Q - T

quadrant edged orifice An orifice having a rounded contour at the inlet edge to yield more constant and predictable discharge coefficient at low flow velocity (Reynolds number less than 10,000).

quadratic programming In operations research, a particular case of nonlinear programming in which the function to be maxi mised or minimised and the constraints are quadratic functions of the controllable variables.

quantisation The subdivision of the range of values of a variable into a finite number of overlapping, and not necessarily equal, subranges or intervals, each of which is represented by an assigned value within the subrange. For example, a person's age is quantised for most purposes with the quantum of one year.

quantisation distortion Inherent distortion introduced when a range of values for a wave attribute is divided into a series of smaller subranges.

quantum noise Noise due to the discrete nature of light-i.e., its quantisation into photons.

quarter amplitude A process control tuning criterion where the amplitude of the deviation (error) of the controlled variable, following a disturbance, is cyclic so that the amplitude of each peak is one quarter of the previous peak.

queuing theory A form of probability theory useful in studying delays or line ups at servicing points.

Quevenne scale A specific gravity scale used in determining the density of milk; a difference of 1° Quevenne is equivalent to a difference of 0.001 in specific gravity, and therefore 20° Quevenne expresses a specific gravity of 1.020.

quick-opening valve Control valve with trim characteristic designed to produce large flow capacity with small amount of valve opening.

radian Metric unit for a plane angle.

radiance The rates of flow of radiant energy with respect to time. Expressed in watts.

Rankine An absolute temperature scale where the zero point is defined as absolute zero (the point where all spontaneous molecular motion ceases) and the scale divisions are equal to the scale divisions in the Celsius system; 0°F equals approximately 459.69°R.

rate action Another name for the derivative control mode.

rated capacity The manufacturers stated capacity rating for mechanical equipment, for instance, the maximum continuous capacity in pounds of steam per hour for which a boiler is designed.

rated flow 1. Design flow rate for a piping system or process vessel. 2. Normal operating flow rate for a fluid passing through a piping system.

rated horsepower The maximum or allowable power output of an engine, turbine or other prime mover under normal, continuous operating conditions.

rated load The maximum design load for a machine, structure or vehicle.

rated supply voltage The supply voltage, or range of voltages, for which the manufacturer has designed the equipment.

ratio controller 1. A controller that maintains a predetermined ratio between two or more variables. 2. Maintains the magnitude of a controlled variable at a fixed ratio to another variable.

ratio of specific heats Specific heat at constant pressure divided by specific heat at constant volume.

reactance A component in an electrical circuit which is due to the presence of capacitative or inductive elements and not resistive elements, and which opposes the flow of electric current.

reactance drop The voltage drop 90° out of phase with the current.

reboiler The heat exchanger at the bottom of a distillation column. The reboiler generates the vapours that ascend through the column from liquid which comes down the column.

receiver gauge A gauge, calibrated in engineering units, which receives the output of a pneumatic transmitter.

rectifying section The section of trays in a distillation column above the feed plate. In this section the vapour is enriched in the light components that are taken overhead.

recursion The property which allows a callable program to call itself.

redox potential The electrochemical potential prevailing in a chemical reaction involving an exchange of electrons (reduction oxidation potential).

reducing atmosphere An atmosphere which tends to: a) promote the removal of oxygen from a chemical compound; b) promote the reduction of immersed materials.

reduction 1. Gain of electrons by a constituent of a chemical reaction. 2. Removal of oxygen from a chemical compound.

redundancy check An automatic or programmed check based on the systematic insertion of components or characters used especially for checking purposes.

redundant Duplication or repetition of elements in electronic or mechanical equipment to provide alternative functional channels in case of failure.

Redwood scale A time based viscosity scale used predominantly in Great Britain; it is similar in concept to the Saybolt scale used in the United States.

reentrancy The property which allows a callable program to be called and executed before it has completed the execution from a previous call. The results of the previous call are not affected.

reflash (multiple input) 1. An auxiliary logic circuit that allows two or more abnormal process conditions to initiate or initiate the alarm state of one alarm point at any time. The alarm point cannot return to normal until all related process conditions return to normal. 2. One type of auxiliary output.

reflectance The fraction of incident light which is reflected by the surface.

reflux The recycle stream that is returned to the top of the column. This stream supplies a liquid flow for the rectifying section that enriches the vapour stream moving up the column. Material in the stream is condensate from the overhead condenser. Reflux closes the energy balance by removing heat introduced at the reboiler.

reflux ratio A quantity usually expressed as the ratio of the reflux flow to the distillate flow. The ratio is used primarily in column design.

refractive index The ratio of phase velocity of a wave in free space to phase velocity of the same wave in the specific medium.

refractometer An instrument for measuring the index of refraction of a transparent substance; measurement can be accomplished in any of several ways, including measuring the critical angle, measuring refraction produced by a prism, observing interference patterns in transmitted light, and measuring the substance's dielectric constant.

Regime I Control valve regime of noise generation in which flow is subsonic and the gas pressure is partially recovered or recompressed.

Regime II Control valve regime of noise generation in which sonic flow exists, with interaction between shock cells and with turbulent choked flow mixing. Pressure recovery is less as the limit of Regime II is approached.

Regime III Control valve regime of noise generation in which no isentropic pressure recovery takes place.

Regime IV Control valve regime of noise generation in which the shock cell structure diminishes as a "Mach disk" is formed.

Regime V Control valve regime of noise generation in which there is a constant acoustic efficiency.

relative flow coefficient The ratio of the flow coefficient (Cv) at a stated travel to the flow coefficient ( Cv) at rated travel.

relative gain An open loop gain determined with all other manipulated variables constant, divided by the same gain determined with all other controlled variables constant.

relative humidity 1. The ratio of the amount of water vapour contained in the air at a given temperature and pressure to the maximum amount it could contain at the same temperature and pressure under saturated conditions. 2. The ratio of the weight of water vapour present in a unit volume of gas to the maximum possible weight of water vapour in unit volume of the same gas at the same temperature and pressure.

reliability 1. The probability that a device will perform its objective adequately, for the period of time specified, under the operating conditions specified. 2. The probability that a component, piece of equipment or system will perform its intended function for a specified period of time, usually operating hours, without requiring corrective maintenance.

relief valve A device used to protect piping and components from overpressure.

relief valve (safety) An automatic pressure relieving device actuated by the pressure upstream of the valve and characterised by opening pop action with further increase in lift with an increase in pressure over popping pressure.

repeatability The ability of a transducer to reproduce output readings when the same measurand value is applied to it consecutively under the same conditions, and in the same direction. NOTE(S): Repeatability is expressed as the maximum difference between output readings; it is expressed as "within percent of full-scale output." Two calibration cycles are used to determine repeatability unless otherwise specified.

repeater 1. Device used to extend the range over which signals can be correctly transmitted and received for a given medium. 2. A device that amplifies or regenerates data signals in order to extend the distance between data stations.

reproducibility 1. In process instrumentation, the closeness of agreement among repeated measurements of the output for the same value of input made under the same operating conditions over a period of time, approaching from both directions. 2. The ability of an instrument to duplicate, with exactness, measurements of a given value. Usually expressed as a percent of span of the instrument.

resealing pressure The inlet pressure at which fluid no longer leaks past a relief valve after it is closed.

reset control action See "control action, integral (reset)."

reset rate See "integral action rate."

reset windup Saturation of the integral mode of a controller developing during times when control cannot be achieved, which causes the controlled variable to overshoot its set point when the obstacle to control is removed.

resonant frequency The wave frequency at which mechanical or electronic resonance is achieved. See also "frequency, resonant." resonant frequency amplification factor The ratio of the maximum sensitivity of a transducer at its lowest resonant frequency to its nominal sensitivity.

response 1. The change in output of a device in relation to a change of input. 2. Defined output for a given input under explicitly stated conditions.

response, dynamic The behavior of the output of a device as a function of the input, both with respect to time.

response, ramp The total (transient plus steady state) time response resulting from a sudden increase in the rate of change from zero to some finite value of the input stimulus.

response, step The total (transient plus steady state) time response resulting from a sudden change from one constant level of input to another.

response time 1. The length of time required for the output of a transducer to rise to a specified percentage of its final value as a result of a step change of measurand.

Reynolds number A dimensionless criterion of the nature of flow in pipes. It is proportional to the ratio of dynamic forces to viscous forces: the product of diameter, velocity and density, divided by absolute viscosity.

rheopectic substance A fluid whose apparent viscosity increases with time at any constant shear rate.

Roentgen A quantity of x ray or gamma ray radiation that produces, in air, ions carrying one electrostatic unit of electrical charge of either sign per 0.001293 gram OI alr.

roentgen equivalent man (rem) The unit of dose in radiation dosimetry; it equals the amount of radiation of any type that produces the same amount of biological damage in human beings as a dose of 1 roentgen of 200 kV x rays.

roentgen equivalent physical (rep) A unit of radiation equal to the amount of radiation of any type that results in energy absorption of 93 ergs/g in soft tissue.

Roentgen rays An alternative term for x rays.

room conditions Ambient environmental conditions, under which transducers must commonly operate, which have been established as follows:

a) temperature: 25 + 10°C (77 + 1 8°F)

b) relative humidity: 90 percent or less

c) barometric pressure: 26 to 32 inches Hg.

NOTE: Tolerances closer than shown above are frequently specified for transducer calibration and test environments.

root mean square value (rms) 1. A statistical averaging technique where all data are individually squared, the average of those squares taken and the square root of that average taken. 2. Equal to 0.707 times the peak.

root valve The first valve located in a sample line after it taps off the process. It is typically located in close proximity to the sample tap.

rotameter A variable area, constant head, indicating-type rate of flow volume meter in which fluid flows upward through a tapered tube, lifting a shaped plummet to a position where upward fluid force just balances the weight of the plummet.

rupture disc A diaphragm designed to burst at a predetermined pressure differential. Symbol rupture disc device A nonreclosing pressure relief device that relieves excessive static inlet pressure via a rupture disc.

Sabin A unit of measure for sound absorption equivalent to one square foot of a perfectly absorptive surface.

safe area 1. Nonhazardous (unclassified) location. 2. An area in which explosive gas/air mixture are not expected to be present so that special precautions for the construction and use of electrical apparatus are not required.

safety ground 1. A connection between metal structures, cabinets, cases, etc. which is required to prevent electrical shock hazard to personnel. 2. Safety ground is not a signal reference point.

safety relief valve An automatic pressure relieving device actuated by the pressure upstream of the valve and characterised by opening pop action with further increase in lift with an increase in pressure over popping pressure.

sample and hold A device which senses and stores the value of an analog signal units subsequently updated.

sampled data control That branch of automatic control theory concerned with the control of variables whose current values are not continuously available for comparison with the setpoint but instead are sampled only at given intervals.

sample vessel An integrally valved, portable sample container designed to obtain pressurised samples at process pressure.

sampling rate For a given measurement, the number of times that it is sampled per second in a time division multiplexed system. Typically, it is at least five times the highest data frequency of the measurement.

sampling theorem Nyquist's result that equispaced data, with two or more points per cycle of highest frequency, allow reconstruction of band limited functions; the theorem states: "If the rms spectrum/G(t), is identically zero at all frequencies above W cycles per second, then g(t) is uniquely determined by giving its ordinates at a series of points spaced l/2Wseconds apart, the series extending through the time domain.

saturation A device characteristic exhibited when a further change in the input signal produces no significant additional change in the output.

Saybolt color scale A standardised color scale used primarily in the petroleum and pharmaceutical industries to grade the yellowness of pale products; it is based on matching the color of a column of the sample liquid with one of a set of color controlled glass disks, as described in ASTM standard D156.

Saybolt Furol viscosimeter An instrument similar to a Saybolt Universal viscosimeter, but with a larger diameter tube for measuring the viscosity of very thick oils.

Saybolt Universal viscosimeter An instrument for determining viscosity by measuring the time it takes an oil or other fluid to flow through a calibrated tube.

scalar quantity Any quantity that can be described by magnitude alone, as opposed to a vector quantity which can only be described by both magnitude and direction.

schedule A term used to define the wall thickness of pipe (schedule 40, 80, 160 and others).

Scientific Apparatus Makers Association (SAMA) A former industrial association in the United States. Reorganised and is now called "Measurement Control & Automation Association (MCAA). " No longer writes or supports SAMA standards.

seal chambers Enlarged pipe sections in measurement impulse lines to provide a) a high area to volume displacement ratio to mini mise error from hydrostatic head difference when using large volume displacement measuring elements, and b) to prevent loss of seal fluid by displacement into the process. Also known as Seal Pots

sealed reference differential pressure transducer A transducer which measures the pressure difference between an unknown pressure and the pressure of a gas in an integral sealed reference chamber.

seal leg The piping from the instrument to the top elevation of the seal fluid in the impulse line. seal on disk A seal ring located in a groove in the disk circumference. The body is unlined in this case [S75.05].

seat The fixed area of a valve into which the moving part of a valve rests when the valve is closed to retain pressure and prevent flow.

segmented ball A closure member that is a segment of a spherical surface which may have one edge contoured to yield a desired flow characteristic.

selective ion electrode A type of oxidation reduction potential electrode that involves use of a metal-metal-salt combination as the measuring electrode, which makes the electrode particularly sensitive to solution activities of the anion in the metal salt.

separable flanges Nonintegral loose flanges held to the vessel/valve by mechanical means such as retaining rmgs or swaged rims.

set point 1. An input variable which sets the desired value of the controlled variable It is expressed in the same units as the controlled variable.

set point control A control technique in which the computer supplies as calculated set point to a conventional analog instrumentation control loop.

set pressure The inlet pressure at which a safety relief valve opens; usually a pressure established by specification or code.

shrinkage A decrease (shrinkage) in drum level due to a decrease in steam bubble volume. This condition is due to a decrease in load (steam flow), with a resulting increase in drum pressure and decrease in heat input.

shutoff head The pressure developed by a centrifugal or axial flow pump at its discharge when the discharge flow is zero.

siemens Metric unit of conductance.

sight glass A glass tube, or a glass faced section of a process line, used for sighting liquid levels or taking manometer readings.

signal common 1. The signal common shall refer to a point in the signal loop which may be connected to the corresponding points of other signal loops. It may or may not be connected to earth ground [S50.1]. 2. The reference point for all voltage signals in a system. Current flow into signal common is mini mised to prevent IR drops which induce inaccuracy in the signal common reference.

signal isolation Signal isolation refers to the absence of a connection between the signal loop and all other terminals and earth ground.

signature The unique vibration characteristics of a machine. This signature changes as physical conditions or force events change within the machine or on its support system.

signature analysis A process can be identified as having a particular signature when operating correctly. This can be noise spectrum or vibration spectrum. Signature analysis involves identifying departures from the reference signature and recognising the source of the departure.

sign bit A single bit, usually the most significant bit in a word, which is used to designate the algebraic sign of the information contained in the remainder of the word.

simmer Detectable leakage from a safety relief valve at a pressure below the popping pressure.

simple apparatus A device which will neither generate nor store more than 1.2 V, 0.1 A, 25 mW, or 20 ZJ. Examples are: switches, thermocouples, light emitting diodes, and resistance temperature devices (RTDs).

sine wave A signal varying with time which can be obtained through projection of a rotating vector of constant magnitude with constant angular velocity on a linear scale.

single element feedwater control A control system whereby one process variable, drum level, is used as the input to the control loop that regulates feedwater flow to the drum to maintain the drum level at set point.

sling psychrometer A device for determining relative humidity that consists of a wet and dry bulb thermometer mounted in a frame that can be whirled about, usually by means of a handle and short piece of chain or wire rope attached to the upper end of the frame.

slip A term commonly used to express leakage in positive displacement flowmeters.

snubber 1. A device which is used to damp the motion of the valve stem. This is usually accomplished by an oil filled cylinder/piston assembly. The valve stem is attached to the piston and the flow of hydraulic fluid from one side of the piston to the other is restricted. 2. A mechanical or hydraulic device for restraining motion. 3. A device installed between an instrument and the process used to protect the instrument from rapid pressure fluctuations.

soft water Water which contains little or no calcium or magnesium salts, or water from which scale forming impurities have been removed or reduced.

solar blind A detector which contains filters to block sunlight, so the detector becomes essentially "blind" to the sun.

solenoid A type of electromechanical operator in which reciprocal axial motion of a ferromagnetic core within an electromagnetic coil performs some mechanical function; common applications include opening or closing valves or electrical contacts.

solenoid valve A shutoff valve whose position is determined by whether or not electric current is flowing through a coil surrounding a moving iron valve stem.

sonic opacity A characteristic of a medium such as one containing a large quantity of particles or small bubbles that results in sound or ultrasound being reflected randomly from the discontinuities rather than being transmitted through the medium.

soot blower A mechanical device for discharging steam or air to clean heat absorbing surfaces.

sound pressure level (SPL) The intensity of a sound wave which, in decibels, equals 20 log (PslPr), where Ps is the pressure produced by the sound and Pr is a stated reference pressure.

sour crude Crude oil containing excessive amounts of sulfur, which liberate corrosive sulfur compounds during refining. Contrast with "sweet crude."

sour gas Natural gas that contains corrosive sulfur bearing compounds such as H2S or mercaptans.

span The algebraic difference between the upper and lower range values.

sparger Liquid distribution device consisting of a length of piping or tubing with holes at spaced intervals along the length; used in spray columns, liquid- vapour contactors, and spray dryers.

special protection A protection technique other than those that have been standardised. This type of protection is referred to by IEC as "Type s".

specific gravity (SG) The ratio of the density of a material to the density of the water at the same conditions. Specific gravity: Gf= liquid at flowing condition referred to water at 60°F; Gg= gas referred to air, both at STP.

specific gravity bottle A small flask used to determine density; its precise weight is determined when empty, when filled with a reference liquid such as water, and when filled with a liquid of unknown density.

specific gravity, gas The density of a gas compared to the density of air.

specific gravity, liquid The density of a liquid compared to the density of water.

specific heat (sp ht) 1. The quantity of heat, expressed in Btu, required to raise the temperature of 1 lb of a substance 1°F. 2. The ratio of the thermal capacity of a substance to that of water. The specific heat at constant pressure of a gas is designated cp. The specific heat at constant volume of a gas is designated cv. The ratio of the two (cp/cv), is called the ratio of specific heats, k.

specific humidity The weight of water vapour in a gas water vapour mixture per unit weight of dry gas.

spectrofluorometer An instrument for determining chemical concentration by fluorometric analysis using two monochromators-one to analyze the wavelength of strongest emission and the other to select the wavelength of best excitation in the sample.

spectrometer A spectroscope which includes an angular scale for measurement of the angular deviation and wavelengths of the components of the spectrum.

spectrophotometric titration Instrumented titration in which the end point is determined by measuring a change in absorbed radiation with a spectrophotometer.

spectroscopic analysis Identification of chemical elements by characteristic emission and absorption of light rays.

stability 1. The ability of a transducer to retain its performance characteristics for a relatively long period of time. 2. The state of a system if the magnitude of the response produced by an input variable, either constant or varied in time, is limited and related to the magnitude of the input variable.

stability of a linear system A linear system is stable if, having been displaced from its steady state by an external disturbance, it returns to that steady state when the disturbance has ceased.

standard atmospheric pressure A reference pressure approximately equal to the mean atmospheric pressure at sea level, because atmospheric pressure varies with elevation and is not constant with time, standard atmospheric pressure is defined arbitrarily as an absolute pressure of 14.695 psi, 30.0 in. of mercury or 760 mm Hg (using mercury of density 13.595 g/cm3).

standard cell A primary cell Whose voltage is accurately known and remains sufficiently constant for instrument calibration purposes; the Weston standard cell has a voltage of 1.018636 volts in 20°C.

standard condition 1. A temperature of 0°C and a pressure of 1 atmosphers (760 torr). Also known as "normal temperature and pressure (NTP)"; "standard temperature and pressure (STP)." 2. According to the American Gas Association (AGA), a temperature of 60°F (1 5-5/9°C) and a pressure of 30 inches of mercury (762 mm). 3. According to the Compressed Gas Institute (CGI), a temperature of 20° C (68°F) and a pressure of 1 atmosphere.

standard deviation The positive square root of the expected value ofthe square of the difference between a random variable and its mean.

standard electrode potential The reversible or equilibrium potential of an electrode in an environment where reactants and products are at unit activity.

standard noise temperature The standard reference temperature used for noise measurements, 290 K.

standard potential The potential of an electrode composed of a substance in its standard state, in equilibrium with ions in their standard states compared to a hydrogen electrode.

standard pressure 1. The arbitrarily selected atmospheric pressure of 1000 millibars to which adiabatic processes are referred for definitions of potential temperature, equivalent potential temperature, etc. 2. A pressure of 1 atmosphere (101.325 Newtons per square meter), to which measurements of quantities dependent on pressure, such as the volume of a gas, are often referred. Also known as "normal pressure."

standard volume The volume of 1 mole of a gas at a pressure of 1 atmosphere and a temperature of 0°C.

standing wave A wave in which, for any of the dependent wave functions, the ratio of its instantaneous value at one point on the wave to its instantaneous value at any other point does not vary with time.

standpipe A vertical tube filled with a liquid such as water.

static head liquid level meter A pressure sensing device, such as a gauge, so connected in the piping system that any dynamic pressures in the system cancel each other and only the pressure difference due to liquid head above the gauge position is registered.

static pressure 1. The pressure of a fluid that is independent of the kinetic energy of the fluid. 2. Pressure exerted by a gas at rest, or pressure measured when the relative velocity between a moving stream and a pressure measuring device is zero.

static RAM Random access memory which requires continuous power but does not need to be refreshed as with dynamic RAM.

static tube A device used to measure static pressure in a stream of fluid. Normally, a static tube consists of a perforated, tapered tube with a branch tube for connecting it to a manometer; a related device called a static pressure tube consists of a smooth tube with a rounded nose that has radial holes in the tube behind the nose.

stationary wave A standing wave in which the energy flux is zero at all points on the wave.

statistically significant number of readings A statistically significant number of readings is a sample whose statistics closely approximate the true statistics of the parameter under consideration. That is, increasing the number of readings or repeatedly performing the data collection procedure will not result in substantially different calculated statistical parameters such as the mean and deviation.

statistical process control (SPC) The use of statistical techniques as a means of controlling the quality of a product or process.

statistical quality control (SQC) Any method for controlling the attributes of a product or controlling the characteristics of a process that is based on statistical methods of inspection.

steady state A characteristic of a condition, such as value, rate, periodicity, or amplitude, exhibiting only negligible change over an arbitrary long period of time.

steady state deviation The system deviation after transients have expired.

steady state model A mathematical model that represents the process at equilibrium (infinite time) conditions.

steady state optimisation A method of optimising some criterion function of a process usually using a steady state model of the process. Linear programming is frequently the optimisation method used and a function approximating the profit of the process is a typical optimising criterion.

steam attemperation Reducing the temperature of superheated steam by injecting water into the flow or passing the steam through a submerged pipe.

steam tracing An arrangement for heating a process line or instrument air line to keep liquids from freezing or condensing-often, a piece of pipe or tubing carrying live steam is simply run alongside or coiled around the line to be heated.

steam trap A device that automatically collects condensate in a steam line and drains it away.

step response Of a system or a component, the time required for an output to go through a specified percentage of the total excursion either before, or (in the absence of overshoot) as a result of a step change to the input NOTE: This is usually stated for 90, 95, or 99 percent change.

stiffness In process instrumentation, the ratio of change of force (or torque) to the resulting change in deflection of a spring like element. NOTE: Stiffness is the opposite of compliance [S51.1].

stilling basin An area ahead of the weir plate large enough to pond the liquid so that it approaches the weir plate at low velocity, also called "weir pond."

stochastic Pertaining to direct solution by trial and error, usually without a step by step approach, and involving analysis and evaluation of progress made, as in a heuristic approach to trial and error methods. In a stochastic approach to a problem solution, intuitive conjecture or speculation is used to select a possible solution, which is then tested against known evidence, observations or measurements. Intervening or intermediate steps toward a solution are omitted. Contrast with "algorithmic" and "heuristic. "

stoichiometric conditions In chemical reactions, the point at which equilibrium is reached, as calculated from the atomic weights of the elements taking part in the reaction; stoichiometric equilibrium is rarely achieved in real chemical systems but, rather, empirically reproducible equivalence points are used to closely approximate stoichiometric conditions.

stoke A unit of kinematic viscosity (dynamic viscosity divided by sample density); the centistoke is more commonly used.

straightening vanes Horizontal vanes inside a fluid conduit or pipe to reduce turbulent flow ahead of a flow meter.

stress corrosion cracking Deep cracking in a metal part due to the synergistic action of tensile stress and a corrosive environment, causing failure in less time than could be predicted by simply adding the effects of stress and the corrosive environment together.

stripper A distillation column that has no rectifying section. In such a column, the feed enters at the top, and there is no other reflux.

stripping section That section of a distillation column below the feed. This section strips the light components from the liquid moving down the column.

supercompressibility The extent to which behavior of a gas departs from Boyle's law.

superheat To raise the temperature of steam above its saturation temperature. The temperature in excess of its saturation temperature.

superheated steam Steam at a higher temperature than its saturation temperature.

superheater A nest of tubes in the upper part of a steam boiler whose function is to raise the steam temperature above saturation temperature.

supervisory control A term used to imply that a controller output or computer program output is used as an input to other controllers.

supervisory control and data acquisition (SCADA) Operator interface and monitoring of (usually remote) control devices by computer.

suppressed range A suppressed range is an instrument range which does not include zero. The degree of suppression is expressed by the ratio of the value at the lower end of the scale to the span.

suppressed weir A rectangular weir in which the width of the approach channel is equal to the crest width, i.e., there are no end contractions.

suppressed zero instrument Any indicating or recording instrument whose zero (no load) indicator position is offscale, below the lower limit of travel for the pointer or marking device.

susceptibility, electric A dimensionless parameter measuring the ease of polarisation of a dielectric. Also known as "dielectric susceptibility."

susceptibility, magnetic The ratio of the magnetisation of a material to the magnetic field strength. Also known as "susceptibility."

sweet crude Crude petroleum containing very little sulfur.

sweet gas Natural gas containing no hydrogen sulfide or mercaptans.

swell 1. An increase (swell) in drum level due to an increase in steam bubble volume. This condition is due to an increase in load (steam flow), with a resulting decrease in drum pressure and an increase in heat input. Swelling also occurs during a cold start up as the specific volume of the water increases. 2. The sudden increase in the volume of steam in the water steam mixture below the water level.

synergism An action where the total effect of two components or agents is greater than the individual effects of the components when simply added together.

syntax In data processing, grammatical rules for software programming that specify how instructions can be written.

Systeme Internationale d'Unites (SI) The current International System of Units.

system, linear One of which the time response to several simultaneous inputs is the sum of their independent time responses. It is represented by a linear differential equation and has a transfer function which is constant for any value of input within a specified range. A system not meeting these conditions is described as nonlinear

tapered tube rotameter A type of variable area flowmeter in which a float that has greater density than the fluid rides inside a tapered tube in such a manner that fluid flowing upward through the tapered section carries the float with it until the upward force exerted by the flowing fluid just balances the downward force due to float weight, as the float rides upward, the annular area around it becomes larger and force on the float decreases; if the tube is made of glass, it can be graduated so that flow rate is read directly by observing float position; otherwise flow rate must be determined from an indirect indication of float position.

tape type liquid level gauge A liquid level gauge consisting of a tape wound around a drum which is attached to a pointer or other level indicator, with one end of the tape attached to a float and the other counterweighted to keep the tape taut.

tare weight In any weighing operation, the residual weight of any containers, scale components or residue that is included in total indicated weight and must be subtracted to determine weight of the live load.

target flowmeter A device for measuring fluid flow rates by means of the drag force exerted on a sharp edged disk centered in a circular flow path due to differential pressure created by fluid flowing through the annulus; usually, the disk is mounted on a bar whose axis coincides with the tube axis, and drag force is measured by a secondary device attached to the bar.

tesla Metric unit for magnetic flux density.

thermal bulb A device for measuring temperature in which the liquid in a bulb expands and contracts with changes in temperature, causing a Bourdon tube element to elastically deform, thereby moving a pointer in direct relation to the temperature at the bulb.

thermal coefficient of resistance The relative change in resistance of a conductor or semiconductor per unit change in temperature over a stated range of temperature. Expressed in ohms per ohm per degree C or F.

thermal conductivity Heat flow per unit cross section per unit temperature gradient.

thermal diffusion Spontaneous movement of solvent atoms or molecules to establish a concentration gradient as a direct result of the influence of a temperature gradient.

thermal electromotive force The electromotive force developed across the free ends of a bimetallic couple when heat is applied to a physical junction between the opposite ends of the couple.

thermal type flowmeter An apparatus in which heat is injected into a flowing fluid stream and flow rate is determined from the rate of heat dissipation; either the rise in temperature or some point downstream of the heater or the amount of thermal or electrical energy required to maintain the heater at a constant temperature is measured.

thermistor A temperature transducer constructed from semiconductor material and for which the temperature is converted into a resistance, usually with negative slope and highly nonlinear.

thermoammeter A device used chiefly to measure radio frequency currents where the current is run through a wire of appropriate size; on this wire is mounted a thermocouple whose output is proportional to the temperature of the wire, which is a function of the R.F. current passing through the wire. Also called a "hot wire ammeter. "

thermocouple Two dissimilar wires joined together that generate a voltage proportional to temperature when their junction is heated.

thermocouple extension wire A matched pair of wires having specific temperature EMF properties that make the pair suitable for use with a thermocouple to extend the location of its reference junction (cold junction) to some remote location.

thermoelectric cooling A method of cooling a chamber based on the Peltier effect, in which an electric current is circulated in a thermocouple whose cold junction is coupled to the chamber; the hot junction dissipates heat to the environment.

thermoelectric heating A method of heating involving a device similar to one used for thermoelectric cooling, except that the direction of current is reversed in the circuit.

thermojunction Either of the two locations where the conductors of a thermocouple are in electrical contact; one, the measuring junction, is in thermal contact with the body whose temperature is being determined, and the other, the reference junction, is generally held at some known or controlled temperature.

thermowell A thermowell is a pressure tight receptacle adapted to receive a temperature sensing element and provided with external threads, flanges or other means for pressure tight attachment to a vessel.

three element feedwater control A control system whereby three process variables (steam flow, feedwater flow and drum level) are used as inputs to the control loop that regulates feedwater flow to the drum to maintain the drum level at set point. This is a cascaded feedforward loop with drum level as the primary variable steam flow as the feedforward input, and feedwater flow (feedback) as the secondary variable.

three mode controller Another name for a "PID controller. "

three term control Proportional-integral-derivative control.

throttling The actions to regulate fluid flow through a valve by restricting its orifice opening. See also "modulating".

time constant 1. The value t in an exponential response term. For the output of a first order system forced by a step or an impulse, t is the time required to complete 63 .2% of the total rise or decay; at any instant during the process, t is the quotient of the instantaneous ratio of change divided into the change still to be completed. In higher order systems, there is a time constant for each of the first order components of the process. 2. In process instrumentation, the value T in an exponential response term A exp (-tlT) or in one ofthe transform factors 1 + sT, 1 + joT, 1/(1 + sT ), 1/(1 +jeT), where: s = complex variable; t = time, seconds; T= time constant;j = square root of-1; @ = angular velocity, radians per second. NOTE: For the output of a first order system forced by a step or an impulse, T is the time required to complete 63.2% of the total rise or decay; at any instant during the process, T is the quotient of the instantaneous rate of change divided into the change still to be completed. In higher order systems, there is a time constant for each of the first order components of the process. In a Bode diagram, break points occur at O = 1/T. 3. The length of time required for the output of a transducer to rise to 63% of its final value as a result of a step change of measurand.

time constant, derivative action Of proportional-plus-derivative control action, a parameter the value of which is equal to 112 Ed whereby is the frequency (in hertz) on a Bode diagram of the lowest frequency gain corner resulting from derivative control action.

time constant, integral action 1. Of proportional-plus-integral control action, a parameter whose value is equal to 112fi whereby is the frequency (in hertz) on a Bode diagram of the highest frequency gain corner resulting from integral control action. 2. It is the reciprocal of integral action rate. NOTE: The use of integral action rate is preferred.

time, derivative action In proportional-plus-derivative control action, for a unit ramp signal input, the advance in time of the output signal (after transients have subsided) caused by derivative control action, as compared to the output signal due to proportional control action only.

titration curve A plot with pH as the ordinate and units of reagent added per unit of sample as the abscissa.

ton 1. A weight measurement equal to 2,000 lb (avoirdupois), short ton; 2,240 lb (avoirdupois), long ton; or 1,000 kg, metric ton. 2. A unit of measurement for refrigerating capacity equal to 200 Btu/min, or about 3517 W; derived from the capacity equal to the rate of heat extraction needed to produce a short ton of ice having a latent heat of fusion of 144 Btu/lb from water at the same temperature in 24 hr.

topworks A nonstandard term for control valve actuator.

torque tube flowmeter A device for measuring liquid flow through a pipe in which differential pressure due to the flow operates a bellows, whose motion is transmitted to a recorder arm by means of a flexible torque tube.

torque type viscometer An instrument that can measure viscosity of Newtonian fluids, non Newtonian fluids, and suspensions by determining the torque needed to rotate a vertical paddle or cylinder submerged in the fluid.

torr Also spelled tor. A unit of pressure equal to the pressure exerted by a column of mercury 1 mm high at 0°C.

total absorption spectrometer An instrument that measures the total amount of x rays absorbed by a sample and compares it to the amount absorbed by a reference sample; the sample may be solid, liquid or gas.

total pressure The sum of the static and velocity pressures.

tracer A colored thread or filament visible in the insulation on an electrical wire so that the wire can be easily identified or traced between connections.

tracer gas A gas used in connection with a leak detecting instrument to find minute openings in a sealed vacuum system.

transfer function A mathematical expression frequently used by control engineers, which expresses the relationship between the outgoing and the incoming signals of a process, or control element. The transfer function is useful in studies of control problems. Transfer functions are generally presented in terms of the Laplace transform.

transient response The response of a transducer to a step change in measurand. NOTE: Transient response, as such, is not shown in a specification except as a general heading, but is defined by such characteristics as time constant, response time, ringing period, etc.

transitional flow Flow between laminar and turbulent flow; generally between a pipe Reynolds number 2000 and 7000.

transverse electromagnetic wave A type of electromagnetic wave having both its electric field vector and its magnetic field vector everywhere perpendicular to the direction of propagation in a homogeneous isotropic medium.

tray A horizontal plate in a distillation column that temporarily holds a pool of descending liquid until it flows into a vertical "downcomer" and onto the next tray. Each tray has openings to permit passage of ascending vapours.

trim The internal parts of a valve which are in flowing contact with the controlled fluid.

trim, anti cavitation A combination of control valve trim that by its geometry reduces the tendency of the controlled liquid to cavitate.

trim, anti noise A combination of control valve trim that by its geometry reduces the noise generated by fluid flowing through the valve.

trim, balanced Control valve trim designed to mini mise the net static and dynamic fluid flow forces acting on the trim.

trim, restricted Control valve trim which has a flow area less than the full flow area for that valve.

trim, reduced Control valve trim which has a flow area smaller than the full flow area for that valve.

trim, soft-seated Valve trim with an elastomeric, plastic or other readily deformable material used either in the closure component or seat ring to provide shutoff with minimal actuator forces.

triple point A temperature at which all three phases of a pure substance solid, liquid and gas-are in equilibrium.

tundish A pouring basin (typically for molten metal).

tuning The adjustment of control constants in algorithms or analog controllers to produce the desired control effect.

turbidity The optical obstruction to the passing of a ray of light through a body of water, caused by finely divided suspended matter.

turbine meter A volumetric flow measuring device using the rotation of a turbine type element to determine flow rate.

turbulent flow A flow regime characterised by random motion of the fluid particles in the transverse direction as well as motion in the axial direction. This occurs at high Reynolds numbers and is the type of flow most common in industrial fluid systems.

turndown The ratio of the maximum plant design flow rate to the minimum plant design flow rate.

Twaddle scale A specific gravity scale that attempts to simplify measurement of liquid densities heavier than water, such as industrial liquors; the range of density from 1.000 to 2.000 is divided into 200 equal parts, so that one degree Twaddle equals a difference in specific gravity of 0.005; on this scale, 40° Twaddle indicates a specific gravity of 1.200.

twisted pair Two insulated wires (signal and return) which are twisted around each other. Since both wires have nearly equal exposure to any electrostatic or electromagnetic interference, the differential noise is slight.

two-element feedwater control A control system whereby two process variables (steam flow and drum level) are used as inputs to the control loop that regulates feedwater flow to the drum to maintain the drum level at set point. The feedforward input is steam flow, with the output of the drum level controller as the primary control signal.

two out of three logic circuit (2/3 logic circuit) A logic circuit with three independent inputs. The output of the logic circuit is the same state as any two matching input states.

two phase A fluid state comprising a mixture of liquid with gas or vapours.

two-wire transmitter Electronic transmitter which uses the power wires (typcally 24vdc) for signal transmission, usually by manipulating the current flow (typically 4-20mA) to represent the desired signal.

Tyndall effect A physical phenomenon in which particles suspended in a fluid can be seen readily if illuminated by strong light and viewed from the side, although they cannot be seen when viewed from the front in the same light beam; forms the basis for nephelometry, which involves measurement of the intensity of side reflected light. Used in such applications as analysing for trace amounts of silver in solution, determining the concentration of small amounts of calcium in titanium alloys, measuring bacterial growth rates, and controlling the clarity of beverages, potable water and effluent discharges.