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Instrumentation Glossary

Instrumentation and control terms explained in plain words, with formulas, typical values, worked examples and links to free calculators.

266terms
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Glossary Sections

32 termsFlow MeasurementFlow measurement terms: flow rate, Reynolds number, beta ratio, K factor, turndown, meter types and gas flow conditions.32 termsPressure MeasurementPressure terms: gauge, absolute and differential pressure, transmitter ranges, gauges, seals and relief valve terms.29 termsLevel MeasurementLevel terms: hydrostatic and DP level, wet and dry legs, radar, ultrasonic, displacer, switches, tank volume and ullage.29 termsTemperature MeasurementTemperature terms: RTD, Pt100, thermocouples, cold junction, thermowells, response time, pyrometers and heat transfer.30 termsControl ValveControl valve terms: Cv and Kv, characteristics, authority, cavitation, choked flow, leakage class, actuators, positioners and fail action.31 termsSafety and SISFunctional safety and hazardous area terms: SIS, SIL, PFDavg, voting, proof testing, LOPA, alarms, zones, gas groups and Ex protection.29 termsSignals and PLCSignal and PLC terms: 4 to 20 mA, HART, Modbus, RS485, fieldbus, isolation, grounding, scaling, scan time and digital I/O.28 termsCalibrationCalibration and metrology terms: accuracy, repeatability, hysteresis, uncertainty, TUR, traceability, guard bands, as found and as left.26 termsAnalyticalProcess analyser terms: pH, ORP, conductivity, dissolved oxygen, turbidity, gas analysis, LEL, zirconia oxygen, CEMS and sample systems.
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Signals and PLC29

Signals and PLC4 to 20 mA Current LoopA 4 to 20 mA current loop sends a measurement as a current between 4 and 20 mA, which is not…Signals and PLCADC ResolutionADC resolution is the number of bits an analog to digital converter uses, which sets the…Signals and PLCAnalog ScalingAnalog scaling converts the raw value from a PLC analog input into engineering units such as…Signals and PLCBinary Coded DecimalBinary coded decimal stores each decimal digit as its own 4 bit binary group, used by…Signals and PLCCable Shield GroundingCable shield grounding connects the screen of an instrument cable to earth, normally at one end…Signals and PLCDry ContactA dry contact is a switch or relay contact with no voltage of its own; the receiving circuit…Signals and PLCElectromagnetic InterferenceElectromagnetic interference is unwanted electrical noise coupled into signal circuits from…Signals and PLCEncoderAn encoder converts shaft rotation into pulses or a digital position, used to measure speed and…Signals and PLCFieldbusFieldbus is a digital network that connects field instruments to the control system, carrying…Signals and PLCGalvanic IsolationGalvanic isolation separates two circuits so no current can flow directly between them, while…Signals and PLCGray CodeGray code is a binary code in which only one bit changes between consecutive values, avoiding…Signals and PLCGround LoopA ground loop occurs when a circuit is earthed at two points with different potentials, letting…Signals and PLCHART ProtocolThe HART protocol superimposes a small digital signal on the 4 to 20 mA loop, allowing…Signals and PLCInterposing RelayAn interposing relay is a relay driven by a PLC output that switches a load the PLC cannot…Signals and PLCLive ZeroLive zero means the lowest signal value (4 mA) is above zero, so a broken loop (0 mA) can be…Signals and PLCLoop ResistanceLoop resistance is the total resistance in a 4 to 20 mA loop, from cable, input resistors and…Signals and PLCMarshalling CabinetA marshalling cabinet is where field cables are terminated and cross wired to the correct…Signals and PLCModbus RTUModbus RTU is a simple master slave serial protocol, usually on RS485, that reads and writes…Signals and PLCModbus TCPModbus TCP carries Modbus messages over Ethernet and TCP/IP, normally on port 502.Signals and PLCNAMUR NE43NAMUR NE43 is a recommendation that defines 4 to 20 mA signal levels for measurement and for…Signals and PLCNAMUR SensorA NAMUR sensor is a two wire proximity sensor that signals by changing its current, used with…Signals and PLCPLC RedundancyPLC redundancy uses duplicate controllers, power supplies or networks so the system keeps…Signals and PLCPLC Scan TimePLC scan time is the time a PLC takes to read inputs, execute the program and update outputs once.Signals and PLCPulse OutputA pulse output sends one pulse per fixed quantity of flow, or a frequency proportional to flow,…Signals and PLCRS485RS485 is a differential serial interface that lets many devices share one twisted pair over…Signals and PLCSignal DampingSignal damping is a filter applied in a transmitter or PLC that smooths a noisy measurement by…Signals and PLCSignal IsolatorA signal isolator copies an analog signal from input to output with galvanic isolation, often…Signals and PLCSinking and SourcingSinking and sourcing describe the direction of current in digital I/O: a sourcing device…Signals and PLCTwo Wire TransmitterA two wire transmitter is powered by the same 4 to 20 mA loop it signals on, so it needs only…

Pressure Measurement32

Pressure MeasurementAbsolute PressureAbsolute pressure is pressure measured relative to a perfect vacuum, so it is never negative.Pressure MeasurementAccumulationAccumulation is the pressure rise above the MAWP of the protected equipment while the relief…Pressure MeasurementAtmospheric PressureAtmospheric pressure is the pressure of the air around us; it falls with altitude and varies…Pressure MeasurementBack PressureBack pressure is the pressure at the outlet of a relief valve, from the discharge system or…Pressure MeasurementBlowdownBlowdown is the difference between the set pressure of a relief valve and the pressure at which…Pressure MeasurementBourdon TubeA Bourdon tube is a curved tube of oval cross section that straightens as pressure rises,…Pressure MeasurementBurst PressureBurst pressure is the pressure at which the sensor or pressure boundary ruptures and process…Pressure MeasurementCapacitive Pressure SensorA capacitive pressure sensor measures pressure from the change in capacitance as a diaphragm…Pressure MeasurementDesign PressureDesign pressure is the pressure used to design equipment, set with a margin above the maximum…Pressure MeasurementDiaphragm SealA diaphragm seal isolates a pressure instrument from the process with a flexible diaphragm and…Pressure MeasurementDifferential PressureDifferential pressure is the difference between two pressures measured at two points.Pressure MeasurementFill FluidFill fluid is the incompressible liquid inside a pressure sensor or diaphragm seal that…Pressure MeasurementGauge PressureGauge pressure is pressure measured relative to the local atmospheric pressure, so it reads…Pressure MeasurementHydrostatic PressureHydrostatic pressure is the pressure created by the weight of a liquid column; it depends only…Pressure MeasurementHydrostatic Test PressureHydrostatic test pressure is the pressure at which piping or vessels are tested with liquid to…Pressure MeasurementManometerA manometer measures pressure from the height difference of a liquid column in a U tube or…Pressure MeasurementMaximum Allowable Working PressureMaximum allowable working pressure is the highest gauge pressure permitted at the top of a…Pressure MeasurementOverpressure LimitOverpressure limit is the highest pressure a sensor can take without damage or loss of its…Pressure MeasurementPiezoresistive Pressure SensorA piezoresistive pressure sensor uses resistors on a diaphragm whose resistance changes with…Pressure MeasurementPressurePressure is force acting per unit area. In fluids it acts equally in all directions at a point.Pressure MeasurementPressure Gauge SiphonA pressure gauge siphon is a looped or U shaped tube fitted below a pressure gauge on steam to…Pressure MeasurementPressure HeadPressure head is pressure expressed as the height of a liquid column that would produce it.Pressure MeasurementPressure SnubberA pressure snubber is a restriction fitted before a gauge or switch to damp pulsation and…Pressure MeasurementPressure Switch DeadbandPressure switch deadband is the difference between the pressure at which a switch trips and the…Pressure MeasurementSet PressureSet pressure is the inlet gauge pressure at which a pressure relief valve is adjusted to start…Pressure MeasurementStatic PressureStatic pressure is the pressure of a fluid measured at right angles to the flow, without the…Pressure MeasurementStatic Pressure EffectStatic pressure effect is the change in a DP transmitter reading caused by line pressure acting…Pressure MeasurementTransmitter TurndownTransmitter turndown is how far a transmitter span is reduced from its sensor upper range limit.Pressure MeasurementUpper Range LimitUpper range limit is the highest value a transmitter sensor can be calibrated to measure.Pressure MeasurementVacuumVacuum is any pressure below atmospheric pressure, from slight suction to near perfect vacuum.Pressure MeasurementWater HammerWater hammer is the pressure surge created when a moving liquid is suddenly stopped, for…Pressure MeasurementZero Elevation and SuppressionZero elevation and suppression shift a transmitter range to cancel the head of liquid in…
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Calibration28

CalibrationAccuracyAccuracy is how close a measurement is to the true value; in specifications it is the maximum…CalibrationAccuracy SpecificationAn accuracy specification states how an instrument maker defines its maximum error, such as %…CalibrationAs Found and As LeftAs found and as left are the calibration readings taken before any adjustment (as found) and…CalibrationCalibrationCalibration is the comparison of an instrument reading with a known reference standard,…CalibrationCalibration CertificateA calibration certificate is a formal document reporting calibration results, the reference…CalibrationCalibration CorrectionA calibration correction is the value added to an instrument reading to obtain the corrected…CalibrationCalibration CurveA calibration curve is a fitted relationship between instrument reading and true value, used to…CalibrationCalibration IntervalA calibration interval is the time between scheduled calibrations, set so the instrument is…CalibrationCoverage FactorCoverage factor k multiplies the combined standard uncertainty to give an expanded uncertainty…CalibrationDeadweight TesterA deadweight tester generates a precise pressure by placing known masses on a piston of known area.CalibrationDecision RuleA decision rule defines how measurement uncertainty is taken into account when stating that an…CalibrationDriftDrift is a slow change in an instrument reading over time when the input stays the same.CalibrationFive Point CalibrationA five point calibration checks an instrument at 0, 25, 50, 75 and 100 % of range, usually…CalibrationGuard BandA guard band tightens the acceptance limit inside the tolerance to allow for calibration…CalibrationHysteresisHysteresis is the difference between readings at the same input when approached from below…CalibrationISO/IEC 17025ISO/IEC 17025 is the international standard for the competence of testing and calibration…CalibrationLinearityLinearity is how closely an instrument output follows a straight line between input and output;…CalibrationLocal GravityLocal gravity is the acceleration due to gravity at a site; it varies with latitude and height…CalibrationMeasurement UncertaintyMeasurement uncertainty is a quantified range around a result within which the true value is…CalibrationPrecisionPrecision is how closely repeated measurements of the same quantity agree with each other,…CalibrationReference StandardA reference standard is the highest accuracy standard at a site, used to calibrate working…CalibrationRepeatabilityRepeatability is the closeness of successive readings of the same input under the same…CalibrationReproducibilityReproducibility is the closeness of results for the same quantity when conditions change, such…CalibrationTest Uncertainty RatioTest uncertainty ratio compares the tolerance of the instrument under test with the uncertainty…CalibrationTotal Probable ErrorTotal probable error combines independent error sources, such as reference accuracy,…CalibrationTraceabilityTraceability links a measurement result to national standards through an unbroken chain of…CalibrationType A and Type B UncertaintyType A and Type B uncertainty are the two ways of evaluating uncertainty: Type A from…CalibrationZero and Span ErrorZero and span error describe an instrument whose reading is offset at the bottom of the range…

Control Valve30

Control ValveAir to OpenAir to open means air pressure on the actuator opens the valve and the spring closes it, so the…Control ValveBall ValveA ball valve uses a rotating ball with a bore to control or stop flow; segmented V ball designs…Control ValveButterfly ValveA butterfly valve is a quarter turn valve with a disc that rotates in the flow path; it is…Control ValveCavitationCavitation is the formation and violent collapse of vapour bubbles when liquid pressure drops…Control ValveChoked FlowChoked flow is the condition where flow through a valve stops increasing even though the…Control ValveControl ValveA control valve is a power operated valve that varies flow in response to a signal from a…Control ValveFail Safe PositionFail safe position is the position a valve goes to on loss of air or signal: fail open, fail…Control ValveFlashingFlashing is when liquid partly turns to vapour across a valve because the outlet pressure stays…Control ValveFlow Coefficient (Cv)Flow coefficient Cv is the flow of water in US gallons per minute through a valve with a…Control ValveGlobe ValveA globe valve is a linear motion valve where a plug moves towards or away from a seat; it is…Control ValveI/P ConverterAn I/P converter changes a 4 to 20 mA current signal into a proportional pneumatic signal,…Control ValveInstalled CharacteristicInstalled characteristic is the actual relationship between valve travel and flow once the…Control ValveKv Flow FactorKv flow factor is the flow of water in m³/h through a valve with a pressure drop of 1 bar; it…Control ValveLiquid Pressure Recovery FactorLiquid pressure recovery factor FL describes how much pressure a valve recovers after the vena…Control ValvePartial Stroke TestingPartial stroke testing moves a shutdown valve a small amount, typically 10 to 20 %, to prove it…Control ValvePiping Geometry FactorPiping geometry factor Fp corrects valve capacity for reducers and other fittings attached…Control ValvePressure Differential Ratio FactorPressure differential ratio factor xT is the ratio of pressure drop to inlet pressure at which…Control ValveSeat Leakage ClassSeat leakage class is the maximum allowed flow through a closed valve, defined by ANSI/FCI 70-2…Control ValveSplit Range ControlSplit range control uses one controller output to operate two valves, each over part of the…Control ValveSpring RangeSpring range is the air pressure range over which a spring diaphragm actuator moves through…Control ValveStroke TimeStroke time is the time a valve takes to move from fully open to fully closed, or the reverse,…Control ValveValve ActuatorA valve actuator is the device that produces the force or torque to move a valve, using air…Control ValveValve AuthorityValve authority is the fraction of the total system pressure drop taken by the control valve,…Control ValveValve CharacteristicValve characteristic is the relationship between valve travel and flow coefficient, such as…Control ValveValve PackingValve packing is the seal around the valve stem that stops process fluid leaking out while…Control ValveValve PositionerA valve positioner is a device on the valve that compares the control signal with the actual…Control ValveValve RangeabilityValve rangeability is the ratio of the largest to the smallest flow coefficient over which a…Control ValveValve StictionValve stiction is static friction that makes a valve stick, then jump, instead of moving…Control ValveValve TrimValve trim is the set of internal parts in contact with the flow that control it, such as the…Control ValveVolume BoosterA volume booster is a pneumatic relay that copies the positioner output at much higher air…
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Safety and SIS31

Safety and SISAlarm ManagementAlarm management is the process of designing, rationalising and maintaining alarms so operators…Safety and SISCause and Effect MatrixA cause and effect matrix is a table showing which inputs (causes) trigger which outputs…Safety and SISCommon Cause FailureCommon cause failure is a single cause that makes redundant channels fail together, such as a…Safety and SISDiagnostic CoverageDiagnostic coverage is the fraction of dangerous failures that automatic diagnostics detect.Safety and SISEquipment Protection LevelEquipment protection level shows how unlikely equipment is to become an ignition source,…Safety and SISFailure RateFailure rate is the number of failures per unit time of a device; in safety it is split into…Safety and SISFault Tree AnalysisFault tree analysis is a top down method that models how combinations of basic failures lead to…Safety and SISFlameproof EnclosureA flameproof enclosure contains any internal explosion and cools escaping gases through flame…Safety and SISGas GroupGas group classifies flammable gases by how easily they ignite and how well flames pass through…Safety and SISHardware Fault ToleranceHardware fault tolerance is the number of hardware faults a subsystem can have while still…Safety and SISHazardous Area ZoneA hazardous area zone describes how likely an explosive atmosphere is at a location: Zone 0, 1…Safety and SISHAZOPHAZOP is a structured team review that uses guide words to find hazards and operability…Safety and SISIndependent Protection LayerAn independent protection layer is a safeguard that can prevent a hazardous scenario on its own…Safety and SISIntrinsic SafetyIntrinsic safety is a protection method that limits electrical and thermal energy in a circuit…Safety and SISLayers of Protection AnalysisLayers of protection analysis estimates the frequency of a hazardous event and checks whether…Safety and SISMean Time Between FailuresMean time between failures is the average operating time between failures of a repairable item.Safety and SISPFDavgPFDavg is the average probability that a safety function fails to act when a demand occurs,…Safety and SISPFHPFH is the average frequency of dangerous failures per hour, used for safety functions in high…Safety and SISProcess Safety TimeProcess safety time is the time from a process fault occurring until the hazardous event…Safety and SISProof Test CoverageProof test coverage is the fraction of dangerous undetected failures that a proof test actually…Safety and SISProof Test IntervalProof test interval is the time between periodic tests that reveal dangerous undetected…Safety and SISRisk Reduction FactorRisk reduction factor is how many times a safety function reduces the frequency of a hazardous…Safety and SISSafe Failure FractionSafe failure fraction is the share of all failures that are either safe or dangerous but detected.Safety and SISSafe StateSafe state is the process condition a safety function must reach to prevent the hazardous…Safety and SISSafety BypassA safety bypass is a temporary override that stops a trip or alarm from acting, for…Safety and SISSafety Instrumented FunctionA safety instrumented function is one protective action of a SIS, from sensing a hazard to the…Safety and SISSafety Instrumented SystemA safety instrumented system is an independent set of sensors, logic solver and final elements…Safety and SISSafety Integrity LevelSafety integrity level is a target level of risk reduction for a safety instrumented function,…Safety and SISSafety Requirements SpecificationA safety requirements specification defines what each safety instrumented function must do and…Safety and SISSpurious TripA spurious trip is an unwanted shutdown caused by a safety system fault rather than a real…Safety and SISVoting ArchitectureVoting architecture describes how many of N redundant channels must act to trip, such as 1oo2…

Temperature Measurement29

Temperature MeasurementAlpha CoefficientAlpha coefficient is the average change in RTD resistance per degree between 0 and 100 °C,…Temperature MeasurementBimetallic ThermometerA bimetallic thermometer uses a coiled strip of two bonded metals with different expansion…Temperature MeasurementCold Junction CompensationCold junction compensation corrects a thermocouple reading for the temperature of the terminals…Temperature MeasurementDew PointDew point is the temperature to which air must be cooled for its water vapour to start condensing.Temperature MeasurementDry Block CalibratorA dry block calibrator is a portable heated or cooled metal block with inserts, used to…Temperature MeasurementEmissivityEmissivity is the ratio of radiation emitted by a surface to that of a perfect black body at…Temperature MeasurementInfrared ThermometerAn infrared thermometer, or pyrometer, measures surface temperature from the infrared radiation…Temperature MeasurementInsertion LengthInsertion length is the length of a thermowell or sensor that projects into the process from…Temperature MeasurementITS-90ITS-90 is the International Temperature Scale of 1990, which defines temperature by fixed…Temperature MeasurementLog Mean Temperature DifferenceLog mean temperature difference is the effective average temperature difference between two…Temperature MeasurementMineral Insulated CableMineral insulated cable has sensor wires packed in compressed magnesium oxide inside a metal…Temperature MeasurementNTC ThermistorAn NTC thermistor is a ceramic semiconductor resistor whose resistance falls strongly as…Temperature MeasurementPt100Pt100 is a platinum RTD with a resistance of 100 Ω at 0 °C, following the IEC 60751 resistance…Temperature MeasurementResponse TimeResponse time is how fast a temperature sensor follows a step change, given as a time constant…Temperature MeasurementRTDAn RTD is a resistance temperature detector: a metal element, usually platinum, whose…Temperature MeasurementRTD Self HeatingRTD self heating is the rise in sensor temperature caused by the measuring current flowing…Temperature MeasurementRTD Tolerance ClassRTD tolerance class is the permitted deviation of an RTD from the standard curve, set by IEC…Temperature MeasurementRTD Wiring (2, 3 and 4 Wire)RTD wiring is the 2, 3 or 4 wire connection of an RTD; more wires cancel more of the lead wire…Temperature MeasurementSeebeck EffectThe Seebeck effect is the voltage generated in a conductor along which there is a temperature…Temperature MeasurementTemperature ClassTemperature class is the maximum surface temperature of electrical equipment in a hazardous…Temperature MeasurementTemperature ScalesTemperature scales are the units used to state temperature: Celsius and Fahrenheit for everyday…Temperature MeasurementTemperature TransmitterA temperature transmitter converts an RTD or thermocouple signal into a robust 4 to 20 mA or…Temperature MeasurementThermal ExpansionThermal expansion is the increase in length or volume of a material as its temperature rises.Temperature MeasurementThermocoupleA thermocouple is two different metal wires joined at one end; it produces a small voltage that…Temperature MeasurementThermocouple Extension CableThermocouple extension cable carries the thermocouple signal to the instrument using wires that…Temperature MeasurementThermocouple Tolerance ClassThermocouple tolerance class is the permitted deviation of a new thermocouple from its…Temperature MeasurementThermocouple TypesThermocouple types are letter codes such as J, K, T, N, E, R, S and B, each a standard pair of…Temperature MeasurementThermowellA thermowell is a closed end tube fitted into the process that protects a temperature sensor…Temperature MeasurementWake FrequencyWake frequency is the frequency of vortices shed by flow past a thermowell; if it approaches…
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Analytical26

AnalyticalAnalyser ShelterAn analyser shelter is a ventilated building or cabinet that protects process analysers and…AnalyticalBuffer SolutionA buffer solution has a stable, known pH and resists change, and is used to calibrate pH electrodes.AnalyticalCell ConstantCell constant is the geometry factor of a conductivity cell, the electrode spacing divided by…AnalyticalConductivityConductivity is the ability of a solution to carry electric current, which rises with the…AnalyticalConductivity Temperature CompensationConductivity temperature compensation converts measured conductivity to the value at a…AnalyticalContinuous Emission Monitoring SystemA continuous emission monitoring system measures pollutants and flow in a stack continuously,…AnalyticalDissolved OxygenDissolved oxygen is the amount of oxygen gas dissolved in water, measured in mg/L or as % of…AnalyticalEmission Oxygen CorrectionEmission oxygen correction converts a measured pollutant concentration to a reference oxygen…AnalyticalExcess AirExcess air is the air supplied to a burner beyond the theoretical amount needed for complete…AnalyticalGas ChromatographA gas chromatograph separates a gas or vaporised liquid sample into components in a column and…AnalyticalLower Explosive LimitLower explosive limit is the lowest concentration of a flammable gas in air that can ignite;…AnalyticalNDIR Gas AnalyserAn NDIR gas analyser measures gases such as CO₂, CO and methane from how much infrared light…AnalyticalNernst SlopeNernst slope is the theoretical change in electrode voltage per pH unit, 59.16 mV at 25 °C,…AnalyticalOxidation Reduction PotentialOxidation reduction potential is the voltage that shows whether a solution tends to oxidise or…AnalyticalpHpH is a measure of how acidic or alkaline a water based solution is, based on the hydrogen ion…AnalyticalpH ElectrodeA pH electrode is a sensor with a pH sensitive glass membrane and a reference electrode,…AnalyticalpH Temperature CompensationpH temperature compensation corrects the electrode slope for sample temperature, so the…Analyticalppm to mg/m³ Conversionppm to mg/m³ conversion turns a gas concentration by volume into mass per volume, using the…AnalyticalReference ElectrodeA reference electrode provides a stable voltage against which the pH or ORP measuring electrode…AnalyticalSample Conditioning SystemA sample conditioning system takes a process sample and makes it suitable for the analyser:…AnalyticalSample Lag TimeSample lag time is the delay for a sample to travel from the process tap to the analyser.AnalyticalSpan GasSpan gas is a certified gas mixture with a known concentration, used to set the upper…AnalyticalTotal Dissolved SolidsTotal dissolved solids is the mass of dissolved substances in water, often estimated from…AnalyticalTurbidityTurbidity is the cloudiness of water caused by suspended particles, measured by how much light…AnalyticalWater HardnessWater hardness is the concentration of calcium and magnesium in water, usually expressed as…AnalyticalZirconia Oxygen SensorA zirconia oxygen sensor measures oxygen in hot flue gas from the voltage across a heated…

Flow Measurement32

Flow MeasurementBernoulli's EquationBernoulli's equation states that pressure, kinetic and elevation energy along a streamline add…Flow MeasurementBeta RatioBeta ratio is the bore diameter of an orifice, nozzle or venturi throat divided by the pipe…Flow MeasurementCompressibility FactorCompressibility factor Z corrects the ideal gas law for real gas behaviour; Z = 1 for an ideal gas.Flow MeasurementCoriolis Flow MeterA coriolis flow meter measures mass flow directly from the twist of vibrating tubes, and also…Flow MeasurementDischarge CoefficientDischarge coefficient is the ratio of actual flow to the theoretical flow through a DP flow element.Flow MeasurementElectromagnetic Flow MeterAn electromagnetic flow meter measures the velocity of conductive liquids from the voltage…Flow MeasurementExpansibility FactorExpansibility factor corrects DP flow equations for the drop in gas density as gas expands…Flow MeasurementFlow ConditionerA flow conditioner is a device placed upstream of a meter that removes swirl and restores a…Flow MeasurementFlow TotalizerA flow totalizer adds up flow rate over time to give the total volume or mass that has passed.Flow MeasurementK FactorK factor is the number of pulses a flow meter sends for each unit volume of fluid passing…Flow MeasurementLaminar FlowLaminar flow is smooth flow in parallel layers with no mixing between them, found at low…Flow MeasurementLow Flow CutoffLow flow cutoff forces the flow reading to zero below a set value, so noise and zero drift do…Flow MeasurementMass Flow RateMass flow rate is the mass of fluid passing a point per unit time. It does not change with…Flow MeasurementMeter FactorMeter factor is the ratio of the true volume from a prover to the volume indicated by the meter…Flow MeasurementOrifice PlateAn orifice plate is a thin plate with a precise bore clamped in a pipe; the DP across it is…Flow MeasurementPermanent Pressure LossPermanent pressure loss is the part of the DP across a flow element that is not recovered…Flow MeasurementPitot TubeA pitot tube measures fluid velocity at a point from the difference between total (stagnation)…Flow MeasurementPressure and Temperature CompensationPressure and temperature compensation corrects gas or steam flow for density changes using live…Flow MeasurementReynolds NumberReynolds number is the ratio of inertial to viscous forces in a flowing fluid. It shows whether…Flow MeasurementRotameterA rotameter is a variable area flow meter where a float rises in a tapered tube until drag…Flow MeasurementSquare Root ExtractionSquare root extraction converts a DP signal into a linear flow signal, because flow through a…Flow MeasurementStandard Volume FlowStandard volume flow is gas flow converted to a reference pressure and temperature, so it…Flow MeasurementStraight Pipe RunStraight pipe run is the length of unobstructed pipe needed before and after a flow meter for…Flow MeasurementTurbulent FlowTurbulent flow is chaotic flow with eddies and mixing across the pipe, found at high Reynolds…Flow MeasurementTurndown RatioTurndown ratio is the maximum flow divided by the minimum flow a meter can measure within its…Flow MeasurementUltrasonic Flow MeterAn ultrasonic flow meter measures velocity from the difference in travel time of sound pulses…Flow MeasurementVelocity ProfileVelocity profile is how flow velocity varies across the pipe, from near zero at the wall to a…Flow MeasurementVena ContractaVena contracta is the point just downstream of an orifice where the jet is narrowest and the…Flow MeasurementVenturi TubeA venturi tube is a DP flow element with a smooth converging inlet, a throat and a diverging…Flow MeasurementVolumetric Flow RateVolumetric flow rate is the volume of fluid passing a point per unit time, equal to pipe area…Flow MeasurementVortex Flow MeterA vortex flow meter counts the vortices shed from a bluff body; their frequency is proportional…Flow MeasurementWeirA weir is a barrier with a notch in an open channel; the height of water above the crest gives…
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Level Measurement29

Level MeasurementBlocking DistanceBlocking distance is the zone just below an ultrasonic or radar sensor where it cannot measure,…Level MeasurementBubbler Level MeasurementBubbler level measurement pushes a small gas flow out of a dip tube; the back pressure equals…Level MeasurementCapacitance Level ProbeA capacitance level probe measures level from the change in capacitance as liquid replaces air…Level MeasurementDielectric ConstantDielectric constant is the relative permittivity of a material; it sets how strongly a liquid…Level MeasurementDished HeadA dished head is the curved end of a vessel, such as a 2:1 ellipsoidal or torispherical head,…Level MeasurementDisplacer Level TransmitterA displacer level transmitter measures level from the buoyancy force on a heavy cylinder partly…Level MeasurementDP Level MeasurementDP level measurement uses a differential pressure transmitter between a bottom tap and a top…Level MeasurementDry LegA dry leg is a low side impulse line that stays full of gas, used for DP level when the vapour…Level MeasurementFalse Echo SuppressionFalse echo suppression teaches a radar or ultrasonic transmitter to ignore echoes from fixed…Level MeasurementFloat Level SwitchA float level switch uses a buoyant float that rises and falls with the liquid to operate a…Level MeasurementGuided Wave RadarGuided wave radar sends microwave pulses down a rod, cable or coaxial probe and measures the…Level MeasurementHydrostatic Level MeasurementHydrostatic level measurement finds liquid level from the pressure at the bottom of a tank and…Level MeasurementInterface LevelInterface level is the height of the boundary between two liquids of different densities in a…Level MeasurementLevel BridleA level bridle is an external vertical pipe connected to a vessel at two points, holding level…Level MeasurementLoad Cell Tank WeighingLoad cell tank weighing measures the mass of a vessel contents from load cells under its supports.Level MeasurementMagnetic Level GaugeA magnetic level gauge shows level with coloured flaps turned by a magnetic float inside a…Level MeasurementOverfill ProtectionOverfill protection is the set of alarms, independent level switches and automatic actions that…Level MeasurementPoint Level MeasurementPoint level measurement detects whether liquid or solid is present at one fixed height, giving…Level MeasurementRadar Level TransmitterA radar level transmitter measures level from the travel time of microwaves reflected from the…Level MeasurementRadiometric Level MeasurementRadiometric level measurement uses a gamma source and a detector outside the vessel; the liquid…Level MeasurementSight GlassA sight glass is a glass gauge on a vessel that lets operators see the liquid level directly.Level MeasurementSpecific GravitySpecific gravity is the density of a liquid divided by the density of water at a reference…Level MeasurementStilling WellA stilling well is a vertical perforated pipe inside a tank that calms the liquid surface for…Level MeasurementTank Strapping TableA tank strapping table converts measured level into volume for a specific tank, from its…Level MeasurementUllageUllage is the empty height or volume above the liquid in a tank, measured from the reference…Level MeasurementUltrasonic Level TransmitterAn ultrasonic level transmitter measures level from the time sound pulses take to travel to the…Level MeasurementVessel Holdup TimeVessel holdup time is how long the volume between two levels lasts at a given flow, used to set…Level MeasurementVibrating Fork Level SwitchA vibrating fork level switch detects liquid when the frequency of its vibrating tines drops as…Level MeasurementWet LegA wet leg is a low side impulse line kept full of liquid, used for DP level where vapour would…
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Glossary A to Z

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Frequently Asked Questions

What is this instrumentation glossary?

A free reference of instrumentation and control terms. Each term has its own page with a short definition, a plain explanation, formulas where they apply, typical values, worked examples and links to calculators.

Which topics are covered?

Flow, pressure, level, temperature, control valves, safety and SIS, signals and PLC, calibration and analytical terms, with more added over time.

Can I use these definitions in training?

Yes, for learning and internal training. Please link back to the term page.

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