U - Z

ultrasonic density sensor A device for determining density from the attenuation of ultrasound beams passing through a liquid or semisolid; a typical application involves immersing an ultrasonic transducer in fully agitated lime slurry, thus avoiding coating and clogging which occurs with other devices.

ultrasonic flowmeter A device for measuring flow rates across fluid streams by either Doppler effect measurements or time of transit determination; in both types of flow measurement, displacement of the portion of the flowing stream carrying the sound waves is determined and flow rate calculated from the effect on soundwave characteristics.

ultraviolet erasable read only memory (UVROM) A type of computer memory that can be erased or changed only by exposure to ultraviolet light.

ultraviolet radiation Electromagnetic radiation having wavelengths shorter than visible light and longer than low frequency x rays-that is, wavelengths of about 14 to 400 nanometers.

Underwriters Laboratories (UL) An independent US testing and certifying organization.

unfired pressure vessel A vessel designed to withstand internal pressure, neither subjected to heat from products of combustion nor an integral part of a fired pressure vessel system.

unified screw thread A US system of standard 60° V threads that are classified coarse, fine and extra fine (UNC, UNF and UNEF) to provide different levels of strength and clamping power.

universal asynchronous receiver transmitter (UART) A semiconductor chip that performs asynchronous communications functions.

USASCII (ASCII) U.S. Standard Code for Information Exchange. The standard code, using a coded character set consisting of 7 bit coded characters (8 bits including parity check), used for information exchange among data processing systems, communications systems, and associated equipment. The USASCII set consists of control characters and graphic characters.

U tube manometer A device for measuring gauge pressure or differential pressure by means of a U shaped transparent tube partly filled with a liquid, commonly water; a small pressure above or below atmospheric is measured by connecting one leg of the U to the pressurised space and observing the height of liquid while the other leg is open to the atmosphere; a small differential pressure may be measured by connecting both legs to pressurised space for example, high and low pressure regions across an orifice or venturi.

UV erasable PROM (EPROM) Memory whose contents can be erased by a period of intense exposure to UV radiation. See UVROM

vacuum breaker A device used in a water supply line to relieve a vacuum and prevent backflow Also known as ''backflow preventer. "

valence A positive number that characterises the combining power of an element for other elements, as measured by the number of bonds to other atoms which one atom of the given element forms upon chemical combination; hydrogen is assigned valence 1, and the valence is the number of hydrogen atoms, or their equivalent, with which an atom of the given element combines.

value, rms (root mean square value) The square root of the average of the square of the instantaneous values.

valve, ball A valve with a rotary motion closure component consisting of a full ball or a segmented ball.

valve body assembly An assembly of a body, bonnet assembly, bottom flange and trim elements. The trim includes a valve plug which opens, shuts or partially obstructs one or more ports.

valve, diaphragm A flexible member which is moved into the fluid flow passageway of a body to modify the rate of flow through the valve.

valve, diaphragm-type A valve with a flexible linear motion closure component which is moved into the fluid flow passageway of the body to modify the rate of flow through the valve by the actuator.

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

valve flow coefficient (Cv) The number of US gallons per minute of 60°F water that will flow through a valve with a one pound per square inch pressure drop.

valve, globe A valve with a linear motion closure component, one or more ports and a body distinguished by a globular shaped cavity around the port region.

valve hand A valve with a manual actuator.

Van Stone nipples (flanges) A pipe nipple made with one enlarged integral end held against another face with a loose flange around the nipple.

vapour filled thermometer A type of filled system thermometer in which temperature is determined from the vapour pressure developed from partial vapourisation of a volatile liquid contained within the system.

vapour pressure 1. The pressure of a vapour corresponding to a given temperature at which the liquid and vapour are in equilibrium. vapour pressure increases with temperature. 2. The pressure (for a given temperature) at which a liquid is in equilibrium with its vapour. As a liquid is heated, its vapour pressure will increase until it equals the pressure above the liquid; at this point the liquid will begin to vapourise.

vapour pressure, Reid The vapour pressure of a liquid at 100°F (311 K) as determined by ASTM Designation D 323 58, "Standard Method of Test for vapour Pressure of Petroleum Products (Reid Method)".

vapour pressure thermometer A temperature transducer for which the pressure of vapour in a closed system of gas and liquid is a function of temperature.

var A unit of measure for reactive power; it is calculated by taking the product of voltage, current, and the sine of the phase angle.

variable inductance pickup A transducer that converts mechanical oscillations into audio frequency electrical signals by varying the inductance of an internal coil.

variable reluctance pickup A transducer that converts mechanical oscillations into audio frequency electrical signals by varying the reluctance of an internal magnetic circuit.

variable reluctance proximity sensor A device that senses the position (presence) of an actuating object by means of the voltage generated across the terminals of a coil surrounding a pole piece that extends from one end of a permanent magnet; coil voltage is proportional to the rate of change of magnetic flux as the object passes through the field near the pole piece.

variance The square of the standard deviation.

vector 1. A quantity having magnitude and direction, as contrasted with a scalar which has quantity only. 2. A one-dimensional matrix.

vee orifice "V"-shaped flow control orifice which allows a characterised flow control as the gate moves in relation to the fixed Vee opening.

velocity head The pressure, measured in height of fluid column, needed to create a fluid velocity. Numerically, velocity head is the square of the velocity divided by twice the acceleration of gravity (V2 .- 2g) [S5 1 .1].

Venn diagram A graphical representation in which sets are represented by closed areas. The closed regions may bear all kinds of relations to one another, such as be partially overlapped, be completely separated from one another, or be contained totally one within another. All members of a set are considered to lie within or be contained within the closed region representing the set. The diagram is used to facilitate the determination of whether several sets include or exclude the same members.

venturi A constriction in a pipe, tube or flume consisting of a tapered inlet, a short straight constricted throat and a gradually tapered outlet; fluid velocity is greater and pressure is lower in the throat area than in the main conduit upstream or downstream of the venturi; it can be used to measure flow rate, or to draw another fluid from a branch into the main fluid stream.

venturi meter A type of flowmeter that measures flow rate by determining the pressure drop through a venturi constriction.

venturi tube A primary differential pressure-producing device having a cone section approach to a throat and a longer cone discharge section. Used for high volume flow at low pressure loss.

vertical orifice installation, vertical orifice run, vertical meter run An orifice plate used in a vertical pipeline.

V/F (boilup to feed ratio) A quantity used to analyze the operation of a distillation column.

vibrating density sensor Any of several devices in which a change in natural oscillating frequency of a device element-cylinder, single tube, twin tube, U tube or vane is detected and related to density of process fluid flowing through the system.

vibrating quartz crystal moisture sensor A device for detecting the presence of moisture in a sample gas stream by dividing the stream into two portions, one of which is dried, then alternately passing the two streams across the face of a hygroscopically sensitised quartz crystal whose wet and dry vibrational frequencies are continuously monitored and compared to the frequency of an uncoated sealed reference crystal.

viscometer An instrument that measures the viscosity of a fluid.

viscosity Measure of the internal friction of a fluid or its resistance to flow.

viscosity, absolute The property by which a fluid in motion offers resistance to shear. Usually expressed as newton seconds/meter2.

viscosity, kinematic The ratio of absolute viscosity to density. The SI unit is the meter2/s.

viscous drag type density meter A type of meter for determining gas density by comparing the drag force on linked impellers driven by flow of a standard gas and the test gas; the balance point is a function of gas density, and the instrument can be calibrated to read directly in density units.

Viton A A fluorocarbon rubber by E.I. du Pont de Nemours co.

volt A unit of electromotive force which when steadily applied to a conductor whose resistance is one ohm will produce a current of one ampere.

voltage, common mode (CMV) That amount of voltage of the same polarity and phase common to both input lines. Common mode voltage can be caused by magnetic induction, capacitive coupling, and resistive coupling.

vortex shedding A phenomenon that occurs when fluid flows past an obstruction; the shear layer near the obstruction has a high velocity gradient, which makes it inherently unstable; at some point downstream of the immediate vicinity of the obstruction, the shear layer breaks down into well defined vortices, which are captured by the flowing stream and carried further downstream.

vortex flowmeter A device that measures flow by sensing the movement of vortices in a pipe or conduit. The instrument usually is constructed with a bluff body (vortex shedder) inserted perpendicular to the flow to allow formation of vortices, and sensor(s) to detect the passing vortices. The vortices are shed from one side of the shedder and then the other side as the fluid flows around the shedder. The shedding frequency (0, flow velocity (v) and the shedder width (d) can be expressed with the formula:
f = Stx V d 4
St is a dimensionless constant called the Strouhal number. If the frequency of vortices generation is known, and the sensor can count the number of vortices generated per unit of time, then the velocity of the fluid can be calculated.

wafer body A body whose end surfaces mate with the pipeline flanges. It is located and clamped between the piping flanges by long bolts extending from flange to flange. A wafer body is also called a flangeless body.

watt (W) Metric unit of power. The rate of doing work or the power expended equal to 107 ergs/second, 3.4192 Btu/hour or 44.27 foot-pounds/minute.

weber (Wb) Metric unit for magnetic flux.

weight (wt) The force with which a body is attracted by gravity. The newton is the unit force in this Standard.

weighting Artificial adjustment of a measurement to account for factors peculiar to conditions prevailing at the time the measurement was taken.

weir An open channel flow measurement device analogous to the orifice plate flow constriction.

weir-type valve A body having a raised contour contacted by a diaphragm to shut off fluid flow.

wet bulb temperature The lowest temperature which a water wetted body will attain when exposed to an air current. This is the temperature of adiabatic saturation.

wet bulb thermometer A thermometer whose bulb is covered with a piece of fabric such as muslin or cambric that is saturated with water; it is most often used as an element in a psychrometer.

wet leg The liquid filled low side impulse line in a differential pressure level measuring system.

white noise 1. Random noise that has a constant energy per unit bandwidth at every frequency in the range of interest. 2. A noise whose power is distributed uniformly over all frequencies and has a mean noise power per unit bandwidth; since idealistic white noise is an impossibility, bandwidth restrictions have to be applied.

Whitworth screw thread A British standard screw threaded characterised by a 55° V form with rounded crests and roots.

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.

Wobbe index The ratio of the heat of combustion of a gas to its specific gravity. For light hydrocarbon gases the Wobbe index is almost a linear function of the gas' specific gravity.

Xrays Short wavelength electromagnetic radiation, having a wavelength shorter than about 15 nanometers, usually produced by bombarding a metal target with a stream of high energy electrons; wavelengths are in the same range as gamma rays, longer than cosmic rays but shorter than ultraviolet; like gamma rays, Xrays are very penetrating and can damage human tissues, induce ionisation, and expose photographic films.

Xray thickness gauge A device used to continually measure the thickness of moving cold rolled sheet or strip during the rolling process; it consists of an Xray source on one side of the strip and a detector on the other thickness is proportional to the loss in intensity as the Xray beam passes through the moving material.

No Entries for the letter "Y".

zap In data processing, meaning to erase or wipe out data.

zero bias A positive or negative adjustment to instrument zero to cause the measurement to read as desired.

zero shift 1. In process instrumentation, any parallel shift of the input-output curve [S51.1]. 2. A change in the zero measurand output over a specified period of time and at room conditions.. NOTE: This error is characterised by a parallel displacement of the entire calibration curve [S37. 1]. 3. A shift in the instrument calibrated span evidenced by a change in the zero value. Usually caused by temperature changes, overrange, or vibration of the instrument.

zero suppression 1. For a suppressed zero range, the amount the measured variable zero is below the lower range value. It may be expressed either in units of the measured variable or in percent of span [S5 1 .1]. 2. The elimination of nonsignificant zeros in a numeral. 3. Biasing the zero output signal to produce the desired measurement. Used in level measurement to counteract the zero elevation caused by a wet leg.

Ziegler-Nichols method A method of determination of optimum controller settings when tuning a process control loop (also called the "ultimate cycle method"). It is based on finding the proportional gain which causes instability in a closed loop.

zone The international method of specifying the probability that a location is made hazardous by the presence, or potential presence, of flammable concentrations of gases and vapours. NOTE: Zone classification has not not yet been defined for dusts and flyings.

Zone 0: Classification of a location in which an explosive concentration of a flammable gas or vapour mixture is continuously present or is present for long periods.

Zone 1: Classification of a location in which an explosive concentration of a flammable or explosive gas or vapour mixture is likely to occur in normal operation.

Zone 2: Classification of a location in which an explosive concentration of a flammable or explosive gas or vapour mixture is unlikely to occur in normal operation and, if it does occur, will exist only for a short time.