NA Numerical aperture.

N The light-gathering ability of a fiber. It defines the max­imum angle to the fiber axis at which light ,vill be accepted and propagated through that fiber. Also, numerical aperture

describes the angular spread of light from a central axis, as in light exiting fiber, being emitted from a source, or entering a detector.

nailing and blocking clip A special fastener for joining fiberboard sheet material to wood framing. [ISA-RP60.11-1991]

NAK Negative acknowledgment. This code indicates that the last block transmitted was in error and that the receiver is expecting a retransmission.

nameplate A plate attach~d to a control valve that bears the name of the manufacturer and

... a listing of the valve's specifications. [ANSI! ISA-75.05.01-2000]

NA mismatch loss In fiber optics, the loss of power at a joint. It occurs when the transmit­ting half has a numerical aperture (NA) that is greater than that of the receiving half, as when coupling light from source to fiber, from fiber to fiber, or from fiber to detector.

NAMUR Normenarbeitsgemeinschaft fUr MeG- und Regelungstechnik in der Chemis­chen Industrie. A committee founded in 1949 by several large chemical companies from Germany, The Netherlands, and Switzerland to standardize industrial practices in process measurement and control. A subcommittee published a set of guidelines for structured batch control in 1986, which were accepted throughout the European chemical industry with strong support worldwide. NAMUR looks at batch automation from the process activity viewpoint and views activities such as charge, mix, heat/ cool, and the like, as repeatable across many processes and indus­tries. It therefore presents a hierarchy of structures, terminology, and definitions for classifying production facilities, processing equipment, recipe structure, as well as batch operational sequences and phases.

NAND 1. A logical operator that has the fol­lowing property: if P is a statement, Q is a statement, R is a statement, and so on, then the NAND of P, Q, R, and so on is true if at least one stat2ment is false. Likewise, it is false if all statements are true. NAND is syn­onymous with "NOT-AND." 2. A logical negation of AND. NAND supplies a logic 0 when all inputs are at logic 1.

nano A prefix that means "one billionth. II

nanometer The unit in which wavelengths of light are expressed. One nanometer is one­billionth of one meter.

nanosecond One billionth of one second. nappe A sheet of liquid that passes through

the notch and falls over the weir crest. narrowband A signal whose bandwidth is

small with respect to the bandwidth of an instrument or receiver.

narrowband channels . Subvoice-grade communication cpannels that are character­ized by a speed range of 100 to 200 bps.

narrowband radiation thermometer A type of temperature-measuring instrument that responds accurately only over a given, rela­tively narrow band of wavelengths. Often, a band is chosen to meet a speCial requirement of the intended application.

National Electrical Code (NEC)/NFPA-70 A set of regulations that governs the construc­tion and installation of electrical wiring and apparatus, established by the National Fire Protection Association. It is widely used by state and local authorities within the United States.

National Institute for Certification in Engi­neering Technologies (NICET) Provides certification for qualification as an "Indus­trial Instrumentation Engineering Technol­ogy Technician."

natural circulation The circulation of water in a boiler. It is caused by differences in water density.

natural draft The convective flow of a gas, as in a boiler, stack, or cooling tower, as a result of differences in density. The warm gas in the chamber rises toward the outlet, drawing in colder, more dense gas through inlets near the bottom of the chamber.

natural frequency See frequency, undamped (frequency, natural). See also, frequency, natu­ral and frequency, resonant.

natural gas A mixture of gaseous hydrocar­bons trapped in rock formations below the earth's surface. The mixture consists chiefly of methane and ethane, with smaller amounts of other low-molecular-weight combustible gases. The mixture sometimes also includes noncombustible gases~such as nitrogen, carbon dioxide, helium, Fd H 2S (called "sour gas"). ;';

natural language Any naturally evolved human language. The term is usually used in comparison to a programming language.

natural radioactivity The spontaneous radio­active decay of a naturally occurring nuclide.

NC Normally closed; numeric control; net­work computer. Usually refers to a switch or

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1 NCAP / nephelometer

relay condition that allows flow until the switch or relay is energized. A technique in the operation of machine tools or similar equipment in which motion is generated in response to numerically ordered commands. A very inexpensive PC-like device that links to networks, contains a minimum of resident software, and has no hard drives, but instead downloads whatever applications it needs for the task at hand and only for as long it needs them. Contrasted with a PC, in which the applications it needs reside on it inter­nally.

NCAP Network-capable applications proces­sor. NCAP "translates" smart-transmitter data to/from the network by STIM an9

. TEDS, as defined by IEEE-PI451. See STIM, TEDS,IEEE-P14S1.

nOt The refractive index of a substance using sodium light as measured at a specific tem­perature.

near letter-quality With computer printers, a dot-matrix character formation that resem­bles the print of earlier cloth-ribbon type­writers.

I NEe National Electrical Code. Regulations

for constructing and installing electrical wir­ing and apparatuses. Established by the NFPA, NEC codes are suitable for manda­tory application by a wide range of state and local authorities. See hazardous area classifica­tions.

neck A reduced section of pipe or tubing that lies between larger-diameter sections or between a pipe and a chamber.

needle valve A type of metering valve with a needle-style plug that is used chiefly for pre­cisely controlling flow. Its essential design feature is a slender, tapered, rodlike control element that fits into a circular or conoidal seat. Operating the valve causes the rod to move into or out of the seat. This gradually changes the effective cross-sectional area of the gap between the rod and its seat. [ANSI! ISA-75.05.01-2000]

negative feedback Returning part of an out­put signal and using it to reduce the value of an input signal.

negative-going edge The edge of a pulse, going from a high to a low level.

negative letter spacing In the typographical composition of screen displays and printing, the subtraction of space between characters individually or en masse. See kerning.

negative temperature coefficient A decrease in resistance with an increase in temperature.

negatron A negatively charged beta particle.

...,

NEMA National Electrical Manufacturers Association. A trade association of electrical equipment manufacturers that develops many manufacturing specifications.

NEMA-7 A standard from the National Elec­trical Manufacturers Association that defines explosion-proof enclosures for use in loca­tions classified as Class I, Groups A, B, C, or D, as specified in the National Electrical Code.

NEMA-12 A.standard from the ational Electrical Manuf~ct~rers Association that defines enclosures th.at provide protection against dirt, dust,' splashes by noncorrosive liquids and salt spray.

NEMA enclosure ratings A system of enclo­sure classes that specify the degree of protec­tion offered from the environment. Similar to IP ratings.

NEMA 1 - Dust-resistant general purpose NEMA 2 - Drip-tight NEMA 3 - Splash-proof (weather-resistant)

EMA 4 - High-pressure hose-proof NEMA 5 - Dust-tight

EMA 6 - Submersible NEMA 7 - Hazardous indoor (NEC Class I Groups

A, S,C, D) NEMA 8' Hazardous indoor / outdoor (NEC Class I

Groups A, B, C, D) NEMA 9 - Hazardous indoor/outdoor (NEC Class

II Groups E, F, G) NEMA 10 - MSHA 30 CFR part 18 1 EMAl1 - General purpose drip and corrosion­

resistant from liquids and gases NEMA 12 - General purpose mdoor use drip-dust­

rust-resistant NEMA 13 - General purpose resistant to dust/rust

and spraying of water / oil/non-corrosive liq­uids.

IEC enclosure classes equivalent to EMA ratings: IPI0 = NEMA 1, IPn = NEMA 2, IP14/54 = NEMA 3/13, IP52 =NEMA 5/12, IP56 + NEMA 4, IP67 = NEMA 7

NEMA standard Consensus standards for electrical equipment and approved by the majority of the members of the ational Electrical Manufacturers Association.

neopheloscope An apparatus for making clouds in the laboratory by expanding moist air or by condensing water vapor.

neoprene A synthetic rubber that is made by polymerizing chloroprene (2-chlorobutadi­ene-I,3). Its color varies from amber to silver to cream. It exhibits excellentfesistance to weathering, ozone, flames, vilrious chemi­cals, and oils.

neper A unit of measure that is determined by taking the natural logarithm of the scalar ratio of two voltages or two currents.

nephelometer A general terms for instru­ments that measure the degree of cloudiness or turbidity.

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nephelometry / neural computing

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local area network or LAN is a system, linked by cables, that occupies one location. A wide area network (WAN) is a widely dis­persed system that is usually connected by telephone lines. 4. In an electric or hydraulic circuit, any combination of circuit elements. 5. An arrangement of nodes and intercon­necting branches. [ISA-TR50.02, Part 9-2000]

network analyzer 1. An instrument that com­bines the fu.nctions of a signal generator and spectrum analyzer NGtv\rork analyzers are used to characterize radio frequency, micro­wave components, and equipment in terms of power level, transmission characteristics, and impedance as a function of frequency. 2. An apparatus that contains numerous elec­tric-circuit elements, which can be readily combined to form models of electric net­works.

network architecture A set of design princi­ples that is used as a basis for designing and implementing a network. Such principles include the organization of functions and the description of data formats and procedures.

network layer Layer 3 in the OSI model. The logical network entity that services the trans­port layer. The network is responsible for ensuring that the data passed to it from the transport layer is routed and delivered through the network.

network management (nm) 1. Network man­agement (NM) permits network manager (NMgr) entities in Fieldbus Foundation to conduct management operations over the network by using network management agents (NMAs). Each NMA is responsible for managing the communications within a device. The NMgr and NMA communicate by using the Fieldbus messaging specifica­tion (FMS) and virtual communications rela­tionship (VCR). 2. The facility by which network communication and devices are monitored and controlled.

network structure A type of alloy micro­structure in which one phase occurs predom­inantly at grain boundaries, enveloping the grains of a second phase.

network topology The physical and logical relationship of nodes in a network. TJ1e sche­matic arrangement of links and nod}s of a network. Networks typically follmJ·the star, ring, tree, or bus topology, or some combina­tion of them.

neural computing Computing that adapts to repeated examples in order to recognize pat­terns. Neural computing is not based on algorithms. It learns mathematical mapping between repeatable cause-and-effect through

nephelometry The applica tion of photometry to the measurement of the concentration of very dilute suspensions.

nephoscope An instrument for determining the direction in which clouds move.

NEPSI National Supervision and Inspection Center for Explosion Protection and Safety of Instrumentation. A Chinese certification and testing laboratory for testing the equipment of different vendors to some common stan­dard.

nest 1. To embed a subroutine or block of data into a larger routine or block of data. 2. To ev,duate an 11th degree polynomial by a particular algorithm that uses (/1 - 1) multiply operations and (11 - 1) addition operations in successIOn.

nested DO loop A DO loop in FORTRA\J that is included within the range of another DO loop.

nesting To embed a programming subroutine or data block in a larger routine or data block. The organization of data in hierarchi­cal structures so repetitive elements can be stored and processed for greater efficiency. As a result, identical elements need to be rep­resented only once in the database.

NetBEUI NetBIOS Enhanced User Interface. The primary network protocol used by Win­dows for Workgroups; supported in all of Microsoft's network products.

NetBIOS Network Basic Input/Output Sys­tem. A high-level session-layer interface that is used by applications as a communications interface to PC-DOS applications.

net fan requirements The calculated operat­ing conditions for a fan, excluding toler­ances.

NetPC Network Pc. More like a compact PC than a network computer (NC), the network PC contains minimum resident software and no hard drive and can download whatever applications it needs for the task at hand only for as long as it needs it.

net positive suction head The minimum dif­ference between the static pressure at the inlet to a pump and the vapor pressure of the liquid being pumped. Below that pressure, fluid is not forced far enough into the pump inlet to be acted upon by the impeller.

network 1. All of the media, connectors, and associated communication elements by which a given set of communicating devices are interconnected. [ANSI/ISA-50.02, Part 2­1992] 2. In data processing, any system that consists of an interconnection of computers and peripherals. Information is transferred between the devices in the network. 3. A

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neural network / NIST

pattern recognition. It cannot handle con­straints and cannot optimize.

neural network An information-processing device that utilizes a very large number of simple modules and in which information is stored by components that at the same time effect connections between these modules. Neural networks have been widely used for pattern-recognition problems. To apply a neural network to a particular problem the user must be trained to adjust the weights of the network.

Network output

Output layer

Input layer

1� Network inputs

Feedjorwnrd Neural Network Architecture

neural network computer (NNC) A com­puter that has no CPU and no memory. It has a learning algorithm and is not necessarily a state machine. Each element (a neuron) is a system that has a transfer function in which a filter is used that assigns a \veight for each input.

neutral atmosphere An atmosphere that tends neither to oxidize nor to reduce immersed materials.

neutral density filter A filter that has uni­form transmission throughout the part of the spectrum where it is used.

neutral filter A light-betlm filter that exhibits constant transmittance at all wavelengths within a specified range.

neutralize To make a solution neutral (nei­ther acidic nor basic, pH of 7) by adding a base to an acidic solution or an acid to a basic solution.

neutral point A point on the titration curve where the hydrogen ion concentration equals the hydroxyl ion concentration.

neutral zone See zone, neutral. neutronA nuclear particle that has a mass

number of one and exhibits zero (neutral) charge.

newton A metric unit for force. Newtonian flow Fluid characteristics that

adhere to the linear relation between shear stress, viscosity, a~d velocity distribution.

i� en en w 0:: f­en 0::

~ :r: en

o RATE OF SHEAR _

Newtonian and Non-Newtonian Fluids

nexus The point in a computer system at which interconnections occur.

NFS Network file server. TCP/IP extension that allows files on remote nodes of a net­work to appear to be locally connected.

nibble A word with four bits, or one half a byte.

NiCad Nickel cadmium. Usually used as plating on brass screws.

nicrosil/nisil A nickel chrome/nickel silicon thermal alloy that is used to measure high temperatures.

nine-light indicator A remote indicator that is used in conjunction with a contact ane­mometer and a wind vane. It consists of a center lamp that is surrounded by eight lamps, equally spaced apart and labeled to indicate compass points. Wind speed is indi­cated by the number of flashes that the center lamp makes in a certain time interval. Wind direction is indicated by the position of an illuminated lamp in the outer.fing.

NIOSH National Institute for{Occupational Safety and Health. An independent U.s. fed­eral agency that is charged with performing research on occupational disease and injury.

nipple A short piece of pipe or tube, usually one that has an external thread at each end.

NIST National Institute of Science and Technology. Formerly the NBS, the NIST is a

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,l"

I� I.

u.s. government agency that provides stan­dard reference materials and calibration ser­vices. NIST-certified instruments are calibrated at NIST, whereas NIST-traceable instruments are factory calibrated against NIST-certified standards.

NO Normally open. Usually refers to a switch or relay condition that does not allow flow unless energized.

noble metal thermocouple A thermocouple whose elements are made of platinum (Pt) or platinum-rhodium (Pt-Rh alloys) and that resist oxidation and corrosion at tempera­tures up to about 1,550°C (2,800°F). Three standard alloy pairs are in common use: Pt vs.Pt-4k)%Rh, Pt vs. Pt-13%Rh, and Pt­6%Rh vs. Pt-30%Rh.

node 1. The end point of a branch in a network or a point at which one or more branches meet. Note: This definition is taken from the fEe Multilingual Dictionary of Electricity. [ANSI/ISA-50.02, Part 2-1992]2. In data pro­cessing, one component of a computer net­work where interconnections occur. 3. Points or lines where very little motion takes place during a condition of resonance.

nodular iron See ductile iron. noise 1. In process instrumentation, an

unwanted component of a signal or variable. Note: It may be expressed in units of the out­put or in percentage of output span. See interference, electromagnetic. [ANSI/ISA-51.1­1979 (R1993)]2. Any spurious variation in the electrical output that is not present in the input. Noise is defined quantitatively in terms of an equivalent parasitic transient resistance that appears between the wiper and the resistance element while the input shaft is being moved. The equivalent noise resistance is established independently of the functional characteristics, in the noise test circuit. The wiper must be excited by a speci­fied DC constant current source. The measur­ing system of the noise test circuit output is an oscilloscope that has a defined frequency bandwidth or time constant. The magnitude of the equivalent noise resistance is mea­sured as ohms variation, while the input shaft is moved at a specified speed and observed as peak-to-peak deflection on the oscilloscope. Note: Noise may be character­ized as generally reproducible, exhibited as a local nonlinearity, or may be the classical sporadic type. Manufacturing cleanliness and improved quality control on processing may significantly reduce noise problems. [ISA-37.12-1982 (R1995)] 3. An unwanted component of a signal or variable that

NUl NOM

obscures the information content. 4. Random variations of one or more characteristics of any entity, such as voltage, current, or data. 5. A random signal of known statistical prop­erties of amplitude, distribution, and spectral density. 6. Loosely, any disturbance that tends to interfere with the normal operation of a device or system. [ISA-RP55.1-1975 (R1983)] 7. Meaningless stray signals in a control system, similar to radio static. Some types of noise interfere with the correctness of an output signal.. .'

noise equivalent power The amount of opti­cal power that must be incident on a detector in order to produce an electrical signal that is equal to the rms level of noise inherent in the detector. Noise equivalent power is the mea­sure of the sensitivity of the sensor.

noise factor In an electronic circuit, the ratio of total noise in the output signal to the por­tion of noise in the input signal under the fol­lowing conditions: a selected input frequency and its corresponding output fre­quency, an input termination whose noise temperature is a standard 290 K at all fre­quencies, a linear system, and noise that is expressed as power per unit bandwidth.

noise figure A calculated or measured math­ematical figure that denotes the inherent noise in a unit, system, or link.

noise floor The lower sensitivity limit of an electronic measuring instrument, expressed in microvolts 00-6 V).

noise generator An instrument that gener­ates noise within a frequency band for the purpose of testing signal-to-noise ratios.

noise immunity 1. The maximum amount of electrical noise that a digital circuit can with­stand at its input without disturbing its per­formance. 2. A device's ability to discern valid data in the presence of noise.

noise quantization Inherent noise that results from the quantization process.

noise sources, control valve Control valve noise can be caused by the (a) turbulent flow of liquid, (b) aerodynamic flow, (c) liquid cavitation flow, or (d) mechanical vibration.

noise temperature At a pair of terminals and at a specified frequency, noise temperature is the temperature of a passive syst~'h that exhibits the same noise power pet;. unit band­width as the actual terminals.

NOM� Natural organic materials. NOM are monitored in water supplies, especially for their reaction to disinfectant, which can cause damaging by-products.

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nominal bandwidth / nonhazardous (unclassified) location�

I�

nominal bandwidth The difference between the upper and lower nominal cutoff frequen­cies of an acoustic, electric, or optical filter.

nominal pressure A numerical designation relating to pressure that is a convenient round number for reference purposes. The International Standards Organization (ISO) uses the term nominnl pressure in the same way that ANSI uses the term class to identify the pressure rating of a piping component. ISO uses the initials PN as an abbreviation for the term followed by a numerical value that designates the pressure rating. All equipment that has the same nominal diame­ter and nominal pressure rating must have the same mating dimensions, as appropriate for the relevant type of end connections. [ANSI/ISA-75.05.01-2000]

nominal size 1. A numerical designation of size that is common to all the components in a piping system other than the components that are designated by outside diameters or by thread size. Nominal size is a convenient round number for reference purposes and is only loosely related to manufacturing dimensions. ISO uses the initials ON as an abbreviation for the term followed by a numerical value designating size. All equip­ment that has the same nominal size and nominal pressure rating must have the same mating dimensions as appropriate for the rel­evant type of end connections. [ANSI/ISA­75.05.01-2000] 2. The standard dimension that is closest to the central value of a toler­anced dimension. 3. A size that is used for general identification.

nominal stress The stress that is calculated by dividing the nominal load by the nominal cross-sectional area. The effect of stress rais­ers is ignored but localized variations caused by general part design are taken into account.

nominal voltage The voltage that manufac­turers give as the recommended operating voltage of their gas-detection equipment. If a range (versus a specific voltage) is given, the nominal voltage must be considered as the midpoint of the range, unless otherwise specified. [ISA-92.02, 03, 04 and 06.01-1998]

nonautomatic Non-self-acting, that is, requir­ing personal intervention for control. As applied to an electric controller, nonauto­matic control does not necessarily imply a manual controller, but only that personal intervention is necessary. [ANSI/ISA­12.01.01-1999]

nonblackbody A term that is used to describe the thermal emittance of real

objects. Real objects emit less radiation than blackbodies at the same temperature. Black­bodies may reflect radiant energy from other sources and may have their emitted radia­tion modified by passing through the medium between the body and a tempera­ture-measuring instrument.

noncondensable gas The portion of a gas mixture (such as vapor from a chemical-pro­cessing unit or exhaust steam from a turbine) that is not easily cordensed by cooling. It normally consists of elements or compounds that have very low, often subzero, boiling . points and vapor pressu~s.

I1oncontact gauging A method for determin­ing physical dimensions without any actual contact occurring bet'ween the measuring device and the object.

noncontacting tachometer Any of several devices for measuring rotational speed with­out any physical contact occurring between a sensor and the rotating element. Examples

>.include stroboscopes or eddy-cur.rent tachometers.

non-countable fault A fault that occurs in parts of electrical apparatuses and in systems that do not comply with the constructional requirements of the relevant part of IEC 60079. [ANSI/ISA-12.02.01-1999 (IEC 60079­11 Mod)

noncritical dimension Any dimension that can be altered without affecting the basic function of a device.

nondestructive read out (NDRO) A method of reading from memory in which the stored value is left intact by the reading process. Plated wire and modern semiconductor ran­dom access memory (RAM) are examples of NORO memory.

nondestructive testing Any testing method that does not damage or destroy the sample. Usually, nondestructive testing consists of stimulating the sample with electricity, mag­netism, electromagnetic radiation, or ultra­sound and then measuring the sample's response.

nonhazardous area An area in which explo­sive gas / air mixtures are not expected to be present. As a result, special precautions for the construction and use of el~trical appara­tus are not required. ~~

nonhazardous location A location that is not designated as hazardous (classified). The term "unclassified location" is also used in the National Electric Code. [ANSI/ISA­RPI2.6-1995]

nonhazardous (unclassified) location 1. A location in which fire or explosion hazards

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are not expected to exist specifically as a result of the presence of flammable gases or vapors, flammable liquids, combustible dusts, or ignitable fibers or flyings. Such a location may also be referred to as a safe area. [ANSI/ISA-12.01.01-1999]2. A location that is not designated as hazardous (classified). [ANSI/ISA-RP12.6-1995]3. A location in which neither a hazardous atmosphere nor a hazardous dust layer is to be expected.

nonimpact printer An electronic device like a copier or laser or inkjet printer that creates images on a (usually paper) surface without contacting it.

nonincendive Equipment and wiring that in its normal operating condition is incapable of igniting a specific hazardous atmosphere or hazardous dust layer. Equipment and wir­ing that have exposed blanketed surface tem­peratures above 80 percent of the ignition temperature in degrees centigrade of the spe­cific hazardous dust layer should not be classed as 110nincendive. The blanketed sur­face temperature must be determined at the outside surface of the enclosure and beneath the surface of a dust accumulation of 0.2 inch or more.

nonincendive circuit A circuit, other than field wiring, in which any arc or thermal effect produced is incapable of igniting the flammable gas-, vapor-, or dust-air mixture under the equipment's intended operating conditions and specified test conditions. [ANSI/ISA-12.01.01-1999]

nonincendive component A component that has contacts for making or breaking an igni­tion-capable circuit. The component's con­tacting mechanism is also constructed so that it is incapable of igniting the specified explo­sive atmosphere. The housing of a nonincen­dive component is neither intended to (1) exclude the flammable atmosphere nor (2) contain an explosion. This type of protection is referred to as "nC" [ANSI/ISA-12.01.01­1999]

nonincendive equipment 1. Equipment that in its normal operating condition would not ignite a specific hazardous atmosphere in that atmosphere's most easily ignited concentra­tion. The electrical circuits may include slid­ing or make-and-break contacts that release insufficient energy to cause ignition. [ANSI/ ISA-S1.11979 (R1993); ISA-RP12.4-1996] Non­incendive equipment also includes wiring that under normal conditions cannot release sufficient energy to ignite a specific hazardous atmospheric mixture by opening, shorting, or grounding. [ANSI/ISA-Sl.l-1979 (R1993)]

nonimpact printer / nonlocking

ote: Equipment that has exposed surface temperatures ab~)Ve 80 percent of the ignition temperature (in C) of the specified hazardous atmosphere is not classified as nonincendive. [ISA-RP12.4-1996]2. Equipment that has elec­trical!electronic circuitry and components that are incapable, under normal conditions, of igniting the flammable gas-, vapor-, or dust-air mixture as a result of arcing or ther­mal effec.t. Thi;; type of protection is referred to as "nA/'.."nC," or :'nR." [ANSI/ISA­12.01.01-1999]

nonincendive field wiring Wiring that enters or leaves the equipment endosure and, under the eqUipment's normal operat­ing conditions, is incapable, because of arc­ing or thermal effects, of igniting a specified flammable gas, vapor-in-air mixture, or com­bustible dust-in-air mixture. Normal opera­tion includes opening, shorting, or grounding the field wiring. [ANSI/ISA­12.12.01-2000; ANSI/ISA-12.01.01-1999]

noninteracting control system A multi-ele­ment control system that is designed to avoid the disturbances to other controlled variables that are caused by the process input adjustments that are made when con­trolling a particular process variable. See con­trol system, noninteracting.

nonlinear distortion A departure from a desired linear relationship between the cor­responding input and output signals of a sys­tem.

nonlinear effects Optical interactions that are proportional to the square or higher powers of electromagnetic field intensities. Nonlin­ear effects generate harmonics of optical fre­quencies and sum-and-difference frequencies when two lightwaves are mixed.

nonlinearity A type of error in an FM system, in which the input to a device does not relate to the output in a linear way. See linearity.

nonlinear optimization See nonlinear pro­gramming.

nonlinear programming 1. In operations research, a procedure for locating the maxi­mum or minimum of a function of variables that are subject to constraints. Either the function, the constraints, or both, are nonlin­ear. Contrast with convex programmiAg and dynamic programming. 2. Synonymo.us with "nonlinear optimization."

nonlinear system Any system whose opera­tion cannot be represented by a finite set of linear differen tial equa tions.

nonlocking Pertaining to code extension characters that change the interpretation of

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nonnuclear safety (NNS) / normally closed (NC)

II ~

one or a specified number of characters. Con­trast with locking.

nonnuclear safety (NNS) Instrumentation that is not included in nuclear-safety-related (equipment). [ISA-67.03-1982; ANSI/ISA­67.02.01-1999]

nonoperating conditions See environmental conditions

nonprocedural programming language A computer programming language that is used to express the parameters of a problem rather than the steps in its solution. An example would be report writer or sort specification languages.

nonprocessor request The system for accom­plishing data transfers between two devices without involving the CPU.

nonreclosing pressure-relief device A device for relieving internal pressure that remains open when it is actuated and must be replaced or reset before it can actuate again.

nonrepeatability See repeatability. nonreturn-to-zero (NRZ) In the coding of

digital data for serial transmission or storage, a method in which logic ONE is represented by one signal level and a logic ZERO is repre­sented by a different signal level.

nonscheduled maintenance 1. Unscheduled maintenance that is specifically intended to eliminate an existing fault. 2. An urgent need for repair or upkeep that was unpredicted or not previously planned and that mLlst there­fore be added to or substituted for previ­ously planned work.

non-sealed apparatus An apparatus that is designed and constructed so as to allow an external atmosphere to enter and exit during the expansion and contraction of the inter­nally contained fluid, during normal opera­tion. [ANSI/ISA-12.26.01-1998 (IEC 60079-6 Mod)]

nonswitched line A communications link that is permanently installed between two points. It is also called "leased line" or "pri­vate line." See leased line.

nontransferred arc In arc welding and cut­ting, an arc that is sustained between the electrode and a constricting nozzle rather than between the electrode and the work.

nontransparent mode The transmission of characters in a defined character format, such as ASCII or EBCDIC, in which all defined control characters and control char­acter sequences are recognized and treated as such.

nonvolatile memory Computer memory that retains data when power is removed.

NOR 1. A logic operator that has the following property: if P is a statement, Q is a statement, R is a statement, and so on, then the NOR of P, Q, R, etc. is true if all statements are false and false if at least one statement is true. "P NOR Q" is often represented by a combination of OR and NOT symbols. "P NOR Q" is also called "NEITHER P NOR Q." NOR is synony­mous with "NOT-OR"; it an inverted OR gate. 2. The logical negation of OR. Supplies' a logic 0 (zero) when any input is at logic 1 (one) .•.. ,

normal atmosphere The pressure that is exerted bv a vertIcal column of 76 cm of mer­cury of d~nsity 13.5951 gl cm3 at a place where the gravitational acceleration, g, is 980.665 cm/sec2

. One Atmosphere =1.01325 x 106 dyne/cm2 (exactly) or = 101.325 kPa (kilopascals exactly) =760.00 mm of Hg = 14.696 psia = 29.921 in. of Hg at O°C (32°F). All of the preceding units are referenced at O°C (32°F). [ANSI/ISA-92.04.01-1996]

normal capacity Normal capacity is 80 per­cent of design capacity. [ISA-RP74.01-1984]

normal condition A condition in which all means for protection against hazards are intact. [ANSI/ISA-82.02.01-1999 (IEC 61010­1 Mod)]

normal conditions Said of equipment that is under normal conditions when it conforms electrically and mechanically with its design specifications and is used within the limits specified by the manufacturer. [ANSI/ISA­12.01.01-1999]

normal (gaussian) distribution The standard expression of the frequency distribution for most common error data. This distribution is the one usually assumed in the absence of conflicting data.

normality A concentration of units that is defined as the number of gram-ions of replaceable hydrogen or hydroxyl groups per liter of solution. A shorter notation of gram equivalents per liter is frequently used.

normalize 1. In programming, to adjust the exponent and fraction of a floating-point quantity such that the fraction lies in a pre­scribed normal standard range. 2. In mathe­matical operations, to reduce a set of symbols or numbers to a normal or s.tindard form. Normalize is synonymous with "standard­ize./I 3. In the heat tre~ting of ferrous alloys, to heat them 50 to 100 F a::,ove the upper transformation temperature, then cool them in still air.

normally closed (NC) 1. Describes a magnet­ically operated switching device such as a contactor or relay that takes the closed (cur­

340

normally open (NO) I not-open position

rent-carrying) position when the operating magnet is deenergized. 2. Describes a control logic element that is true when its controlling element is false. The element may exist either as hardware or software. 3. A valve that has the means to move to and/or hold in its closed position without energy supply from an actuator. See fail-closed. [ANSI/ISA­75.05.01-2000]

normally open (NO) 1. Describes a magneti­cally opeJ;~ted switching device such as a contactor or relay that takes the open posi­tion when the operating magnet is deener­gized. In this position the device will not carry current. 2. Describes a control logic ele­ment that is true when its controlling ele­ment is true. The element may exist either as hardware or software. 3. A valve that can move to and / or hold wide-open position without ~nergy supply from the actuator. See fail-open. [ANSI/ISA-75.05.01-2000]

normal-mode interference See interference, normal-mode. It may be expressed as a dimen­sionless ratio, a scalar r:atio, or in decibels as 20 times the 10glO of that ratio. [ANSI/ISA­51.1-1979 (R1993)]

normal-mode rejection The ability of a cir­cuit to discriminate against a normal mode voltage. Note: normal-mode rejection may be expressed as a dimensionless ratio, a scalar ratio, or in decibels as 20 times the IOg10 of that ratio. [ANSI/ISA-51.1-1979 (R1993)]

normal-mode rejection ratio The ability of an instrument to reject interference, usually of the line frequency (50-60 Hz), across its input terminals (versus common mode). See CO/1l­

mon mode rejection ratio. normal-mode voltage An extraneous volt­

age that is induced across the circuit path (transverse mode voltage). See also voltage, normal mode.

normal operating conditions See operating conditions, normal.

normal operation An intrinsically safe appa­ratus or associated apparatus is in normal operation when it conforms electrically and mechanically to the design specification pro­duced by its manufacturer. Note: Normal operation includes the application Urn from unspecified apparatus. [ISA-12.02.01-1999 (lEC 60079-11 Mod)]

normal operational conditions Equipment is in normal operational conditions when it conforms electrically and mechanically to its design specifications and is used within the limits specified by the manufacturer. These conditions involve (1) supply voltage, cur­rent, and frequency; (2) environmental con­

ditions (including process interface); (3) all tool-removable parts in place (e.g., covers); (4) all operator-accessible adjustments at their most unfavorable settings; and (5) opening, shorting, or grounding of nonincen­dive field wiring. [ANSI/ISA-12.12.01-2000]

normal service, motors Normal service for motors includes continuously operating at nameplate ratings, including starting condi­tions. [ANSI/ISA-12.16.01-1998 (IEC 60079-7 Mod)]

Nor~al Thermome'tricScale The first inter­national standard t~mperaturescale, adopted in 1887. It was based on the funda­mental interval of 100

0

between the ice point of pure water and the condensing point of pure water Ir'apor.

normal use Equipment operation, including stand-bv, that conforms to the manufac­turer's instructions or is performed for the obvious intended purpose. Note: In most cases, "normal use" also implies normal con­dition because the instructions for use will warn against using the equipment when it is not in normal condition. [ANSI/ISA­82.02.01-1999 (IEC 61010-1 Mod)]

NOT Logic output B exists if, and only if, logic input A does not exist. [ANSI/ISA-5.2­1976 (1<1992)]

NOT-AND Same as NAND. not at intermediate position A position that Iis either above or below the specified inter­

mediate position. [ANSI/ISA-5.2-1976 (R1992)]

notation 1. The act, process, or method of representing facts or quantities by a system or set of marks, signs, figures, or characters. 2. A system of such symbols or abbreviations that is used to express technical facts or quantities, as in mathematical notation. 3. An annota tion.

notch 1. A V-shaped indentation in an edge or surface. 2. An indentation of any shape that acts as a severe stress raiser.

notch filter In digital signal processing (DSP), a filter that attenuates signals in a cer­tain frequency band and passes all others.

notching Cutting out various shapes from the edge of a metal strip, blank, or part.

notch width The horizontal distan<::(oetween the opposite sides of a weir notch.t

not-closed position A position that is more than 0 percent open. A device that is not closed mayor may not be open. [ANSI/ISA­5.2-1976 (R1992)]

NOT-IF-THEN Same as exclusive OR. not-open position A position that is less than

100 percent open. A device that is not open

341

NUT-Ul{ I nucleus

mayor may not be closed. [ANSI/lSA-5.2­1976 (R1992)]

NOT-OR Same as NOR. NOVRAM Nonvolatile random access mem­

ory. A type of nonvolatile semiconductor computer memory.

NOx Nitrogen oxides, which are a consider­ation in EPA (U.s.) regulations. Compounds that are formed by the oxidation of atmo­spheric nitrogen under high-energy condi­tions. Nitrogen oxides are released primarily

.from burning fossil fuels. nozzle 1. A short flanged or welded-neck

connection on a drum or shell that allows fluids to enter or exit. 2. A projecting spout through which a fluid flows. 3. A stream­lined device for accelerating and directing fluid flow into a region of lower fluid pres­sure. 4. A particular type of restriction that is used in a flow system to facilitate the mea­surement of flow by pressure drop across a restriction.

nozzle efficiency The efficiency of a nozzle in converting potential energy to kinetic. It is commonly expressed as the ratio of actual to ideal change in kinetic energy at a specific pressure ratio.

I nozzlelflapper A fundamental part of pneu­

matic signal processing and pneumatic con­trol operations. Basically, the device converts a displacement of the flapper into a pressure signal.

NPDES National Pollutant Discharge Elimi­nation System. A U.s. federal permissions­granting program for discharging effluent to surface waters, mandated under the Clean Water Act. Over fifteen thousand municipal and forty-eight thousand industrial facilities report monthly on the amount and content of discharge.

NRZ Non-return-to-zero code. High level = logic 1, Low level = logic O. See also Ilonre­tum-to-zero (NRZ).

NT New technology. This term is used to describe the Microsoft computing platform Windows NT.

NTC Negative Temperature Coefficient. An "NTC resistor (thermistor)" will make a large, abrupt change in resistance when an undercurrent or low temperature cools it below some specific point. It effectively "switches off," or in reality acts like a solid­state fuse.

n-type semiconductor A semiconductor material that has been doped with an ele­ment that has excess electrons. See p-type semiconductor.

nuclear emulsion A photographic emulsion that is specially designed to record the tracks of ionizing particles.

nuclear fluorescence thickness gauge A device for determining the weight of an applied coating by exciting the coated mate­rial with gamma rays and measuring the low-energy fluorescent radiation that results.

nuclear radiation The emission of charged and uncharged particles as well as of electro­magnetic radiation from atomic nuclei. [lSA­37.1-1975 (R1982)].

nuclear safety-related (NSR) 1. The activation or control of systems or components that are essential to the emergency shutdown of a reactor shutdown, to isolating containment, to cooling the reactor core, and to removing con­tainment and reactor heat. Also included are any actions that are otherwise essential in pre­venting or mitigating a significant release of radioactive material into the environment or that provide reasonable assurance that a nuclear power plant can operate without pos­ing health or safety risks. [lSA-67.03-1982] 2. That which is essential to the following: (1) providing emergency shutdown of a reactor, (2) providing isolation of containment, (3) providing cooling of the reactor core, (4) pro­viding for the removal of containment or reac­tor heat, (5) preventing or mitigating a significant release of radioactive material into the environment or providing reasonable assurance that a nuclear power plant can operate without undue risk to public health and safety, (6) providing for the maintenance of safe shutdown conditions. Nuclear safety­related is used in the relevant standard to meet the requirements of Title 10, Part 50, Code of Federal Regulatiolls (lOCFR50), Appen­dix A. [ANSI/lSA-67.02.01-1999; lSA­RP67.04.02-2000]

nucleonics Technology that focuses on atomic nuclei, such as nuclear reactors, parti­cle accelerators, radiation detectors, and radioisotope applications.

nucleus� 1. The positively charged core of an atom. It contains almost all of the mass of the atom but occupies only a small fraction of its volume. 2. A number of atoms or molecules that are bound together with 4ileratomic forces. These forces are sufficie,ntly strong to make a small particle of a new phase stable in a mass that otherwise consists of another phase. Creating a stable nucleus is the first step in phase transformation through a nucleation-and-growth process. 3. That por­tion of the control program that must always be present in main storage. Also, the main

342

nucleus counter I NyqUIst dIagram�

storage area that is used by the nucleus and other transient routines of a control program.

nucleus counter An instrument that mea­sures the number of condensation or ice nuclei in a sample volume of air.

nuclide A species of atom that is character­ized by a unique combination of the charge, the mass number, and the quantum state of its nucleus.

nude vacuum gauge A hot-filament ioniza- • tion gauge that is mounted entirely within· the vacuum system whose pressure is being measured.

null A condition, such as of balance, that results in a minimum absolute value of out­put. [ISA-37.l-l975 (R1982)]

null-balance recorder An instrument that records a measured value by means of a pen or printer that is attached to a motor-driven slide. The position of the slide is determined by cOH.tinuously balancing the current or voltage in the measuring circuit against the current or voltage from a sensing element.

null indicator An indicating device such as a galvanometer that is used to determine when voltage or current in a circuit is zero. A null indicator is used chiefly to balance bridge circuits. Also known as null detector.

null modem A communication device that interfaces between some local peripheral that normally requires that a modem and a com­puter be nearby. This peripheral drives a modem to interface to that communication device. An imitation modem in both direc­tions.

number 1. A mathematical entity that may indicate the quantity or amount of units. 2. Loosely, a numeral. 3. See binary IHll1lber and randol1l numbers.

number system 1. A systematic method for representing numerical quantities. Any quantity is represented as the sequence of coefficients of the successive powers of a par­ticular base with an appropriate point. Each succeeding coefficient from right to left is associated with and usually multiplies the next higher power of the base. 2. The follow­ing are the names of the number systems Wit. h b ases 2 h t roU?;ih 20

i2, binary 18, octal or H, quaterdenary octonaryI

13, ternary 19, nO\'enary 15, quindenary

!4, quaternary j10, decimal 16,sexadecimal-or hexadecimal

i5, quinary 111, undecimal 17, septendecimal

16, senary !12, duodecimal 18,octodenary

17, septenary 19, novemdenary113, terdenary 20, vicenary.

Also, 32, duosexadecimal (or duotricinary) and 60, sexagenary. The binary, octal, deci­mal, and sexadecimal systems are widely used in computers. See decilllalllllmber and binary number. Related to positiollalnotation and clarified by octal digit and binary digit.

numerical analysis The study of methods for obtaining useful quantitative solutions to mathematical problems, regardless of whether an analytic solution exists or not. The study of"the errors and bounds on errors in obtaining such solutions"

numerical aperture The sine of the half-angle over which an optical fiber or optical system can accept light rays, multiplied by the index of refraction of the medium that contains the rays.

nurri'erical control A control system for machine tools and some industrial processes. Numerical values that corre~;pond to the desired positions of tools or controls are used to control the operation automatically.

numerical keypad Typical of a computer keyboard, a set of 0 through 9 keys, separate from the main alphabetical keyboard, that are arranged like numerical keys on a 10-key adding machine.

numeric word A word that consists of digits and possibly space characters and special characters.

NURBS Nonuniform rational b-splines. S­splines that are used to ensure continuity among objects. A mathematical description of geometry that makes it possible to easily manipulate entities and surfaces.

nutating-disk flowmeter A type of posi­tive-displacement flowmeter in which the advancing volume of fluid causes a measur­ing disk to wobble (nutate). This passes a precise volume of fluid through the meter with each revolution of the disk.

nutation Rocking back and forth or periodi­cally repeating a circular, elliptical, conical, or spiral path. Usually involves relati\'ely small degrees of motion.

N-value The exponent in the following power function: V(D = KTN This is the cali­bration function for a ratio thermometer. The N-value and mean effective wavelength can be used l J express the opera ting c~facteris­tics of a given ratio thermometer. f;'

nylon A plastics material that is used to make filaments, fibers, fabric, sheet, and extru­sions. A generic name for a type of long-chain polymer in which there are recur­ring amide groups within the main chain.

Nyquist diagram A plot in the complex plane of the open-loop transfer function as

I I

343�

NyqUIst trequency I l'''lyqUIst rate

the complex frequency is varied along the Nyquist contour. It is used to determine the stability of a control system.

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Nyquist Diagram

Nyquist frequency One-half of the sampling� frequency in a sampled data system.�

• Nyquist rate In digital signal processing�

(DSP), the lowest sampling rate that is� needed in order to completely reconstruct a� signal without distorting caused by aliasing.� The rate is equal to twice the highest fre­�quency component of the signal.

344�