture of gases and/or vapours. 2. An explosive mixture of dust in air. hazardous (classified) location A location where fire or explosion hazards may exist due to flammable gases or vapours, flammable liquids, combustible dust, or easily ignitable fibers or flyings.

hazardous material Any substance that requires special handling to avoid endangering human life,

health or well being. Such substances include poisons, corrosives, and flammable, explosive or radioactive chemicals.

head loss Pressure loss in terms of a length parameter such as inches of water or

millimeters of mercury.

head pressure Expression of a pressure in terms of the height of fluid. P = ypg, where p is

fluid density and y is the fluid column height.

heart cut Chromatography technique is frequently used in the analysis of trace components that are not readily separated from a large composition peak. It utilises two columns and a diverting valve, and reduces the ratio of large components to small ones.

heat Energy that flows between bodies because of a difference in temperature; same as thermal energy.

heat available The thermal energy above a fixed datum that is capable of being absorbed for useful work. In boiler practice, the heat available in a furnace is usually taken to be the higher heating value of the fuel corrected by subtracting radiation losses, unburned combustible, latent heat of the water in the fuel formed by the burning of hydrogen, and adding the sensible heat in the air for combustion, all above ambient temperatures.

heat balance An accounting of the distribution of the heat input and output.

heat content The amount of heat per unit mass that can be released when a substance undergoes a drop in temperature, a change in state or a chemical reaction.

heat exchanger A vessel in which heat is transferred from one medium to another.

heat flux The quantity of thermal energy transferred to a unit area per unit time.

heat transfer, coefficient of Heat flow per unit time across a unit area of a specified surface under the driving force of a unit temperature difference between two specified points along the direction of heat flow. Also known as "over all coefficient of heat transfer. "

heat tracing The technique of adding heat to a process or instrument measurement line by placing a steam line or electric heating element adjacent to the line.

henry Metric unit for inductance.

Henry's law A principle of physical chemistry that relates equilibrium partial pressure of aa substance in the atmosphere above a liquid solution to the concentration of the same substance in the liquid; the ratio of concentration to equilibrium partial pressure equals the Henry's law constant, which is a temperature sensitive characteristic; Henry's law generally applies only at low liquid concentrations of a volatile component.

hertz Unit for frequency of a periodic phenomenon measured in cycles per second.

heuristic Pertaining to a method or problem solving in which solutions are discovered by evaluation of the progress made toward the final solution, such as a controlled trial and error method. An exploratory method of tackling a problem, or sequencing of investigation, experimentation, and trial solution in closed loops, gradually closing in on the solution. A heuristic approach usually implies or encourages further investigation, and makes use of intuitive decisions and inductive logic in the absence of direct proof known to the user. Thus, heuristic methods lead to solutions of problems or inventions through continuous analysis of results obtained thus far, permitting a determination of the next step. A stochastic method assumes a solution on the basis of intuitive conjecture or speculation and testing the solution against known evidence, observations, or measurements. The stochastic approach tends to omit intervening or intermediate steps toward a solution. Contrast with stochastic and algorithmic.

heuristic program A program that monitors its performance with the objective of improved performance.

hexadecimal (hex) A number of representation system of base 16 and the digit symbols from 0 to 9 and A to F. The hex number system is very useful in cases where computer words are composed of multiples of four bits (that is, 4 bit words, 8 bit words, 16 bit words, and so on).

high level language A programming language whose statements are translated into more than one machine language instruction. Examples of high level languages are BASIC, FORTRAN, COBOL, and PASCAL.

humidity, relative The ratio of the water vapour pressure actually present to the water vapour pressure required for saturation at a given temperature, expressed in percent.

hunting An undesirable oscillation of appreciable magnitude, prolonged after external stimuli disappear. Note: In a linear system, hunting is evidence of operation at or near the stability limit; nonlinearities may cause hunting of well defined amplitude and frequency. In automatic control, hunting is generally caused by the gain or reset of the controller being set too high.

hydrocracker A chemical reactor in which large hydrocarbon molecules are fractured in the presence of hydrogen.

hydrogen damage Any of several forms of metal failure caused by dissolved hydrogen, including blistering, internal void formation, and hydrogen induced delayed cracking.

hydrostatic test Determining the burst resistance or leak tightness of a fluid component or system by imposing internal pressure.

hygrometer An instrument for directly indicating humidity. hygrometry Any process for determining the amount of moisture present in air or another gas.

hygroscopic Having a tendency to absorb water. Having the ability to accelerate the condensation of water vapour.

hypergolic A term related to spontaneous ignition upon contact.

hysteresis 1. That property of an element evidenced by the dependence of the value of the output, for a given excursion of the input upon the history of prior excursions and the direction of the current traverse. NOTE: 1 It is usually determined by subtracting the value of dead band from the maximum measured separation between upscale going and downscale going indications of the measured variable (during a full range traverse, unless otherwise specified) after transients have decayed. This measurement is sometimes called "hysteresis error". NOTE 2: Some reversal of output may be expected for any small reversal of input; this distinguishes hysteresis from dead band. 4. A phenomenon demonstrated by materials which make their behavior a function of the history of the environment to which they have been subjected. 5. The tendency of an instrument to give a different output for a given input, depending on whether the input resulted from an increase or decrease from the previous value.