M - P

manipulated variable 1. In a process that is desired to regulate some condition, a quantity or a condition that is altered by the control in order to initiate a change in the value of the regulated condition.

Manufacturing Automation Protocol (MAP) A specification for a suite of communication standards for use in manufacturing automation developed under the auspices of General Motors Corporation.

material balance 1. The procedure of accounting for the mass of material going into a process versus the mass leaving the process.

mathematical programming In operations research, a procedure for locating the maximum or minimum of a function subject to constraints. Refer also to "convex programming," "dynamic programming," "integer programming," "linear programming," "nonlinear programming," and "quadratic programming."

Maxwell The CGS unit of magnetic flux.

MByte 1,048,576(220)bytes.

mean time between failures (MTBF) The limit of the ratio of operating time of equipment to the number of observed failures as the number of failures approaches infinity. The total operating time divided by the quantity (n + 1), where n is the number of failures during the time considered.

mean time to failure (MTTF) The average or mean time between initial operation and the first occurrence of a failure or malfunction, as the number of measurements of such time on many pieces of identical equipment approaches infinity.

mechanical compliance Displacement of a mechanical element per unit force; it is the mechanical equivalent of capacitance in an electrical circuit.

mechanical hygrometer A hygrometer that uses an organic material, such as a bundle of human hair, to sense changes in humidity.

meter prover A device for checking the accuracy of a flow meter.

meter run A flowmeter installed and calibrated in a section of pipe having adequate upstream and downstream length to satisfy standards of flowmeter installation.

MeV Mega electron volts; a unit of energy equivalent to the kinetic energy of a single electron accelerated through an electric potential of 1 million volts.

mho A customary unit of conductance and admittance generally defined as the reciprocal of one ohm, or the conductance of an element whose resistance is one ohm; the equivalent SI unit Siemen is preferred.

microwave Electromagnetic radiation having a wavelength of 1 to 300 mm (frequency between infrared and radio waves).

microwave amplification by the stimulated emission of radiation (MASER) The microwave equivalent and predecessor of the laser. It produces coherent microwaves.

mile A non metric unit of length; a statute mile, used for distances over land, is defined as 5280 ft.; a nautical mile, used for distances along the surface of the oceans, is defined as one minute of arc measured along the equator, which equals 6080.27 fit or 1.1516 statute miles.

millimetre 1. A unit of length equal to 0.001 meter. 2. A millimeter of mercury, abbreviated mm Hg, is a unit of pressure equivalent to the pressure exerted by a column of pure liquid mercury one mm high at 0°C under a standard gravity of 980.665 cm/s2; it is roughly equivalent to 1/760th of standard atmospheric pressure.

modulating The actions to keep a quantity or quality in proper measure or proportion.

modulation 1. The process or the result of the process by which some characteristic of one wave is varied in accordance with some characteristic of another wave (AM, amplitude modulation; PM, phase modulation; FM, frequency modulation). 2. The action of a control valve to regulate fluid flow by varying the position of the closure component.

modulus of elasticity In any solid, the slope of the stress-strain curve within the elastic region; for most materials, the value is nearly constant up to some limiting value of stress known as the elastic limit; modulus of elasticity can be measured in tension, compression, torsion or shear; the tension modulus is often referred to as "Young's modulus. "

Monel High nickel, high copper alloys used for their corrosion resistant properties to certain conditions.

Monte Carlo method A trial and error method of repeated calculations to discover the best solution of a problem. Often used when a great number of variables are present, with interrelationships so extremely complex as to forestall straightforward analytical handling.

multiplexing The transmission of a number of different messages simultaneously over a single circuit.

NAK (negative acknowledgment) This code indicates that the last block transmitted was in error and that the receiver is expecting a re-transmission.

nano A prefix which means one billionth.

nappe A sheet of liquid passing through the notch and falling over the weir crest.

natural draft Convective flow of a gas as in a boiler, stack or cooling tower-due to differences in density. 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"; see also "frequency, resonant."

needle valve. Its essential design feature is a slender tapered rodlike control element which fits into a circular or conoidal seat. Operating the valve causes the rod to move into or out of the seat, gradually changing the effective cross sectional area of the gap between the rod and its seat.

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

neoprene A synthetic rubber which exhibits excellent resistance to weathering, ozone, flames, various chemicals and oils.

neper A unit of measure 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.

nephelometry The application of photometry to the measurement of the concentration of - very dilute suspensions.

net positive suction head The minimum difference between the static pressure at the inlet to a pump and vapour 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 In data processing, any system consisting of an interconnection of computers and peripherals. Information is transferred between the devices in the network. LAN (Local Area Network) is a system at one location linked by cables. WAN (Wide Area Network) is a widely dispersed system usually connected by telephone lines.

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

neutralise 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.

neutron A nuclear particle with a mass number of one and exhibiting zero (neutral) charge.

newton Metric unit for force.

Newtonian flow Fluid characteristics adhering to the linear relation between shear stress, viscosity and velocity distribution.

NFPA US National Fire Protection Association.

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

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

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 1550°C (2800°F); three standard alloy pairs are in common use Pt vs. Pt-10%Rh, Pt vs. Pt-13%Rh, and Pt-6%Rh vs. Pt 30%Rh.

noise 1. In process instrumentation, an unwanted component of signal or. See "interference, electromagnetic". 2. Any spurious variation in the electrical output not present in the input. 3. An unwanted component of a signal or variable which 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 tending to interfere with the normal operation of a device or system

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 quantisation Inherent noise that results from the quantisation process.

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

noise temperature At a pair of terminals and at a specified frequency, the temperature of a passive system exhibiting the same noise power per unit bandwidth as the actual terminals.

noninteracting control system A multi element control system designed to avoid disturbances to other controlled variables due to the process input adjustments which are made for the purpose of controlling a particular process variable.

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

nonlinear programming 1. In operations research a procedure for locating the maximum or minimum of a function of variables which are subject to constraints, when either the function or the constraints, or both, are nonlinear

nonlinear system Any system whose operation cannot be represented by a finite set of linear differential equations.

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

nonreturn to zero (NRZ) Coding of digital data for serial transmission or storage whereby a logic "1" is represented by one signal level and a logic "0" is represented by a different signal level.

normal capacity Normal capacity is 80 percent of design capacity

normality Concentration units 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.

normalise 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 standard form; synonymous with "standardise."

normally closed (NC) 1. A switch position where the usual arrangement of contacts permits the flow of electricity in the circuit. 2. In a solenoid valve, an arrangement whereby the disk or plug is seated when the solenoid is deenergised. 3. A field contact that is closed for a normal process condition and open when the process condition is abnormal. 4. A valve with means provided to move to and/or hold in its closed position without actuator energy supply. 5. Relay contacts that are closed when the coil is not energised. See also "field contact." normally closed (NC) valve A valve with means provided to move to and/or hold in its closed position.

NOVRAM Nonvolatile random access memory (nonvolatile semiconductor computer memory).

nozzle 1. A short flanged or welded neck connection on a drum or shell for the outlet or inlet of fluids; also a projecting spout through which a fluid flows. 2. A streamlined device for accelerating and directing fluid flow into a region of lower fluid pressure. 3. A particular type of restriction used in flow system to facilitate flow measurement by pressure drop across a restriction

nozzle/flapper A fundamental part of pneumatic signal processing and pneumatic control operations. Basically, the device converts a displacement of the flapper to a pressure signal

Oersted The CGS unit of magnetic field strength; the SI unit, ampere turn per meter, is preferred.

offset 1. A sustained deviation of the controlled variable from set point. This characteristic is inherent in proportional controllers that do not incorporate reset action. 2. Offset is caused by load changes. 3. The steady state deviation when the set point is fixed. NOTE: The offset resulting from a no load to a full load change (or other specified limits) is often called "droop" of load regulation." See also "deviation, steady state". 4. A constant and steady state of deviation of the measured variable from the set point.

ohms per volt A standard rating of instrument sensitivity determined by dividing the instrument's electrical resistance by its full scale voltage.

on-off control A simple form of control whereby the control variable is switched fully on or fully off in response to the process variable rising above the set point or falling below the set point respectively. Cycling always occurs with this form of control.

opacity The reciprocal of optical transmissivity.

open-loop control 1. A control system which does not take any account of the error between the desired and actual values of the controlled variables. 2. An operation in which computer evaluated control action is applied by an operator. 3. A system in which no comparison is made between the actual value and the desired value of a process variable.

optical time domain reflectometer A device that sends a very short pulse of light down a fiber optic communication system and measures the time history of the pulse reflection. The reflection indicates fiber dispersion and discontinuities in the fiber path, such as breaks and connectors. The time it takes for the light pulse to travel to and from the discontinuity indicates how far it is from the test set.

orifice 1. The opening from the whirling chamber of a mechanical ato miser or the mixing chamber of a steam ato miser through which the liquid fuel is discharged. 2. A calibrated opening in a plate, inserted in a gas stream for measuring velocity of flow. A specially designed orifice plate holding device. orifice flange taps The 1/2 in. or 3/4 in. pipe taps in the edge of an orifice flange union. orifice flange union Two unique flanges used to hold an orifice plate primary element with specific design dimensions established by the AGA (American Gas Association).

orifice meter A general term used to describe any recording differential pressure measuring instrument.

orifice plate A disc or platelike member, with a sharp edged hole in it, used in a pipe to measure flow or reduce static pressure.

orifice run The differential pressure producing arrangement consisting of selected pipe, orifice flange union and orifice plate. An orifice run has rigid specifications defined by the AGA (American Gas Association).

orifice type variable area flowmeter A flow measurement device consisting of a tube section containing an orifice and a guided conically tapered float that rides within the orifice; flow of a fluid through the meter positions the float in relation to flow rate, with float position being determined magnetically or by other indirect means.

OSI (open system interconnection.) OSI reference model A seven layered model of communications networks defined by ISO. The seven layers are:

Layer 7 Application: provides the interface for application to access the OSI environment.

Layer 6 APresentation: provides for data conversion to preserve the meaning of the data.

Layer 5 Session: provides user to user connections.

Layer 4 Transport: provides end to end reliability.

Layer 3 Network: provides routing of data through the network.

Layer 2 Data Link: provides link access control and reliability.

Layer l Physical: provides an interface to the physical medium.

Ounce A U.S. unit of weight; one ounce (avoirdupois) equals 1/16 pound, and is used for most commercial products; one ounce (troy) equals 1/12 pound, and is used for precious metals.

Oval gear flowmeter A type of positive displacement flowmeter that operates by trapping a precise volume of fluid between an oval, toothed rotor and the meter housing as the rotor revolves in mesh with a second rotor; volume flow of an incompressible fluid is indicated directly by determining rotor speed.

overrange In process instrumentation, of a system or element, any excess value of the input signal above its upper range value or below its lower range value

overrange limit The maximum input that can be applied to a device without causing damage or permanent change in perfommance.

override control 1. Generally, two control loops connected to a common final control element-one control loop being nommally in control with the second being switched in by some logic element when an abnormal condition occurs so that constant control is maintained. 2. A technique in which more than one controller manipulates a final control element. The technique is used when constraint control is important.

overshoot 1. The amount of output measured beyond the final steady output value, in response to a step change in the measurand. NOTE: (S) Expressed in percent of the equivalent step change in output. 2. A transient response to a step change in an input signal which exceeds the normal or expected steady state response. 3. The maximum difference between the transient response and the steady state response.

packed column A distillation column filled with packing (commonly Raschig rings) to mix the descending liquid with the ascending vapours. Packing is often used instead of trays in columns for certain applications (such as gas adsorption) or very low pressure drop systems.

packet access device (PAD) An interface between a terminal or computer and a packet switching network.

packet switching network System where messages are broken down into smaller units called ackets, each individually addressed, and are routed through the network.

packet switching system (PSS) In a wide area network, a method of sending data between computers.

packing A sealing system consisting of deformable material of one or more mating and deformable elements contained in a packing box which may have an adjustable compression means to obtain or maintain an effective pressure seal.

packing box The chamber, in the bonnet, surrounding the stem and containing packing and other stem sealing parts.

paddle wheel level detector A device for detecting the presence or absence of bulk solids at the device location; it consists of a motor that slowly rotates a paddle in the absence of material, and rotates itself against a momentary switch when material is at or above the paddle location.

PAM The process (or the results of the process) in which a series of pulses is generated having amplitudes proportional to the measured signal samples. PAM/FM Frequency modulation of a carrier by pulse amplitude modulated information. PAM/FM/FM Frequency modulation of a carrier by subcarriers that are frequency modulated by pulse amplitude modulated (PAM) information.

Pareto analysis Using a Pareto chart to identify which problems to focus upon, a cause and effect analysis can be conducted.

Pareto chart A display of the number of failures of components by part number in descending order of failure rate or number of failures observed. Data may also be shown taking into account the total cost of each failure.

parity A code that is used to uncover data errors by making the sum of the " 1 " bits in a data unit either an odd or even number. parity bit A binary digit appended to a group of bits to make the sum of all the bits always odd (odd parity) or always even (even parity); used to verify data storage and transmission.

parity check A check that tests whether the number of ones or zeroes in an array of binary digits is odd or even.

parse To break a command string into its elemental components for the purpose of interpretation.

Parshall flume A venturi type device for measuring flow in an open channel at flow rates up to 6 million m3/day; it consists of a converging upstream section, a downward sloping throat and an upward sloping discharge section, and may be made of any suitable structural material, usually concrete.

partial pressure The portion of total pressure in a closed system containing a gas mixture that is due to a single element or compound.

parts per million (ppm) Represents parts per million and should be given on a weight basis. The abbreviation shall be ppm (w/w). If inconvenient to present data on a weight bases (w/w), it may be given in a volume basis; (v/v) must be stated after the term ppm

pascal Metric unit for pressure or stress.

peer to peer protocol Communication protocol between peer entities.

Peltier effect The principle in solid state physics that forms the basis of thermocouples if two dissimilar metals are brought into electrical contact at one point, the difference in electrical potential at some other point depends on the temperature difference between the two points.

period 1. The time required for one complete cycle of oscillation. 2. Of a periodic function, the smallest increment of the independent variable that can be repeated to generate the function. 3. Of an undamped instrument, the time between two successive transits of the pointer through the rest position in the same direction following a step change in the measured quantity. periodic duty A type of intermittent duty involving regularly repeating load conditions. periodic function An oscillating quantity whose values repeatedly recur for equal increments of the independent variable.

peripheral 1. A supplementary piece of equipment that puts data into, or accepts data from the computer (printers, floppy disc memory devices, videocopiers). 2. Any device, distinct from the central processor, that can provide input or accept output from the computer.

permanent pressure drop The unrecoverable reduction in pressure that occurs when a fluid passes through a nozzle, orifice or other throttling device.

pH The symbol for the measurement of acidity or alkalinity. Solutions with a pH reading of less than 7 are acid; solutions with a pH reading of more than 7 are alkaline on the pH scale of 0 to 14, where the midpoint of 7 is neutral.

phase-locked loop (PLL) A feedback mechanism commonly used in receivers and signal generators that uses a phase detector to compare the output phase of a voltage-controlled oscillator to that of a reference signal.

phase margin The difference between 180° and the absolute value of the open loop phase angle for a stable feedback system at that frequency where the gain is unity.

phase modulation Modulation of a sinusoidal carrier wave in which the angle of the modulated wave differs from the angle of the carrier wave by an amount proportional to the instantaneous amplitude of the modulating wave.

phase shift 1. Of a transfer function, a change of phase angle with test frequency, as between points on a loop phase characteristic. 2. Of a signal a change of phase angle with transmission. 3. Difference between corresponding points on input and output signal wave shapes, disregarding any difference in magnitude. 4. The time difference between the input and output signal or between any two synchronised signals, of a control unit, system, or circuit, usually expressed in degrees or radians. 5. A change in phase angle between the sinusoidal input to an element and its resulting output.

phase shift keying (PSK) A form of PCM achieved by shifting the phase of the carrier; e.g., + 90 degrees to represent "ones" and "zeros."

phase velocity The velocity of an equiphase surface along the normal of a traveling single frequency plane wave.

phasor A rotating vector.

ph meter An instrument for electronically measuring electrode potential of an aqueous chemical solution and directly converting the reading to pH (a measure of hydrogen ion concentration, or degree of acidity).

phon A unit of loudness level equivalent to a unit pressure level in decibels of a 1000 Hz tone.

phosphorescence Emission of radiant energy-often in the visible light range following excitation due to absorption of shorter wavelength radiation; phosphorescent emission may persist for a long time after the exciting radiation stops.

phot The CGS unit of illuminance, which equals one lumen power cm2; the SI unit, lux, is preferred.

photocell A device that alters its electrical resistance in proportion to the amount of light that impinges on it.

PI control Proportional-plus-integral control, used in combination to eliminate offset. Also called "proportional-plus-reset control."

PID action A mode of controller action in which proportional integral, and derivative action are combined.

PID control Proportional-plus-integral-plus-derivative control, used in processes where the controlled variable is affected by long lag times.

piezoelectric effect The generation of an electric potential when pressure is applied to certain materials or conversely a change in shape when a voltage is applied to such materials. The changes are small, but piezoelectric devices can be used to precisely control small motions of optical components.

pig An in line scraper for removing scale and deposits from the inside surface of a pipeline; a holder containing brushes, blades, cutters, swabs, or a combination is forced through the pipe by fluid pressure.

pigtail A 270° or 360° loop in pipe or tubing to form a trap for vapour condensate. Used to prevent high temperature vapours from reaching the instrument. Used almost exclusively in static pressure measurement.

pile An assemblage of thermoelectric elements, dissimilar metal plates or fissile material components so arranged to produce electrical or thermal power as in a thermopile, storage battery or atomic reactor.

pilot circuit That portion of a control circuit or system which carries the control signal from the signal generating device to the control device.

pinch or clamp valve A valve consisting of a flexible elastomeric tubular member connected to two rigid flow path ends whereby modulation and/or shut off of flow is accomplished by squeezing the flexible member into eventual tight sealing contact.

pipe elbow meter A variable head meter used to measure flow around the bend in a pipe.

pipelining The process of increasing data processing speed by simultaneously executing a number of basic instructions.

piping and instrumentation drawing (P&ID) 1. Show the interconnection of process equipment and the instrumentation used to control the process. In the process industry, a standard set of symbols is used to prepare drawings of processes. The instrument symbols used in these drawings are generally based on Instrument Society of America (ISA) Standard S5. 1. 2. The primary schematic drawing used for laying out a process control installation.

Pirani gauge A pressure transducer used to measure very low gas pressure based on measurement of the resistance of a heated wire filament; resistance varies in accordance with thermal conduction of the gas, which in turn is related to gas pressure. Used primarily for pressures less than one atmosphere.

piston meter A type of fluid flowmeter; it is a variable area, constant head device in which the flow rate is indicated by a pointer attached to a piston, which in turn is positioned by the buoyant force of the fluid.

piston type variable area flowmeter Any of several flowmeter designs in which fluid passing through the meter exerts force on a piston such that the piston moves against a counterbalancing force to expose a portion of an exit orifice, the amount exposed being directly related to volume flow.

pitot tube 1. An instrument for measuring stagnation pressure of a flowing liquid; it consists of an open tube pointing upstream, into the flow of fluid, and connected to a pressure indicator or recorder. 2. An instrument which will register total pressure and static pressure in a gas stream, used to determine its velocity.

pitot venturi tube A combination of a venturi device and a pitot tube.

plug meter A device for measuring flow rate in which a tapered rod extends through an orifice; when the rod is positioned so that the effective area of the annulus is just sufficient to handle the fluid flow, the rate of flow is read directly from a scale.

plug valve 1. A valve with a closure member that may be cylindrical, conical or a spherical segment in shape. It is positioned, open to closed, with rotary motion. 2. A type of shutoff valve consisting of a tapered rod with a lateral hole through it. As the rod is rotated 90° about its longitudinal axis, the hole is first aligned with the direction of flow through the valve and then aligned crosswise, interrupting the flow.

poise The CGS unit of dynamic viscosity, which equals one dyne second per cm2; the centipoise (cP) is more commonly used.

Poiseuille flow Laminar flow of gases in long tubes at pressures and velocities such that the flow can be described by Poiseuille's equation.

polar diagram A diagram showing the relative effectiveness of an antenna system for either transmitting or receiving. Principally shows directional characteristics .

positioner A position controller, which is mechanically connected to a moving part of a final control element or its actuator, and automatically adjusts its output pressure to the actuator in order to maintain a desired position that bears a predetermined relationship to the input signal. The positioner can be used to modify the action of the valve (reversing positioner), extend the stroke/controller signal (split range positioner), increase the pressure to the valve actuator (amplifying positioner) or modify the control valve flow characteristic (characterised positioner).

positioner, amplifying A pneumatic positioner in which the input control signal is amplified to a proportionately higher pressure, needed to drive the actuator, e.g., 3-15 psig input/6-30 psig output.

positioner, characterised A positioner in which the valve position feedback is modified to produce a nonlinear response.

positioner, double-acting 1. A positioner is double acting if it has two outputs, one with "direct" action and the other with "reversed" action 2. A positioner with two outputs, suited to a double-acting actuator.

positioner, electro-pneumatic A positioner which converts an electronic control signal input to a pneumatic output.

positioner, reversing A positioner which converts the input control signal into an output which is directionally opposite to the input.

positioner, single-acting 1. A positioner is single acting if it has a single output. 2. A positioner with one output, suited to a single action spring opposed actuator.

positioner, split range A positioner which drives an actuator full stroke in proportion to only a part of the input signal range.

positive draft Pressure in a furnace, gas chamber or duct that is greater than ambient atmospheric pressure.

positive feedback 1. A closed loop in which any change is reinforced until a limit is eventually reached. 2. Returning part of an output signal and using it to increase the value of an input signal.

potential energy Energy related to the position or height above a place to which fluid could possibly flow or a solid could fall or flow.

power factor The ratio of total watts to the total root-mean-square (rms) volt-amperes.
NOTE: If the voltages have the same waveform as the corresponding currents, power factor becomes the same as phasor power factor. If the voltages and currents are sinusoidal and for polyphase circuits, for symmetrical sets, Fw= cos (Liz - ,8)

precession The change in orientation of a rapidly spinning body, such as a gyrowheel, that occurs when its axis of spin rotates about a line perpendicular to a plane defined by the original position of the axis of spin and the axis of torque for the moment producing the change in orientation.

precipitate To separate materials from a solution by the formation of insoluble matter by chemical reaction.

predictive control 1. A type of automatic control in which the current state of a process is evaluated in terms of a model of the process and controller actions modified to anticipate and avoid undesired excursions. 2. Self tuning. 3. Artificial intelligence.

pressure Measure of applied force compared with the area over which the force is exerted.

pressure, absolute 1. The pressure measured relative to zero pressure (vacuum). 2. The pressure above the absolute zero value of pressure that is theoretically obtained in empty space or at the absolute zero for temperature, as distinguished from gauge pressure.

pressure, design The pressure used in the design of a vessel or device for the purpose of determining the minimum permissible thickness or physical characteristics of the parts for a given maximum working pressure (MWP) at a given temperature.

pressure, differential 1. The difference in pressure between two points of measurement. 2. The difference in pressure between two points of a system.

pressure drop 1. The differential pressure in pascals at a maximum linear flow measured between points four pipe diameters upstream and four pipe diameters downstream from its ends, using a specified liquid, and using pipe size matching the fittings provided [RP3 1 .1]. 2. The difference in pressure between two points in a system, caused by resistance to flow.

pressure, gauge 1. Pressure measured relative to ambient pressure. 2. The amount by which the total absolute pressure exceeds the ambient atmospheric pressure.

pressure, leak The pressure at which some discernible leakage first occurs in a device.

pressure level In acoustic measurement, P = 1 log (Ps/Pr), where P is the pressure level in bels, Ps is the sound pressure, and Pr is a reference pressure, usually taken as 0.002 dyne/cm2.

pressure, maximum working (MWP) The maximum total pressure permissible in a device under any circumstances during operation, at a specified temperature. It is the highest pressure to which it will be subjected in the process. It is a designed safe limit for regular use. NOTE: MWP can be arrived at by two methods: a) designed-by adequate design analysis, with a safety factor; b) tested-by rupture testing of typical samples.

pressure rating The maximum allowable internal force per unit area of a pressure vessel, tank or piping system during normal operation.

pressure regulating valve A valve that can assume any position between fully open and fully closed, or that opens or remains closed against fluid pressure on a spring loaded valve element, to release internal pressure or hold it and allow it to build up, as desired.

pressure regulator An in line device that provides controlled venting from a high-pressure region to a lower pressure region of a closed compressed gas system to maintain a preset pressure value in the lower pressure region.

pressure relief device A mechanism that vents fluid from an internally pressurised system to counteract system overpressure; the mechanism may release all pressure and shut the system down (as does a rupture disc) or it may merely reduce the pressure in a controlled manner to return the system to a safe operating pressure (as does a spring loaded safety valve).

pressure, rupture The pressure, determined by test, at which a device will burst. NOTE: This is an alternate to the design procedure for establishing maximum working pressure (MWP). The rupture pressure test consists of causing the device to burst.

pressure, static The steady state pressure applied to a device; in the case of a differential pressure device, the process pressure applied equally to both connections.

process reaction method A method of determination of optimum controller settings when tuning a process control loop. The method is based on the reaction of the open loop to an imposed disturbance.

proof pressure The maximum pressure which may be applied to the sensing element of a transducer without changing the transducer performance beyond specified tolerances. NOTE: 1. In the case of transducers intended to measure a property of pressurised fluid, proof pressure is applied to the portion subject to the fluid. 2. Differential pressure transducer specifications should indicate whether the specified differential proof pressure is applicable at ambient or maximum specified reference pressure, or both, and whether a reverse differential proof pressure, at ambient or maximum specified reference pressure, or both, is additionally applicable.

propeller meter An instrument for measuring the quantity of fluid flowing past a given point; the flowing stream turns a propeller like device, and the number of revolutions are related directly to the volume of fluid passed.

proportional band 1. The change in input required to produce a full range change in output due to proportional control action. NOTE 1: It is reciprocally related to proportional gain. NOTE 2: It may be stated in input units or as a percent of the input span (usually the indicated or recorded input span). The preferred term is proportional gain. 2. The amount of deviation of the controlled variable from set point required to move the final control element through the full range (expressed in % of span). 3. An expression of gain of an instrument (the wider the band, the lower the gain).

proportional control A control mode in which there is a continual linear relationship between the deviation computer in the controller, the signal of the controller, and the position of the final control element.

proportional control action Corrective action which is proportional to the error, that is, the change of the manipulated variable is equal to the gain of the proportional controller multiplied by the error (the activating signal).

proportional control mode 1. A controller mode in which the controller output is directly proportional to the controlled variable error. 2. Produces an output signal proportional to the magnitude of the input signal. 3. In a control system proportional action produces a value correction proportional to the deviation of the controlled variable from set point.

proportional, integral and derivative 1. Three-mode controller. 2. Refers to a control method in which the controller output is proportional to the error, its time history, and the rate at which it is changing. The error is the difference between the observed and desired values of the variable that is under control action. 3. Proportional-plus-integral-plus-derivative control, used in processes where the controlled variable is affected by long lag times.

proportional-integral-derivative (PID) control A combination of proportional, integral and derivative control actions. Refers to a control method in which the controller output is proportional to the error, its time history, and the rate at which it is changing. The error is the difference between the observed and desired values of the variable that is under control action. Also called "three mode control.

proportional-plus-reset control A mode of control in which there is a continuous linear relation between the value of the controlled variable and the position of the final control element (proportional) plus an additional change in the position of the final control element based on both the amount and duration of the change in the controlled variable (reset). Same as "PI control."

pseudo random The property of satisfying one or more of the standard criteria for statistical randomness but being produced by a definite calculation process. Related to "uniformly distributed random numbers. "

pseudo random number sequence A sequence of numbers, determined by some defined arithmetic process, that is satisfactorily random for a given purpose, such as by satisfying one or more of the standard statistical tests for randomness. Such a sequence may approximate any one of several statistical distributions, such as uniform distribution or normal (Gaussian) distribution.

psychrometer A device consisting of two thermometers, one of which is covered with a water saturated wick, used for determining relative humidity; for a given set of wet bulb and dry bulb temperature readings, relative humidity is read from a chart. Also known as "wet and dry bulb thermometer."

purge meter A device designed to measure small flow rates of liquids and gases used for purging measurement piping.

purging 1. The addition of air or inert gas (such as nitrogen) into the enclosure around the electrical equipment at sufficient flow to remove any hazardous vapours present and sufficient pressure to prevent their re entry. 2. Elimination of an undesirable gas or material from an enclosure by means of displacing the undesirable material with an acceptable gas or material.

pycnometer A container of precisely known volume that is used to determine density of a liquid by weighting the filled container and dividing the weight by the known volume. Also spelled "pyknometer."

pyroelectric detectors Detectors of visible, infrared, and ultraviolet radiation which rely on the absorption of radiation by pyroelectric materials. Heating of such materials by the absorbed radiation produces electric charges on opposite sides of the crystal, which can be measured to determine changes in the amount of radiation incident on the detector. These detectors usually also exhibit piezoelectric properties and may require isolation from acoustic or acceleration phenomena.

pyrometer Any of a broad class of temperature measuring instruments or devices. Some typical pyrometers include thermocouples, radiation pyrometers, resistance pyrometers and thermistors, but usually not thermometers. It is a temperature transducer that measures temperatures by the EM radiation emitted by an object, which is a function of the temperature.