HI A fieldbus network that operates at 31.25 kbit/
second. Generally, it is a
lower-speed and lower-cost
network than H2.
HI field device A field
bus device that is connected
directly to an HI fieldbus.
Typical HI field devices are valves and trans
mitters.
HI repeater An HI repeater is an active, bus powered, or non-bus-powered device that is used to extend the range over which signals can be correctly transmitted and received for a given medium. A maximum of four repeat ers and/or active couplers can be used between any two devices on an HI fieldbus network. HZ A proposed ISA SP50 (fieldbus) standard for communications at the controller level of hosts. It has been superseded by the HSE (High Speed Ethernet) protocol. hacking In computers, writing programs or using computer communicating for fun. Through misuse in the news media it has come to mean only the indiscriminate and unauthorized accessing of other computers over public networks. See cracking. hair-line cracks Fine, random cracks in a coating such as paint or in any rigid surface. hairline register In the composition of screen displays and printing, alignment within ±1/ 2 row of dots. half-adder A logic circuit that accepts two binary input signals and produces corre sponding sum and carry outputs. Two half-adders and an OR gate can be combined to realize a full-adder. See also full adder. half-adjust To round a number so that the least significant digit or digits determine whether or not a one (1) is to be added to the digit that is next higher in significance. After the adjustment is made, if required, the digit or digits used as the criterion will be dropped. For example, in 432.784, using the terminal 4 as the criterion yields 432.78 as the half-adjusted value. The number 432.785 half-adjusts to 432.79 since the terminal digit is "one half, or more." half-and-half solder A lead-tin alloy (50Pb 50Sn) that is used primarily to join copper tubing and fittings. half bridge In strain gauges, such as in force or pressure sensors, the state in which only two of four legs of a Wheatstone bridge con figuration are active in the development of a signal.
H
half cycle In alternating circuits, the time it takes to complete one-half of a full cycle at the operating frequency. half duplex Communications in both direc tions (transmit and receive), but in only one direction at a given instant in time. See duplex, half and full duplex. half-duplex transmission "One-way-at-a time" communication. Both devices can transmit and receive data but only one at a .: t i m e . , . ~.,<. half-height drive A 5-1/4-inch disk drive that is 1-5/8 inches wide when installed. half-life The time span necessary for half the atoms of a nuclide to disintegrate. half-thickness The thickness of an absorbing medium that will depreciate the intensity of a radiation beam by one half. halftone In electronic publishing and screen displays, the reproduction of continuous tone images through a screening process. The image is converted into dots of various sizes and equal spacing between centers (as with conventional printing). Also called "amplitude modulated (AM) screening." See amplitude modulation. half-wave plate A polarization retarder that causes the light of one linear polarization to be retarded by a half wavelength 180 rela tive to the phase of the orthogonal polariza tion. half-wave rectifier 1. An electronic circuit that converts an AC input signal into a DC output signal. Current flows in the output circuit during only one half of each cycle of the input signal. 2. A rectifier that feeds cur rent during the half cycle when the alternat ing current voltage is in the polarity at which the rectifier has low resistance. During the other half cycle the rectifier passes no cur rent. halide A compound that contains fluorine, bromine, chlorine, or iodine. [ISA-71.04 1985] Hall effect An electromotive force (emf) that is 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 cur rent and the magnetic field vector,~he mag nitude of the emf is proportional to the product of current intensity, magnetic force, and the sine of the angle between the current direction and the magnetic field vector. halogen Anyone of the four chemical ele ments chlorine, fluorine, bromine, or iodine. [ISA-71.04-1985] 0
239
II
HALT I hard-drawn wire HALT Highly Accelerated Life Test. Typi cally, the rapid ramp-soak cycling of elec tronic equipment between the extremes of its temperature rating over two to three days. The purpose of the test is to quickly identify failures that would otherwise take years to surface. hammer 1. A hand tool that is used for strik ing a workpiece in order to shape it or drive it into another object. 2. A machine element· tha..t consists of an arm and a strikinOg h€ a d, such as for ringing a bell. Or a machine ele ment that consists of a guided striking head, often carrying one half of a die set, for shap ing metals by forging. Hamming code An error-correcting code, with or without parity, that allows a data device to detect and correct single-bit errors in coded digital data. ) 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 in order to produce another legiti mate character. Hand D curve The measurement of photo graphic emulsion shown as a curve in which density is expressed as a function of the loga rithm of exposure. hand-held equipment Portable equipment that is intended to be supported by one hand during normal use. [ANSI/ ISA-82.02.01-1999 (lEe 61010-1 Mod)] handhole An opening in a pressure part that provides access, but usually not exceeding 6 inches in the longest dimension. handhole cover A hand hole closure hand jack A manual hydraulic or mechanical override device that uses a lever to stroke a valve or to limit its travel. [ANSI/ISA 75.05.01-2000] hand lance A manually manipulated length of pipe that carries air, steam, or water for the purpose of blowing ash and slag accumula tions from heat-absorbing surfaces. hand set Timers that are set manually after each operation. The operator turns the set pointer to the required time interval, and during timing the timer motor drives the pointer back to zero. handshake The recognition between hvo computers that they are able to communi cate. handshaking The exchange of predeter mined signals between two devices in order to establish a connection. Handshaking is usually part of a communications protocol.
handwheel A mechanical manual override device that uses a rotary wheel to stroke a valve or to limit its travel. [ANSI/ISA 75.05.01-2000] handwheel, in-yoke mounted In-yoke gear types are designed with a worm gear drive that is contained in a lubricated housing. The gearbox is integral with the yoke, which is usually elongated in order to provide space (orJhe worm gear assembly. With this type of handwheel,"stops may be set in either or both directions to limin~e travel of the v.alve stem. This type of handwheel is declutch able. [ANSI/ISA-75.05.01-2000] handwheel, shaft-mounted, declutchable A shaft-mounteq worm gear drive that can be declutched from the power actuator. [ANSI/ ISA-75.05.01-2000] handwheel, side-mounted Bellcrank lever types are externally mounted on the control valve yoke..,\hey can provide a limit to the extent that a va.tve stem will travel in either direction, but not in both directions. [ANSI/ ISA-75.05.01-2000] handwheel, top-mounted The handwheel is mounted on top of the valve actuator case. This type of handwheel does not have a clutch. It is usually used to restrict the motion of the valve stem in one direction only. [ANSI/ISA-75.05.01-2000] hard card A type of computer hard disk on which data is placed on a card rather than a spinning disk. hard-clad silica fibers Silica optical fibers that are coated with hard plastic material, not with the soft materials typically used in plastic-clad silica. hard copy Output in permanent (paper) form, usually through a printer or chart recorder, as compared with the impermanent form of output of a disk or a display termi nal. hard disk A computer storage medium that has a large storage capacity as compared to floppy disks. Disks that provide gigabytes of storage space are common. hard-disk management Since hard-disk life is limited, there are four basic things that will enhance disk life and use: (1) using subdirec tories rather than having all ~rk files in one directory; (2) deleting files that are no longer needed; and (3) periodically running a "scan disk" utility to check for lost clusters; (4) run ning a defragmentation program every three months. hard-drawn wire Heavily cold-drawn metal wire that has relatively high tensile strength and low ductility.
240
hardening / Hastelloy C
I'
hardening Producing increased hardness in a metal by quenching it from high tempera ture, such as when hardening steel. Another method of hardening, for certain aluminum or other nonferrous alloys, is done by precip itation-hardening (aging) a dilute alloy. hard error A repeatable error that exhibits a stuck-at-one or stuck-at-zero condition within a digital device. hard facing 1. A material that is applied to valve internals to resist fluid erosion and/ or to reduce the chance of galling between mov ing parts, particularly at high temperatures. [ANSI/ISA-75.05.01-2000] 2. A material that is harder than the surface to which it is applied. It is used to resist fluid erosion and/ or to reduce the chance of galling between moving parts, particularly at high tempera ture. [ANSI/ISA-75.05.01-2000] hard lead Any of a series of lead-antimony alloys of low ductility. Typically, hard lead contains 1 percent to 12 percent Sb. hardness A measure of the amount of cal cium and magnesium salts in a boiler water. Usually expressed as grains per gallon or ppm as CaC03. hard plating A thin metal deposit, sometimes electroplated, that is used to provide a sur . face coating or induce surface hardening. Hard plating is many orders of magnitude . thinner than hard facing. [ANSI/ISA 75.05.01-2000] hardware 1. Physical equipment that is directly involved in performing industrial process measuring and controlling functions. [ANSI/ISA-51.1-1979 (R1993)]2. Physical equipment (associated with a computer), as opposed to programs, procedures, rules, and associated documentation. [ISA-TR50.02, Part 9-2000] hardware priority interrupt See priority inter rupt and software priority interrupt. hard water Water that contains calcium or magnesium in amounts that require an excessive amount of soap to form a lather. hard-wired Electrical connections accom plished without the use of software or firm ware. [ANSI/ISA-84.01-1996] harmonic Having a frequency that is a multi ple of the basic cyclical quantity to which it is related. harmonic analyzer 1. An instrument for mea suring the magnitude and phase of the har monic segments of a cyclical function from a graph. 2. An electronic instrument that mea sures the amplitude and frequency of an AC signal, including those of its harmonics.
241
harmonic content The distortion in a trans ducer's sinusoidal output, which takes the form of harmonics other than the fundamen tal component. Note: It is usually expressed as a percentage of rms output. [ISA-37.1-1975 (R1982)] harmonic conversion transducer A trans ducer in which the output frequency is a multiple of the input frequency. harmonic disfortion 1. Distortion that is char~!=tedted by the appearance in the out put of harmonics other than the fundamental component when the input wave is sinusoi dal. [ISA-RP55.1-1975 (R1983)]2. Distortion that is caused by the presence of harmonics of a desired signal. 3. The generation of unwanted harmonics by a device as expressed by the amplitude of the harmonics as a percentage or ratio (in dB) of the pri mary signal. harmonic generattQ!1... 1. The multiplication of the frequency of a'lightwave through nonlin ear interactions of the lightwave with certain materials. Generating the second harmonic is equivalent to <;lividing the wavelength in half. 2. An eledronic means for multiplying frequency. It IS usually accomplished with the assistance of nonlinear devices. harmonics Vibration frequencies that are integral multiples of the fundamental. HART "Highway Addressable Remote Transducer." An open, "smart" field instru mentation protocol developed by Rose mount that has been adapted by several other companies, creating a de fl/eta standard fieldbus. Not considered a true fieldbus, it imposes a Bell 202 FSK, 1200 bps digital sig nal on a twisted pair of wires carrying 4-20 mA input. Hartley information unit In information the ory, a unit for the logarithmic measurement of the decision content of a set of 10 mutually exclusive events, expressed as the logarithm to the base 10. For example, the decision con tent of an eight-character set equals log 8, or 0.903 Hartley. hashing The generation of a meaningless number from a group of records, which can be used as a location address. . Hastelloy B An International Nic-tl Co. alloy that has a nominal composition of nickel (Ni), 66.7 percent; iron (Fe), 5 percent; molyb denum (Mo), 28 percent; and vanadium (V) 0.3 percent. Hastelloy C An International Nickel Co. alloy that has a nominal composition of nickel (Ni), 59 percent; iron (Fe), 5 percent;
Hay bridge / HCFC
I
molybdenum (Mo), 16 percent; tungsten (W), 4 percent; and chromium (Cr), 16 percent. Har br~d?e ~ general-purpose AC bridge ClrcUlt m whIch two opposing sides of the bridge are fixed resistances. The unknown leg is a combination of resistance and induc tance, and the remaining side consists of a variable resistor and a variable capacitor. hazard A chemical or physical condition that has the potential for causing injury to people or the environment. [ANSI/ISA-84.01-1996] hazardous area An area in which explosive gas/air mixtures are, or may be expected to be, present in quantities such that special precautions are required for the construction and use of electrical apparatus. hazardous area classifications There are three hazardous area classifications: Division 1 (hazardous), Division 2 (normally nonhaz ardous), and nonhazardous. 1. Division 1 (hazardous): A location in which concentra tions of flammable gases or vapors exist (a) continuously or periodically during normal operations; (b) frequently during repair or maintenance or because of leakage; or (c) becaus.e of eq~ipment breakdown or faulty operation, which could cause the simulta neous failure of electrical equipment. (See the National Electrical Code-2002, Paragraph 500-4(a), for a detailed definition.) 2. Division 2 (normally nonhazardous): Locations in which the atmosphere is normally nonhaz ardous and may become hazardous only of the ventilating system fails, pipe lines are opened, or other unusual situations arise. (See National Electrical Code, Paragraph 500-4(b), for a detailed definition.) 3. Non hazardous. Areas not classified as Division 1 or Division 2 are considered nonhazardous. Note: It is safe to have open flames or other continuous sources of ignition in nonhazard ous areas. [ISA-RP12.4-1996] hazardous atmosphere 1. A combustible mixture of gases and/or vapors. 2. An explo sive mixture of dust in air. hazardous (classified) location A location in which fire or explosion hazards may exist because of an explosive atmosphere of flam mable gases or vapors, flammable liquids, combustible dust, or easily ignitable fibers or flyings. Note: See also explosive atmosphere. [ANSI/ISA-12.01.01-1999; ISA-RP12.4-1996; ANSI/ISA-RP12.6-1995] hazardous dust layer Any accumulation of combustible dust that will propagate or cause a fire . hazardous live Something that is capable of rendering an electric shock or electric burn in
Divisions 1 - Continuous hazard (> 1000 hrs/yr): Zone o for gases, Zone Z (10) for dusts; and intermittent hazard (l0-1000 hrs/yr): Zone 1 for gasses, Zone Z (10) for dusts. Division 2 - Hazard under abnormal conditions (0.1-10 hrs/yr): Zone 2 for gases, Zone Y (11) for dusts. Class I - Areas in whic.h 0ammable gases or vapors
may be present in au In suffiCIent quantities to be
explosive.
Group A - Atmospheres containing acetylene.
Group B - Atmosph~res such as butadiene, ethvlene
. oXide, propylenl/ oxide, acrolein, or hydrogen (or
gases and v~pors equivalent in hazard).
Group C - Atmospheressllch as cyclopropane, ethyl
ether, ethylene, or gases and vapors equivalent in
hazard.
Group D - Atmospheres such as acetO:1e alcohol
ammonia, benzene, benzol, butane, g~soline, hex
ane, lacquer-solvent vapors, naptha, natural gas,
propane, or gases and vapors equivalent in hazard
ous.
Class II - Areas made hazardous bv the presence of combustible dust. ' Group E1 - Atmospheres containing combustible metal dusts, regardless of resistivitv. Group E2 - Atmospheres containing combustible dust
of slmdarly hazardous characteristics having resis
tivitv of less than 100Kn -cm
Group'E3 - Atmospheres contai~ing combustible elec trically conductive dusts. ' Group F1 - Atmospheres containing combustible, car
bon black, charcoal, or coke dusts having more than
8 percent total volatile material.
Group. F2 - Atmospheres containing combustible dusts haVing an explslOn hazard with resistivity <lOon cm and ~1 X 108n -cm. . Group G1 - Atmospheres containing combustible dust ha\'ing resistivity ~100kn -cm. Group G 2 - Atmospheres containing combustible elec trically nonconductive dusts.
Hazardous Area Classifications normal condition or in single-fault condition. [ANSI/ISA-82.02.01-1999 (IEC 61010-1 Mod)] hazardous location An area where flamma ble gases, vapors, dusts, fibers, or filings cre ate the possibility of explosion and fire. hazardous material Any substance that requires special handling so as to avoid endangering human life, health, or well bei:,g. Such substances include poisons, cor roSIves, and flammable, explosive, or radio active chemicals. hazardous waste Under the Resource Co:,se~vat.ion and Recovery Act (U.S.), any sohd, hqUld, ga~, or co~binathQn of wastes that because of ItS chemical, physical, or infectious characteristics may pose a hazard when managed improperly. hazemeter See transmissollleter. HCFC Hydrochlorofluorocarbon. A consider ation in EPA (U.5.) regulations. Any of sev eral substances that are used as alternatives to chlorofluorocarbons (CFCs) but that are
242
HCS I heat content also thought to deplete the Earth's protective ozone layer. HCS hybrid control system. A control system that includes both "classic" distributed control and programmable control components for processes or operations that need to have the benefit of connecting each system onto the same network. HOLC High-level Data Link Control. A standard, international bit-oriented communication路 protocol defined by CCITT for ISO and used in . HOTV high-definition television. A television that has a pixel resolution of 1,920 x 1,080. Open Systems Interconnection (OSI) applications. See CCITT and ISO. . HDX half duplex. A mode of communication in which two-directional data will flow only one direction at a time (2-wire). head The portion of a computer disk drive that reads, writes, or erases any magnetic storage medium. head crash In data processing, the malfunction of the read and write head in a disk drive. head end A passive component in a broadband transmission network that translates one range of frequencies (transmit) to a different frequency band (receive). Head end allows devices on a single cable network to send and receive signals without interference. ,header [Eng] 1. In digital transmission, control information that is added to the begin, ning of a message. [Comp] 2. In engineering, a conduit or chamber that receives fluid flow from a series of smaller conduits connected to it or that distributes fluid flow among a series of smaller conduits. 3. In data processing, data that is placed at the beginning of a file for identification purposes. A header typically contains the destination address, the source address, and the message number. 4. Information about the purpose, source, and version of the recipe, such as recipe and product identification, creator, and issue date. [ANSI/ISA-88.01-1995] head gap 1. The space between the reading or recording head and the recording medium, such as a tape, drum, or disk. 2. The space or gap that is intentionally inserted into the magnetic circuit of the head in order to force or direct the recording flux into the recording medium. headline, "head" In the typographical composition of screen displays and printing, the display type. It usually appears at the top of the screen or document to identify the topic.
243
head loss The pressure loss in flow systems expressed in terms of a length parameter such as inches of water or millimeters of mercury. head margin In the typographical composition of screen displays and printing, unused space abo\'e the first line on a page. head pressure The expression of a pressure in terms of the height of fluid: P =ypg, where p is fluid density and y is the fluid column heig~t. : "~ head pulley The pulley at the discharge end of the belt conveyor. The power drive for the belt is generally applied to the end pulley. [ISA-RP74.01-1984] health physics The technology that is associated with the measurement and control of radiation doses in' humans. heartbeat 1. A signal quality error (SQE) in the CSMA/CD communication media access method. 2. A test conducted between the transceiver/media attachment unit (MAD) and the data terminal equipment (DTE) to ensure that the collision-detection circuit in the transceiver / MAD is working. heart cut A chromatography technique that is frequently used to analyze trace components that are not readily separated from a large composition peak. It utilizes two columns and a diverting valve, and reduces the ratio of large components to small ones. heat Thermal energy. It is expressed in units of calories or Btus. Energy that flows between bodies because of a difference in temperature. heat-absorbing filter A glass filter that transmits most visible light, but strongly absorbs infrared light. 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. That value is then corrected by subtracting radiation losses, unburned combustibles, the latent heat of the water in the fuel formed by the burning of hydrogen. It is further corrected by adding the sensible heat in the air for combustion, all above ambient t~peratures. . heat balance An accounting of the distribution of the heat input and output. See illustration on next page. 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.
I
heated equipment / heavy duty cable Btu per hour per cubic foot of furnace volume.
ATOMIZING ....R lPOT(I<iT\Al & K1N'ETFTlC) A10MlLlNG STEAM I TOTAl ENERO')
1.
~~I~~~~~
, -_ _
LATENT HEAT
I
--1.1 ---1---,
I
--
(AVERAGING
FLUE GAS
(F.o..N POWt:.R)
1--_
:>QTl:'NTlAlENERGY,
~:~~~~Oy
STEAM TQTAt.B':"U
~--____+ lINCl l(INtTlCf:NERG .... )
____+
~~~j~:~~~CY
U"'I6URNED C!'+Er.lICAL lJO,TENrl-lEAT
SENSl8LE rlEAI
COJdUSTlQHAlR
SENSIS...E HE.AT LATENTt-tEAT
!i
KINET,CENfRGY iF"'" POWER)
C)
(FLOW
------.
PQTEN11AlENEffGY
L-
.I
-"
"""""""
SENSI6LEl-l€Af v.-:-eNT HEAT
KINETIC ENERGY PQTENTtlol ENE~GY (PI)MPPQWER/
••
CALORIMETER (ANALYSIS TRANSMITTER DETERMINES HEATING VALUE OF GAS)
t
(CUBIC FEET ~ ~
SOOT BlOWING STEA"" OR "IR SENSIBLE "EATI.. ASrl UNilURNED CAR80N If< "-$H.
FEEOWATER TorN-STU
AT
HE::~T~-'EY-~ [--~-"' ~
(MULTIPLYING RELAY)
",-XY
1 " " ' - - BTU PER HOUR "" Cuft. .
e
• INPUT! C!"lANGe. IN STORED ENE.RGY' OUTPUT
• STORED ENERGY INCREASES AS I"lRlNC MTE 1NCf<EAS£S • SOOT BLOWING lOSS IS PEFIIQOIC
Heat Balance heated equipment Equipment on which resistance heat-trahng units are installed. [ANSI/ISA-12.16.01-1998 (IEC 60079-7 Mod)] h~af exchanger A vessel in which heat is transferred from one medium to another. heat flux The quantity of thermal energy that is transferred to a unit area per unit time. [ISA-37.1-1975 (R1982)] heating panel A surface heater whose output is a defined number of watts per unit area. It is comprised of a series or parallel, connected elements that have sufficient flexibility to conform to the area to be heated. [ANSI! ISA-12.16.01-1998 (IEC 60079-7 Mod] heating surface Those surfaces that are exposed to products of combustion on one side and water on the other. Heating surface area is measured on the side receiving the heat. [ASME Power Test Code] heat rate The ratio of heat input to work output of a thermal power plant. It is a measure of power plant efficiency.
SMOOTHED HEAT RATE ACTUAL HEAT RATE
SHOWING EFFECT OF VALVE THROTTLING 4 SEQUENTIAL VALVES
HEAT RATE BTU/kWhr
INCREMENTAL HEAT RATE
kW
heat release The total quantity of thermal energy above a fixed datum that is introduced into a furnace by the fuel. Heat release is considered to be the product of the hourly fuel rate and its high heat value, expressed in
...!lliL
HEAT RATE (BOARD·MOUNTED RECORDER)
'BTU STANDS FOR BRITISH THERMAL UNIT. THE AMOUNT OF HEAT NEEDED TO RAISE THE TEMPERATURE OF ONE POUND OF WATER BY ONE DEGREE FAHRENHEIT AT SPECIFIC CONDITIONS (THE WORDS IN PARENTHESES ARE FOR EXPLANATION AND
Heat Release heat sink In thermodynamics, a body that can absorb thermal energy. In mechanical design, a finned piece of metal that is used to dissipate heat from the components mounted on it. Any device (usually a static device) that is used primarily to absorb heat and thereby protect another component from damage caused by excessive heat. 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. heat-tracing cable Part of a resistance-heating system, which is comprised of one or more heating elements, suitably insulated and protected. They are usually used to heat pipes and other equipment. [ANSI/ISA12.16.01-1998 (IEC 60079-7 Mod)] heat transfer, coefficient of The 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 "overall coefficient of heat transfer." heat treatment Controlled heating and cooling so as to alter the properties or structure of a metal, alloy, or glasslike material. heavier-than-heavy key ~e remaining components in the bottoms sfream other than the heavy key. See heavy key. Heaviside bridge A type of AC bridge for making mutual inductance measurements ~hen the inductance of the primary winding IS already known. heavy duty cable Generally, a type of fiberoptic or electrical cable that is designed to withstand unfriendly conditions, such as
244
heavy ends I hexagonal-head bolt
\
lose encountered outdoors. Some varieties re armored to withstand hostile conditions. IVy ends The fraction of a petroleum mix.lre that has the highest boiling point. IVy key The component in multicompo.ent distillation that is removable in the bot)ms stream and has the highest vapor 'ressure of the components at the bottoms. If lore reboiler heat is added, the heavy-key omponent is the first component to be put 1 the overhead product. IVy oil A viscous fraction of petroleum or oal-tar oil. It has a high boiling point. IVy spot The imagina.ry vector sum of the tonuniform mass distribution within a rotatl.g body. The corrective procedure of balanc:lg places a weight that compensates for the )rces caused by the heavy spot. IVy water (deuterium oxide) A Vquid ompound, D 20, whose chemical properties re similar to H 20 (light water). It occurs in a atio of 1 part in 6,000 in fresh water. :tare A metric unit of land measure that is qual to 10,000 m 2, or approximately 2.5 cres. :tare-meter A metric unit of volume, comnonly used in irrigation work. It equals 0,000 m 3 and represents the amount of vater needed to cover an area of one hectare :) a depth of one meter. ~l blo..:k A block or plate attached to a die hat keeps the punch from deflecting too ouch. ght gage A mechanical device, usually quipped with a vernier or micrometer scale, hat is used for measuring precise distances bove a reference plane. iarc welding See gas-tungsten arc welding ~TAW).
llige turbidimeter A variable-depth :lstmment for visually determining the loudiness of a liquid as a result of the presnee of finely divided suspended matter. p In data processing, an on-screen infor:lation resource that a user can activate to nswer questions. INe Abbreviation for Helium-neon red ~ser.
try The metric unit for inductance. nry's law A principle of physical chemis,y that relates the equilibrium partial presme of a substance in the atmosphere above liquid solution to the concentration of the arne substance in the liquid. The ratio of oncentration to equilibrium partial pressure quais the Henry's law constant, which is a ~mperature-sensitive characteristic. Henry's
245
law generally applies only at low-liquid concentrations of a volatile component. hermetically sealed device A device that is sealed against the entrance of an external atmosphere and in which the seal is made by fusion, for example, soldering, brazing, welding, or the fusion of glass to metal. [ANSI! ISA-12.12.01-2000] hertz(Hz) A unit of measure for the frequency of a periodic phenomenon, measured in cycles per second. heterodyne ~>combinationof the. AC signals of two different frequencies. The)' are coupled so as to produce beats whose frequency is the sum or difference of the frequencies of the original signals. . heterodyne conversion transducer A trans:" ducer in which the output frequency is the sum and difference of the input frequency and the local oscillator frequency. heterogeneous radiation A beam of radiation that contains rays of s~v~.ral.different wavelengths or particles of different energies or types. heterojunction A junction between semiconductors that differ in doping levels and also in their atomic compositions. An example would be a junction between layers of CaAs (gallium arsenide) and CaAlAs (gallium aluminum arsenide). A double heterojunction laser contains two such junctions; a single heterojunction laser contains only one. heuristic Of an approach that is based upon common-sense rules (rules of thumb) as well as trial and error. Contrasted with an approach based on comprehensive theory that is found in algorithms using mathematically provable procedures. Heuristic programs are characterized by being selflearning. That is, they get better through experience, and arrive at a good result, if not always the best result. The heuristic approach is often used in expert systems. heuristic program A program that monitors its performance with the objective of improved performance. hex A number representation system of base 16. The hex number system is very useful in cases where computer words are composed of multiples of four bits (that is, 4-bit wore+;, 8-bit words, 16-bit words, and so on). â&#x20AC;˘. hexadecimal notation A numbering system that uses 0, 9, A, B, C, D, E, and F with 16 as a base. hexagonal-head bolt A standard threaded fastener that has an integral hexagon-shaped head.
G
H
G'
H..
hexagonal nut I high-level language
I
hexagonal nut A hexagon-shaped fastener with internal threads that is used with a mating, externally threaded bolt, stud, or machine screw. hex code A low-level code in which the machine code is represented by numbers that use a base of 16. Heydweiller bridge A type of AC bridge circuit that is suitable for determining the m~tu~l inductance ?etween two interacting windings, both haVing unknown inductances. HFC Hart Communication Foundation. Formed in, 1993 as an independent, nonprofit organization whose specific mission is to carryon. the work of the HART user group: to coordinate and support the application of HART technology. HGEO High-gain em~ssive display. A flat, LCD-like panel that contains color phosphors similar to those of CRTs. However, HGED displays are modified to operate at extremely low voltage (under 100) and are activated by a matrix grid instead of a beamsteered, high-voltage electron gun. Unlike LCDs, a matrix needs no transistors to address pixels. Phosphors are instead excited by electrons that are guided to the grid by a patented process developed by Telegren, Inc. HGED costs less than one-tenth of LCD and has no viewing angle restrictions. HGED has the brightness, color palette, resolution, and refresh rates needed for high-definition video, without the concern that the user will be bombarded by X-ray emissions from nearfield viewing. hierarchical An approach used in many technologies, where actions, scope of work, and so on are arranged in hierarchies that establ~sh priorities and appropriate routings. Used In process control, machine vision, system networks, databases, sequence-of-video screen views, and planning, among others hierarchical distributed control A hierarchy of computer systems in which one computer acting as supervisor controls several lowerlevel computers. See illustration. hierarchy The specified rank or order of items. Thus, a series of items classified by rank or order. high-alloy steel An iron-carbon alloy that contains at least 5 percent, by weight, of additional elements. high brass A commercial wrought brass that contains 65 percent copper and 35 percent zinc. high-carbon steel A plain carbon steel with a carbon content of at least 0.6 percent.
SUPERVISORY CONTROL SYSTEM
MODEL PREDICTIVE CONTROLLER
TI
AI
PI
DISTRIBUTED CONTROL SYSTEM
INSTRUMENTATION AND REGULAT9RY CONTROLS
Hierarchical Distributed Control high density digital recording (HOOR) The technique that combines the good features of non-r~turn-to-zero (NRZ) and biphase codes, t~ achIe~e a pa~king density of up to 33,000 bits per Inch per track in instrumentation tape recording. hi~h-f~eque~cybias A sinusoidal signal that IS mIxe~ WIth th~ data signal during the magnetIc tape direct-recording process for the purpose of increasing the linearity and dynamIC range of the recording medium. Bias frequency is usually three to four times the highest data frequency to be recorded. h~gh-frequencyheati~g See electronic heating. hIgh-gas pre~sure SWitch A switch to stop . the burner If the gas pressure is too high. high-heat value See calorific value. high integrity Not liable to become defective in such a way as to cause a risk of hazard within the sense of the relevant ANSI/ISA standard. A high-integrity part is considered one that is not subject to failure when tests under fault conditions are made. [ANSI/ ISA-82.02.01-1999 (IEC 61010-1 Mod)] high-level computing device (HLCD) A microprocessor-based device that is used to perform computerlike functions. high-level data link control (HOLC) The standard for communications protocol developed by the International Standards Organization (ISO). . high-level human interferenc~1HLHI) A device that allows a human to interact with the total distributed control system over the .shared communications facility. hIgh-level language A programming language whose statements are translated into r:'0re than one machine-language instructIon. Examples of high-level languages are BASIC, FORTRAN, COBOL, and Televent.
246
high-level operator Interlace (HLUl) I hOlel high-level operator interface (HLOI) A type of high-level human interference (HLHI) that is designed for use by a process operator. highlighting A term that encompasses vari ous attention-getting techniques, such as blinking, intensifying, underscoring, and color coding. [ISA- 5.5-1983] high limiting control See control, high-limit mg. high-low bias test Same as marginal check. highly accelerated temperature and humidity stress test (HAST) A test that subjects devices to a controlled temperature (25 200°C) and humidity (40 - 95%) environ ment. high order Pertaining to the weight or signif icance that is assigned to the digits of a num ber. For example, in the number 123456, the highest-order digit is I, the lovyest-order digit is 6. The three higher-order bits of a binary word are another example. highpass filter A filter that passes high fre quencies above the cutoff frequency, with lit tle attenuation. high potential (hi pot> test A test that consists in applying a voltage in excess of normal between a product's circuitry and any exposed parts. Basic hipot tests include insu lation resistance, dielectric absorption, and step voltage. high-pressure boiler A boiler that furnishes steam at pressure in excess of 15 pounds per square inch. Also, a boiler that furnishes hot water at temperatures in excess of 250°F or at pressures in excess of 160 pounds per square inch. high-resolution graphics A finely defined graphical display on a computer monitor screen. high-strength alloy A metallic material that has a strength considerably above that of most other alloys of the same type or classifi cation. high-temperature alloy A metallic material that is suitable for use at 500°C (930'F) or above. This classification includes iron-base, nickel-base and cobalt-base superalloys as well as the refractory metals and their alloys. These retain enough strength at elevated temperature to be structurally useful and generally endure metallurgical changes without weakening or embrittling the mate rial. high-temperature device In the context of electrical instruments in hazardous loca tions, a high-temperature device is one in which the maximum operating temperature (including ambient temperature effect)
exceeds 80 percent of the auto-ignition tem perature in degrees Celsius (C) of the gas or vapor involved. [ANSI/ISA-12.01.01-1999] high-temperature hot-water boiler A water heating boiler that operates at pressure exceeding 160 psig or temperatures exceed ing 250°F. high-voltage stress test A semiconductor memory test that uses accelerated device aging by cycling through all addresses for several seconds at both high~ and low-logic levels using a voltage at 1-213 nominal at 12S°C. . . hinge A mechanical device that connects two members across a joint yet allows one mem ber to pivot about an axis that runs along the joint. . hi pot test An insulation AC current-limited wherein a 0-600 volt-rated wire insulation is tested for 1,000 v + (2 X nominal voltage rat ing) for a test time of 60 seconds. Note: This test is not generally r~quired within a control center and should not be performed unless specified. HIS Human resources information system. A computer database for manpower s~hedul ing used in enterprise information services (EIS). histogram A type of statistical quality control (SQc) chart. It is a graphical representation of a distribution function by means of rectan gles whose widths represent intervals into which the range of observed values is divided and whose heights represent the number of observations occurring in each interval. historical data In process control, operating data that is saved within a specific device, equipment, or system. See archival (archive). hit 1. An Internet term for successfully gain ing access to a file on a Web page. 2. In data processing, the isolation of a matching record. hitch pin See cotter pin. hit rate The number of successful matches in a computer search. HMO Head-mounted display. Worn by an engineer or operator, the HMD fits on a pair of high-tech glasses. It uses virtual imaging to present the illusion of a full-size viq,o screen to the wearer while he or she is tour ing a plant to make modify the process. hold An external inpClt that is used to stop the AID process and freeze the input. Binary-coded decimal (BCD) hold is an external input that is used to freeze the BCD output while allowing the AID process to continue operation.
247
•
hOldIng beallll horn relay contact
I
110lding beam An electron beam for reactivating the charge on the surface of an electronic device. holding time The length of time that a communication channel is in use for each transmission. hold station Usually refers to circuit card(s) that have capacitive elements for holding electrical charges that represent values calculated by controllers. The charges are held until some multiplexing operation 路s~rfds the held value(s) as signals to the field. Outp1it signal conditioning circuitry is sometimes still referred to as "hold station." hold time ~In any process cycle, an interval during which no changes are imposed on the system. Hold til}1e is usually used to allow a chemical or metallurgical reaction to reach completion, or to al19w a physical or chemical condition to stabilize before proceeding to the next step. Hollerith card A punched card used in digital computing. It is named for Herman Hollerith, vvho developed a computing method using punched cards to compile the 1890 U.S. census. Hollerith code A widely used system of encoding alphanumeric information onto cards. Hellce, Hollerith cards are the same as punch cards. See Hollerith card. holographic diffraction grating A diffraction grating in \vhich the pattern of light-diffracting lines was recorded holographically rather than mechanically ruled into the surface. holographic optical elements Holograms that have been made to diffract light in the same pattern as other optical components. It is possible to produce (usually by computer synthesis) a hologram that mimics the f1lnction of the lens. In some applications, such holographic optical elements are less costly than conventional optics. home In personal computers, a key that places the cursor at the upper left-hand position on the screen or the upper left-hand position of the entire file. home page The first HTML page users see when they open their browsers and gain access to the Internet. homogeneous radiation A beam of radiation that contains rays whose wavelengths all fall within a narro\v band of wavelengths. Or a beam of radiation that contains particles of a single type that have abollt the same energy. homojunction A junction bet\veen semiconductors that differ in doping levels but not in atomic composition. An example would be a
junction bet\veen n-type and p-type gallium arsenide. homologous pair In optical spectroscopy, two lines so chosen that the ratio of their radiant powers changes little \vith variations in the input conditions. hone To remove a small amount of material by using fine-grit abrasive stones and thereby obtaining an exceptionally smooth surface finish or very close dimensional tol. eran<;es. '.. Hookean behavior A condition in liquid .. .expansion when the fractional change in volume is proportional to the hydrostatic stress. If a liquid is under such stress it evidences ideal elastic beha vior. hook gauge路' An instrument that consists of a pointed metal hook mounted on a micrometer slide. It is used to measure the level of a liquid in an evaporation pan. The level with respect to a reference height is determined vvhen the point of the hook just breaks the liqllid surface. HOOPS Hierarchical Object-Oriented Picture System. An emerging standard in 3-D graphics. hopper, card A device that holds cards and makes then1 available to a card-feed mechanism. Synonymous with "input magazine." [ISA-RP55.1-1975 (R1983)] hopper scale A weighing device that consists of a bllik container or hopper suspended on load cells or a lever system and used to batch-weigh bulk solids. Hopper scales are often used in connection with automated batch processing or wit!1 continuous receiving or shipping operations. horizontal boiler A water-tube boiler in which the main bank of tltbes is straight and on a slope of 5 to 15 degrees from the horizontal. horizontal return-tubular boiler A fire-tube boiler that consists of a cylindrical shell, in which tubes inside the shell are attached to both end closures. The products of combustion pass under the botton1 half of the shell and return through the tubes. horn A device for directing and intensifying sOllnd waves that consists of a tube whose cross section increases from <~e end to the other. hom antenna The flared end of a radar \vaveguide. Its dimensions are chosen to provide the efficient radia tion of electromagnetic energy into the surro1lnding environment. horn relay contact See auxiliary output.
248
horsepower I HTTl' .---------------------------------------
---h
e:f Unit of measure for the ability to °dTsepowk It is equivalent to 33,000 pounds o wor . h one f . . tproug oot'In one mmute. moving . 1Jter) [Comp]1. The pnmary com. h ost (comp . -:> multielement system. The system pu ter In "'" · ues commands, has access to the most th a t ISS . t flt d ata, an d'IS th e most versatJ'1 e proImpor . a sys tem. C ompare WIt .h . aelement m cessmg . rtlputer [Cont] 2. The pnmary or contarge t co . a mu I'tip Ie-part system. ' computer In troII mg h . . 'des pnmary serVICes .suc as compuIt proVI · database access, speCIal programs, ta t lon, . 1 . ",d generatIOns, or programmmg ancomma.· . In some earlIer DCSs, the system guages. fi o-ured by programming the host, was con 0 th d wl1loaded over the network to each en Oller file. The term host implies that tHe' con tro . . te r IS requll~d for the system to be compu f . . IunctIona f' l'Ity I tely unctIOna.I P a~tIa comPbe may e ,.-:>chievable without the host. See " A process f or rust-proo f h guest. alvamzmg t d' ~ I? g and steel products by applying a mg Iron of meta 11"IC zmc. · coatIng . h t d' 'og A process for coating parts by Pl °b . iJ1lmersing them in J molten metal bnt~ ~hel1 withdrawing them and allowing a , tal to solidify and cool. th erne A th I' . th hot 'unctioo ermocoup e JunctIOn at'1S J d t J1leasure an unknown temperature use 0 . .) (measu ring Junction. I d' d contro 1 systems, hot-standPY n Istn'b ute. ually means a duplIcate (redundant) th IS us "d ' . su ch as a contro II er th at carnes 1 entId eVIce II . d b f . · forrJ1atIon an can e unctIona y 1 .. h ca I In 't hed into rep acement serVICe WIt SWI c 'ally no mterruptIon . . 0 f th ose funcessent1 L
Ih
O
tions. ing The a b'l' 1 Ity to remove an d H o t swap P 1 . ' WIt . hean e ectromcs b oard or d eVICe rep la c . fl' . out reI1l0"I~g power rom tle equ1pment m h' h it reSIdes. Also, a hardware and softW IC ro tocol that is defined bv the PCMCIA warep , P nal Computer Memory Card Interna(· ersolAssociation) whereby PC cards can be t tOna . d Into . . h out an d mserte soc k ets WIt remov e d . ;,.,0" down or rebooting the Pc. poweru<o .. . e A measunng dev1ce h o t WIT instrument ' reaction. . af a tha t depe~ d s on t he h eatIn? . carrymg a current for ItS operatIOn. WIre Ad n:mlstratIve '" housekeeping or over h ea d tions or functions that are Fnecessary Ito opera maintain contro Iafa" SItuation. or examp e, orrtputer program, housekeeping C for a . . bl es IveS settmg up constants an d vana . InVO used In . th e program. Synonymous WIt .h ta b e " "red tape. . housing A protective enclosure or case.
249
HP-IB Hewlett-Packard Interface Bus. An IEEE 488 standard bus that is parallel, multipart, 300 Kbs to 1 Mbs, 15 to 28 stations on up to 50 feet. Originally developed by Hewlett-Packard for laboratory instrumentation and later called "GPIB" for General Purpose Interface Bus. HRF Hazard Reduction Factor. The inverse of PFD (probability of failure on demand), which is the chance,that a control system will fail when in 'energized mode. Hand Station' Provides for a direct process control signal to the final element. HSE High-speed Ethernet (lOOMbits/sec). An industrial network protocol that is based on commercial Ethernet physical layer components. 'It uses TCP /IP and UDP Data/Session layers for communications at the controller level of a host, and it supports all functions of the Fieldbus HI laver. HSE field device A high-speed 'field device is a fieldbus device that is connected directly to a high-speed Ethernet (HSE) fieldbus. Typical HSE field devices are HSE linking devices, HSE field devices running function blocks (FBs), and host computers. HSE linking device An HSE linking device is a device that is used to interconnect HI fieldbus segments to high-speed Ethernet (HSE) in order to create a larger network. HSE switch An HSE switch is standard Ethernet equipment. It used to interconnect multiple high-speed Ethernet (HSE) devices such as HSE linking devices and HSE field devices in order to form a larger HSE network. HSM Hierarchal Storage Management. A digital memory and data backup system. HSV Hue-saturation-value (or brilliance or luminance). The color space that used in some graphic programs for screen displays. HSWA Hazardous and Solid Waste Amendments. Amendments passed in 1984 to the Resource Conservation and Recovery Act (U.5.), which establish regulations for land disposal and underground storage tanks for hazardous materials. HTML Hypertext markup language. A computer authoring language for publishing documents through the World W~e Web on the Internet by using both text and twodimensional graphics. HTML Hypertext Markup Language. A format used to create World Wide Web documents. HTTP Hypertext transport protocol. An Internet computer communication encoding
II
,
hub I hydraulic gauge
JI I ,i
standard for exchanging multimedia documents across the Web. hub In digital communications, a wiring concentrator or repeater that brings together connections from multiple network nodes in a star topology. hue The main attribute of a given color that distinguishes it from other colors. Huggenberger tensometer A magnifying extensometer that employs a compound lever system to intensify the changes taking place in a 10 to 20 mm gauge length by about 1,200 times. hum An 40desirable by-product in an alternating current (AC) power supply. human-factors engineering A branch of engineering in which the capabilities and limitations of human beings are integrated into design models ilf order to enhance the overall performance of a system that uses both humans and machines. humidification Artificially increasing the moisture content of a gas. humidistat An instrument for measuring and controlling relative humidity. PERFORATED COVER
SWITCH PUSH ROD
COTTON THREADS
TENSION SPRING
RANGE ADJUSTMENT
humidity, absolute The mass of water vapor that is present in a unit volume of air or other fluid. [ISA-37.1-1975 (R1982)] humidity, relative The ratio of the water vapor pressure actually present to the water vapor pressure required for saturation at a given temperature, expressed in percentage terms. [ISA-37.1-1975 (R1982)] humidity element The part of a hygrometer that senses the amount of water vapor in the atmosphere. humidity test A corrosion test for comparing the relative resistance of specimens to a highhumidity environment at constant temperature. hunting 1. A continuing cyclic motion that is caused by friction, in which the positioner attempts to find the set position. 2. An undesirable oscillation of appreciable magnitude
that continues long after external stimuli disappear. Note: In a linear system, hunting is evidence of the system operating at or near the stability limit. Nonlinearities may cause hunting of well-defined amplitude and frequency. [ANSI/ISA-51.1-1979 (R1993)] In data processing, the system usually contains a standard, a method for determining deviation from this standard, and a method for in.ouencing the system, such that the differ"ence between s~andard and the state of the system is brought to zero. In auroinatic control, hunting is generally caused by the gain or reset of the controller being set too high. See dither. HVLC high volume, low concentration. Usually refers to pollutant measurement for EPA (U.s.) regulations. hybrid circuit A miniaturized assembly that combines discrete components and/ or monolithic circuits mounted on an insulating substrate. hybrid computer 1. A computer for data processing that uses both analog representation and discrete representation of data. 2. A computing system that uses an analog computer and a digital computer working together. hybrid control system A control system that includes both "classic" distributed control and programmable control components for processes or operations that need the benefits of having each system connected onto the same network. hybrid T Series T and shunt T junctions that are located at the same point in a waveguide and are designed to restrict energy flow to specified channels. hydraulic Referring to any device, operation, or effect that uses the pressure or flow of oil, water, or any other low-viscosity liquid. hydraulic actuator A fluid device that converts the energy of an incompressible fluid into motion. [ANSI/ISA-75.05.01-2000] hydraulic circuit A fluid-flow circuit that operates somewhat like an electric circuit. hydraulic engineering A branch of civil engineering that deals with the design and construction of such structures as dams and other flood-control devices, sewers and sewage-disposal plants, water-dnven electric power stations, and water treatment and distribution systems. hydraulic fluid A light oil or other low-viscosity liquid that is used in a hydraulic circuit. hydraulic gauge A gauge that is designed for service at extremely high pressure.
250
....
hydraulic valve / hysteresis hydraulic valve A fluid-powered device that converts the energy of an incompressible fluid into motion. [ANSI/ISA-75.05.01-2000] See also actuator, hydraulic type. hydrocarbon A chemical compound of hydrogen and carbon. hydro cracker A chemical reactor in which large hydrocarbon molecules are fractured in the presence of hydrogen. hydroelectric plant An electric power-gener'ating station in which the power is produced by generators driven by hydraulic turbines. hydrogen damage Any of several forms of metal failure that are caused by dissolved hydrogen. These include blistering, internal void formation, and hydrogen-induced delayed cracking. hydrokineter A device for recirculating or initiating the flow of wateJ by using a jet of steam or water at a higher pressure than the water was initially flowing. hydrometer An instrument for directly indicating the density or specific gravity of a liquid. hydrophilic Water receptive. hydrophobic Water repellent. hydrophone A transducer that reacts to water-borne sound waves. hydropneumatic Of a device that is operated by both liquid and gas power. hydrostatic head The pressure that is created by a height of liquid above a given point. hydrostatic-head gauge A pressure gauge that is unique from others in the graduation of scale; usually expressed in feet. 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 1. Referring to a material that will readily absorb and retain moisture. [ISARP60.11-1991] 2. Having a tendency to absorb water. [ISA-71.04-1985] Having the ability to accelerate the condensation of water vapor. Dry material that has the ability to absorb water vapor from the surrounding atmosphere. hygrothermograph An instrument that records both temperature and humidity on the same chart. hypercard A programming environment that organizes information into what appears functionally to the user to be stacks of index
cards. These stacks can then be manipulated and resorted to create new applications. hypergolic Referring to spontaneous ignition upon contact. hyperlink In networked computers, a path between two documents that allows the user to point and click on specific words in one document and be moved to some other relevant document, wherever it physically is located on the network or Internet. Synonymous with hypertext. .. hypermedia A hypertext function that has been expanded so that documents contain links not only to other pieces of text but also to other forms of media: sounds, images, and video. Images themselves can be selected to link to sounds or documents, and vice versa. hypermodel A behavioral model of the analog-digital interface in a mixed-mode simulator. hypertext Basically the same as regular digital communication text-i.e., it can be stored, read, searched, or edited-but also containing connections (links) within that text to other documents. For example, while viewing a word or phrase in one document, the user can access and retrieve another document using that word or phrase. Synonymous with hyperlink. hypsometer An instrument that determines elevation above a reference plane (such as sea level) by measuring the boiling point of a liquid and then from that measurement finding atmospheric pressure. hysteresimeter A device for measuring a lagging effect related to physical change, such as the relationship between magnetizing force and magnetic induction. hysteresis 1. That property of an element such that the value of the output, for a given excursion of the input, depends upon the history of prior excursions and the direction of the current traverse. Note 1: Hysteresis is usually determined by subtracting the value of dead band from the maximum measured separation between upscale-going and downscalegoing indications of the measured variable (during a full-range traverse, lmless otherwise specified) afte~ transie~ts have deca~ed. Thi.s measurement IS sometimes called 'J'ysteresls error" or "hysteretic error." Note 2: Some reversal of output may be expected for any small reversal of input. This distinguishes hysteresis from dead band. [ANSI/ISA-51.11979 (R1993)] 2. The maximum difference in output value for any single input value during a calibration cycle, excluding errors caused by dead band. This difference is some-
251
I
hysteresis / hysteresis plus dead band times called "hysteretic error." [ANSI/ISA75.05.01-2000] 3. The maximum difference in output, at any measurand value within the specified range, when the value is approached first with an increasing and then with a decreasing measurand. Note: Hysteresis is expressed in percentage of full-scale output during anyone calibration cycle. Friction error is included with hysteresis unless dithering is specified. [ANSrj.rSA-37.1-1975 (R1982)] 4. A phenomenon demonstrated by materials that make their behavior a function of the history of the environment to which they have j;)een 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.
'.-
HYSTERETIC ERROR ONLY
DEAD BAND ONLY
HYSTERESIS (HYSTERETIC ERROR PLUS DEAD BAND)
Hysteresis hysteresis plus dead band The maximum difference for the same input between the upscz:le and downscale output values during a full-range traverse in each direction. This is the summation of hysteresis and dead band. [ANSI/ISA-75.05.01-2000]
252
~
....