Guide to Instrumentation and Process Control: Glossary

Absolute accuracy The accuracy stated as a definite amount, i.e., not as a percentage.

Absolute position measurement Position measured from a fixed point.

Absolute pressure--Pressure measured with reference to a perfect vacuum.

Accelerometer A sensor for measuring acceleration or the rate of change of velocity.

Accuracy A measure of the difference between the indicated value and the true value.

Actuator A device that performs an action on one of the input variables of a process according to a signal received from the controller.

ADC An analog-to-digital converter that converts an analog voltage or current into a digital signal.

Alarm A warning that a variable has exceeded set limits.

Alternating current--Current that flows in one direction during one half of a regular time period and the opposite direction during the other half.

Ammeter An instrument for measuring electrical current or electron flow.

Ampere The unit of current or electron flow.

Amplifier An electrical circuit that increases the magnitude of a signal.

Analog A continuously varying signal.

Aneroid barometer A barometer which uses an evacuated capsule as a sensing element.

Anticipatory action See Derivative action.

Aqueous solution A solution containing water.

Atmospheric pressure The pressure acting on objects on the earth's surface caused by the weight of the air in the earth's atmosphere, normally measured at sea level.

Barometer An instrument used for measuring atmospheric pressure.

Bellows A pressure sensor that converts pressure into linear displacement.

Bernoulli equation A flow equation based on the conservation of energy which includes velocity, pressure, and elevation terms.

Beta ratio The ratio of the diameter of a restriction to the diameter of the pipe containing the restriction.

Bimetallic A thermometer with a sensing element made of two dissimilar metals with different thermal coefficients of expansion.

Binary Two values, or a numbering system using the base 2.

Bit A binary digit.

Bourdon tube A pressure sensor that converts pressure to movement. The device is a coiled metallic tube that straightens when pressure is applied.

Bridge A network of passive components arranged so that small changes in one of the components can be easily measured.

British thermal unit A measure of heat energy, i.e., the amount of heat required to raise 1 lb of water 1°F at 68°F and atmospheric pressure.

Buffer amplifier A circuit for matching the output impedance of one circuit to the input impedance of another.

Buoyancy The upward force on an object floating or immersed in a fluid caused by the difference in pressure above and below the object.

Byte Eight bits of binary information.

Calorie A measure of heat energy, i.e., the amount of heat required to raise the temperature of 1 g of water by 1°C.

Capacitance A measure of a device's ability to store electrical charge.

Capacitance probe An instrument using the capacitance between two metal plates for measuring fluid level.

Capacitor A device that can store electrical charge.

Cell A simple power source that provides emf, usually by means of a chemical reaction.

Celsius One of the commonly used temperature scales.

Coefficient of heat transfer A term used in the calculation of heat transfer by convection.

Coefficient of thermal expansion A term used to determine the amount of linear expansion due to heating or cooling.

Comparator A device which compares two signals and outputs the difference.

Concentric plate A plate with a hole located at its center (orifice plate) used to measure flow by measuring the differential pressures on either side of the plate.

Conduction The movement of heat energy in a material by the transfer of energy from one molecule to another.

Conductivity probe An instrument using two electrodes to measure fluid level.

Continuity equation A flow equation which states that, if the overall flow rate is not changing with time, the flow rate past any section of the system must be constant.

Continuous level measurement A level measurement that is continuously updated.

Controlled variable The variable measured to indicate the condition of the process output.

Controller The element in a process control loop that evaluates any error of the measured variable and initiates corrective action by changing the manipulated variable.

Convection The movement of heat by the motion of warm or hot material.

Converter A device that changes the format of a signal but not the type of energy used as the signal carrier, i.e., voltage to current.

Correction signal The signal to the manipulated variable.

DAC A device that converts a digital signal into an analog voltage or current.

Dead weight tester A device for calibrating pressure-measuring devices which uses weights to provide the forces.

Decibel (dB) A unit used to compare amplitude or power levels.

Density The amount of mass in a unit volume.

Derivative action--Action that is proportional to the rate at which the measured vari able is changing.

Dew point--The temperature at which the water vapor in a mixture of water vapor and gas becomes saturated and condensation starts.

Dielectric constant The factor by which the capacitance between two plates changes when a material fills the space between the plates.

Differential amplifier An amplifier that amplifies the difference between two inputs.

Digital Signals having two discrete levels.

Dry-bulb temperature The temperature indicated by a thermometer whose sensing element is dry.

Dynamic pressure That part of the total pressure in a moving fluid caused by the fluid motion.

Dynamometer An instrument used for measuring torque or power.

Eccentric plate An orifice plate with a hole located below its center to allow for the pas sage of suspended solids.

Effective value The dc voltage or dc current that would produce the same power in a load as the ac voltage or ac current being measured.

Electromagnetic flow meter A flow-measuring device which senses the change in a magnetic field between two electrodes as a fluid flows between them.

Electromagnetism The relationship between magnetic fields and electric current.

Electromotive force (emf) The force that causes electrons to move, and is measured in volts.

Error signal The difference in value between a measured signal and a set point.

Fahrenheit One of the commonly used temperature scales.

Farad The unit of capacitance.

Feedback (1) The voltage fed from the output of an amplifier to the input in order to control the characteristics of the amplifier. (2) The measured variable signal fed to the controller in a closed-loop system, so that the controller can adjust the manipulated variable to keep the measured variable within set limits.

Fiber optics The transmission of information through optical cables using light signals.

Flow nozzle A device placed in a flow line to provide a pressure drop that can be related to flow rate.

Flow rate The amount of fluid passing a given point in a given interval of time.

Flume An open-channel flow-measuring device.

Form drag The force acting on an object due to the impact of fluid.

Foundation fieldbus Process control bus used in the United States.

Free convection Movement of heat as a result of density differences.

Free surface The surface of the liquid in an open-channel flow that is in contact with the atmosphere.

Frequency The number of cycles completed in 1 s.

Gauge pressure The measured pressure above atmospheric pressure.

Gas thermometer A temperature sensor that converts temperature to pressure in a constant volume system.

Hall-effect sensor A transducer that converts a changing magnetic field into a proportional voltage.

Head Sometimes used to indicate pressure, i.e., 1 ft of "head" for water is the pressure under a column of water 1 ft high.

Heat A form of energy related to the motion of atoms or molecules.

Heat transfer The study of heat energy movement.

Henry (H) The unit of inductance.

Hertz (Hz) A measure of frequency in cycles/second.

Hot-wire anemometry A velocity-measuring device for gas or liquid flow that senses temperature changes, due to the cooling effect of gas or liquid moving over a hot element.

Humidity A term to indicate the amount of water vapor present in the air or a gas.

Humidity ratio The mass of water vapor in a gas divided by the mass of dry gas in the mixture.

Hydrometer An instrument for measuring liquid density.

Hydrostatic paradox The fact that pressure varies with depth in a static fluid, but is the same throughout the liquid at any given depth.

Hydrostatic pressure The pressure caused by the weight of static fluid.

Hygrometer A relative humidity-measuring device.

Hygroscopic A material that absorbs water and whose conductivity changes with moisture content.

Hysteresis The non-reproducibility in an instrument caused by approaching a measurement from opposite directions, i.e., going from low up to the value, or high down to the value.

Impact pressure The sum of the static and dynamic pressure in a moving fluid.

Impedance An opposition to ac current or electron flow caused by inductance and/or capacitance.

Incremental position measurement An incremental position measurement from one point to another, absolute position is not recorded, and position is lost if the power fails.

Indirect level-measuring device A device that extrapolates the level from the measurement of another variable, i.e., liquid level from a pressure measurement.

Inductance An electrical component that opposes a change in current or electron flow.

Inductor A device that exhibits inductance.

Instrument A device used to measure a physical variable.

Integral action The action designed to correct for long-term loads.

Kelvin The absolute temperature scale associated with the Celsius scale.

Kirchoff's current law The sum of the currents flowing at a node is zero.

Kirchoff's voltage law The algebraic sum of voltages around a closed path is zero.

Ladder logic The programmable logic used in PLCs to control automated industrial processes.

Lag time The time required for a control system to return a measured variable to its set point.

Laminar flow A smooth flow in which the fluid tends to move in layers.

LED Light emitting diode

Linearity A measure of the direct proportionality between actual value of the variable being measured and the value of the output of the instrument to a straight line.

Load The process load is a term used to denote the nominal values of all variables in a process that affect the controlled variable.

Load cell A device for measuring force.

Loudness A subjective quantity used to measure relative sound strength.

LVDT A linear variable transformer that measures displacement by conversion to a linearly proportional voltage.

Magnetorestrictive element (MRE)--A magnetic field sensor that converts a changing magnetic field into a proportional resistance.

Manipulated variable The variable controlled by an actuator to correct for changes in the measured variable.

Measured variable The variable measured to indicate the condition of the process output.

Meniscus The convex or concave surface of a column of liquid in a tube.

Moment The effect of a force acting at a given perpendicular distance from a point.

Natural convection The movement of heat as a result of density differences.

Newtonian fluid A fluid in which the velocity varies linearly across the flow section between parallel plates.

Node A junction of three or more conductors.

Noise The term usually used to indicate unwanted or undesirable sounds.

Nutating disk meter A flow-measuring device using a disk that rotates and wobbles in response to the flow.

Offset The nonzero output of a circuit when the input is zero.

Ohmmeter An instrument used to measure resistance.

ON/OFF control A system in which a process actuator has only two positions, i.e., on and off.

Open-channel flow--The flow in an open conduit (e.g., as in a ditch).

Operational amplifier A circuit used to amplify electronic signals.

Orifice plate A plate containing a hole which when placed in a pipe causes a pressure drop which can be related to flow rate.

Over pressure The term used to describe the maximum amount of pressure a gauge can withstand without damage or loss of accuracy.

Overshoot The overcorrection of the measured variable in a control loop.

Parabolic velocity distribution Occurs in laminar flow when the velocity across the cross-section takes on the shape of a parabola.

Parallel transmission Simultaneous transmission of a number of binary bits.

Pascal Pressure reading units (SI), i.e., newtons per square meter Pascal's law The pressure applied to an enclosed fluid is transmitted to every part of the fluid.

Percent of reading The accuracy given in terms of the percentage of the reading.

Percentage full-scale accuracy The accuracy determined by dividing the accuracy of an instrument by its full-scale output taken as a percentage.

Period A fixed amount of time during which alternating current is completing one full cycle and is the inverse of the frequency in Hertz.

pH A term used to indicate the activity of the hydrogen ions in a solution, it helps to describe the acidity or alkalinity of the solution.

Phase A term used to describe the state of matter, i.e., solid, liquid, or gas.

Phons A unit for describing the difference in loudness levels.

Photodiode A sensor used to measure light intensity by measuring the leakage across a pn junction.

P/D --Proportional control with derivative and integral action.

P&ID --Stands for piping and instrument diagrams.

Piezoelectric effect-- The electrical voltage developed across certain crystalline materials when a force or pressure is applied to the material.

Pitot-static tube --A device used to measure the flow rate using the difference between dynamic and static pressures.

PLC --Programmable logic controller.

Pneumatic --System that employs gas for control or signal transmission.

Poise --The measurement unit of dynamic or absolute viscosity.

Potentiometer (Pot) --An adjustable resistance device.

Precision-- The smallest division that can be read on an instrument.

Pressure --The magnitude of a force divided by the area over which it acts, i.e., psi or Pa.

Pressure differential --The difference in pressure amplitudes at two locations.

Process --A sequence of operations carried out to achieve a desired end result.

Process control --The automatic control of certain process variables to hold them within given limits.

Processor --A digital electronic computing system that can be used as a control system.

Pro-bus--Process control bus used in Europe Proportional action A controller action in which the controller output is directly proportional to the measured variable error.

Psychrometric chart --A chart dealing with moisture content in the atmosphere.

Pyrometer --An instrument for measuring temperature by sensing the radiant energy from a hot body.

Radiation --The emission of energy from a body in the form of electromagnetic waves.

Range --The lowest to the highest readings that can be made by a sensing device.

Rankine --The absolute temperature scale associated with the Fahrenheit scale.

Rate action --See Derivative action.

Reactance --The opposition to an ac current or electron flow caused by a capacitor or an inductor.

Relative humidity --The amount of water vapor present in a given volume of a gas, expressed as a percentage of the amount that would be present in the same volume of gas under saturated conditions at the same pressure and temperature.

Reluctance --The opposition in a material to carrying magnetic flux, it is the magnetic equivalence to resistance.

Repeatability --A measure of the closeness between several consecutive readings of a value.

Reproducibility --The ability of an instrument to produce the same reading of a variable with repeated readings.

Reset action --See Integral action.

Resistance --A measure of the opposition to electron or current flow in a material.

Resistance thermometer (RTD) --A temperature sensor that provides temperature readings by measuring the resistance of a metal wire (usually platinum).

Resistivity --A temperature-dependent "constant" that reflects a material's resistance to electron flow.

Resistor --A component that exhibits resistance.

Resolution --The minimum detectable change of a variable in a measurement.

Reynolds Number --A dimensionless number indicating whether the flow is laminar or turbulent.

Rotameter --A flow-measuring device in which a float moves in a vertical tapered tube.

Saturated --The condition when the maximum amount of a material is dissolved in another material at the given pressure and temperature conditions, i.e., water vapor in a gas.

Sealing fluid --An inert fluid used in a manometer to separate the fluid whose pressure is being measured from the manometer fluid.

Segmented plate --An orifice plate with a hole located so as to allow suspended solids to pass through.

Sensitivity --The ratio of the change in output to input magnitudes.

Sensor --A device that can convert a physical variable into a measurable quantity.

Serial transmission --A sequential transmission of digital bits.

Set point --The reference value for a controlled variable in a process control loop.

Signal conditioning --The conversion of a signal to a format that can be used for trans mission.

Single-point level measurement-- Indicates when a particular level has been reached.

Sling psychrometer-- A device for measuring relative humidity.

Smart sensor --Integration of a processor directly into the sensor assembly to give direct control of the actuator and digital communication to a central controller.

Sone --A unit for measuring loudness.

Sound pressure level-- The difference between the maximum air pressure at a point and the average air pressure at that point.

Span --The difference between the lowest and highest reading for an instrument.

Specific gravity --The ratio of the specific weight of a solid or liquid material and the specific weight of water, or for a gas, the ratio of the specific weight of the gas and the specific weight of air under the same conditions.

Specific heat --The amount of heat required to raise a definite amount of a substance by one degree, i.e., 1 lb 1°F or 1 g 1°C.

Specific humidity --The mass of water vapor in a mixture divided by the mass of dry air or gas in the mixture.

Specific weight --The weight of a unit volume of a material.

Static pressure --The part of the total pressure in a moving fluid not caused by the fluid motion.

Stoke--The measurement unit of kinematic viscosity.

Strain gauge--A sensor that converts information about the deformation of solid objects when they are acted upon by a force into a change of resistance.

Sublimation --Passing directly from solid to vapor or vapor to solid.

Telemetry-- The electrical transmission of information over long distances usually by radio frequencies.

Temperature-- The term used to describe the hotness or coldness of an object.

Thermal conductivity-- A measure of the ability of a material to conduct heat.

Thermal expansion --The expansion of a material as a result of its being heated.

Thermal time constant --The time required for a body to heat or cool by 63.2 percent of the difference between the initial temperature and the aiming temperature.

Thermistor --A temperature sensing element made from a metal oxide that usually has a negative temperature coefficient.

Thermocouple-- A temperature sensing device that uses dissimilar metal junctions to generate a voltage proportional to the differential temperature between the metal junctions.

Thermometer --An instrument used to measure temperature.

Thermopile-- A number of thermocouples connected in series.

Time constant (electrical) --The amount of time needed for a capacitance C, to discharge or charge through a resistance R, by 62.3 percent of the difference between the initial voltage and the aiming voltage; the product of RC gives the time constant in seconds.

Torque --The name given to a force moment that tends to create a twisting action.

Torr The pressure caused by the weight of a column of mercury 1 mm high.

Total flow --The amount of flow past a given point over some length of time.

Total pressure --The sum of the static and dynamic pressures in a moving fluid.

Transducer --A device that changes energy from one form to another.

Transfer function An equation that describes the relationship between the input and output of the function.

Transmission --The transferring of information from one point to another.

Transmitter --A device that conditions the signal received from a transducer so that it is suitable for sending to another location with minimal loss of information.

Turbine flow meter --A flow-measuring device using a turbine wheel.

Turbulent flow --An agitated flow in which there are random velocity fluctuations on top of the average flow.

U-tube manometer-- A glass tube in the shape of the letter U that is used to measure pressure or pressure differences.

Ultrasonic probe --An instrument using high-frequency sound waves to measure fluid levels.

Vacuum (pressure) --The amount that the measured pressure is below atmospheric pressure.

Velocity --A measure of speed, and in a flow is the average speed across the flow and the direction of movement of a liquid.

Vena contracta-- The narrowing down of the fluid flow stream as it passes through an obstruction.

Venturi tube --A specially shaped restriction in a section of pipe that provides a pressure drop which can be related to flow rate.

Viscometer (viscosi-meter) --An instrument for measuring viscosity.

Viscosity --The term describing the resistance to flow of a fluid.

Volt --The unit of electromotive force.

Voltage --An electromotive force that causes electrons or a current to flow.

Voltage drop --The difference in voltage between two points.

Vortex --Swirling or rotating fluid motion.

Wavelength --The time for an alternating source to complete a full cycle.

Weir --An open-channel flow-measuring device.

Wet-bulb temperature --The temperature indicated by a thermometer whose sensing element is kept moist.

Wheatstone bridge --The most common electrical bridge circuit used to measure small changes in the value of an element.

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