Primary standards for gas flow Primary standards for gas flow metering & calibrationmetering & calibration

NIDHIN MANOHARNIDHIN MANOHAR

calibrationcalibration

Correlation of readings of any equipment Correlation of readings of any equipment with a standard of known uncertaintywith a standard of known uncertainty

Why calibration of FlowmetersWhy calibration of Flowmeters

Initial calibration by manufacturersInitial calibration by manufacturers To confirm uncertainty by usersTo confirm uncertainty by users Improvement of uncertaintyImprovement of uncertainty After major repairsAfter major repairs Joint calibration under disputeJoint calibration under dispute Legal/contractual obligationsLegal/contractual obligations

Calibration MethodsCalibration Methods

Weighing MethodsWeighing Methods Comparison Method using Reference Comparison Method using Reference

flowmetersflowmeters Estimation using geometrical parametersEstimation using geometrical parameters

Basic types of flow standardsBasic types of flow standards

Primary standardsPrimary standards Secondary standardsSecondary standards Working standardsWorking standards

Primary standardsPrimary standards

A flow measuring system in which the flow A flow measuring system in which the flow of a fluid is established to within a known of a fluid is established to within a known degree of uncertainty by the application of degree of uncertainty by the application of “First principles” of Physics.“First principles” of Physics.

Eg :Gravimetric systemEg :Gravimetric system Basic ParametersBasic Parameters:Length,Mass,Time:Length,Mass,Time

Primary parameters in gas flowPrimary parameters in gas flow

LengthLength Mass Mass Time Time VolumeVolume

Though Volume is a derived quantity,it is Though Volume is a derived quantity,it is considered as a primary measure in gas considered as a primary measure in gas flow measurements.flow measurements.

Eg : ProversEg : Provers

Mass of a quantity of gas is relatively small Mass of a quantity of gas is relatively small when compared to liquids.Hence it is easier when compared to liquids.Hence it is easier to accurately determine the volume rather to accurately determine the volume rather than it’s mass.than it’s mass.

Common primary methodsfor gas flow are Common primary methodsfor gas flow are Volume- Time based systemsVolume- Time based systems

Mass flowrate:Vol.flowrate * DensityMass flowrate:Vol.flowrate * Density

Reference standards (gas flow)Reference standards (gas flow)

A flowmeter having good long term A flowmeter having good long term repeatability but for which an experimental repeatability but for which an experimental calibration is required.calibration is required.

Eg: Sonic nozzlesEg: Sonic nozzles Turbine flowmeterTurbine flowmeter PD meters etcPD meters etc

TraceabilityTraceability

Traceability is the method of establishing Traceability is the method of establishing the calibration chain with National the calibration chain with National Standards.Standards.

UncertaintyUncertainty

An estimate characterizing the range of An estimate characterizing the range of values within which the true value of a values within which the true value of a measurand liesmeasurand lies

Solenoid valve

Digiruler

Tank

S.S. pipe

Support columnBell

Resistance

detectortemperature

Pressure

Digirulerdisplay

connector

Range : 0 to 40 m /h

Make : SIM Brunt, Italy

Uncertainty : ±0.2%

Medium : Air3

Data Acquisition System

RTD indicator

ScaleDigirulersensor head

Limit switch

Solenoid valveBall valve

Blower

Pulley

Strips

Cross bar

BELL PROVER

Data Acquisition System

Diaphragm gas meter

Rh

Rh - Relative humidity

V1

V2

V1 - Flow control valve

V2 - On/Off valveRTD indicator

ScaleDigirulersensor

Limit switch

Solenoid valveBall valve

Blower

Pulley

Strips

Cross bar

Solenoid valve

DigirulerTank

head

S.S. pipe

Support columnBell

Resistance

detectortemperature

Pressure

Digirulerdisplay

connector

Flexible hose

T

Toatmosphere

(meter under calibration)

P - Pressure

T - Temperature

Pt t

t - Test

DIAPHRAGM GAS METER UNDER CALIBRATION

(Test meter)

Mass flowmeter

Nitrogencylinder

Pressureregulator

5000 sccm

50000 sccm

500 sccm

50 sccm

Mass flow controller calibrator

Power supply

Signalconditioner

A200interface

Cylinder pressure

Computer

Key board

Printer

Schematic of calibration setup

CALIFLOW A200

Temperature

Nitrogen at 4 bar (valve operation)

Soap film burettesSoap film burettes

It contains an accurately calibrated burette It contains an accurately calibrated burette of Known volumeof Known volume

At the bottom of the burette is a flexible At the bottom of the burette is a flexible bulb filled with soap solutionbulb filled with soap solution

At the bottom there is a gas inlet tubeAt the bottom there is a gas inlet tube Time taken by the soap film to travel a Time taken by the soap film to travel a

known volume of burette is notedknown volume of burette is noted Estimate flow rate (V/t)Estimate flow rate (V/t)

Application of SFB’sApplication of SFB’s

Very small flow ratesVery small flow rates Especially for leakage flow measurementsEspecially for leakage flow measurements Suitable for small operating pressures,then Suitable for small operating pressures,then

only soap film will be stable & film will not only soap film will be stable & film will not break.break.

Only for air/gas applications.Only for air/gas applications.

Calibration of SFBCalibration of SFB

To be done by gravimetric methods using To be done by gravimetric methods using distilled water of known properties & distilled water of known properties & precision balances.precision balances.

Accuracy of SFB’s :0.1-0.2 %Accuracy of SFB’s :0.1-0.2 %

Nozzle assembly

Critical Flow Venturi

Multifunction

T1 to T12 RTD with indicatorBFV - Pneumatically actuated

butterfly valve

(100NB)

T,P

Vacuumpump

pressureindicator

ToGate valve (25NB)

BFV

T10

T9

T8

P1,P2T11,T12

T5T4

T3

T2

T1

T6

T7

P3

P - Pressure

T - Temperature

RTD / Pressure on rear side

Rh

Nozzle

Compact capacitancegauge (Dual gauge)

RTD - Resistance Temperature Detector

Rh- Relative humidity

FS - Flow straightener

FS

PVTt facility for calibration of Critical Flow Venturi Nozzles

Volume Vessel

Po

Multifunctionpressureindicator

P

PVT t MethodPVT t Method

Volume of tank :Approx: 2m3Volume of tank :Approx: 2m3 Sonic Nozzles upto 90 m3/hr can be Sonic Nozzles upto 90 m3/hr can be

calibrated in this facility.calibrated in this facility. Larger collection time ensures greater Larger collection time ensures greater

accuracy.accuracy. Ensure sonic flow during collection.Ensure sonic flow during collection. It is a primary method of calibration.It is a primary method of calibration.

Gravimetric gas flow standardGravimetric gas flow standard

A system that weighs the collected gas A system that weighs the collected gas measures the mass flow rate without having measures the mass flow rate without having to perform density correctionsto perform density corrections

Direct weighing systems are more accurate Direct weighing systems are more accurate than volumetric systemsthan volumetric systems

But due to the relatively low density of But due to the relatively low density of most gases,very large quantities of gas must most gases,very large quantities of gas must be collectedfor an accurate mass be collectedfor an accurate mass measurement.measurement.

Schematic of Primary Standard Gravimetric System

Weighing balancesWeighing balances

Although there are electronic electronic Although there are electronic electronic balances that can detect microgram or balances that can detect microgram or nanogram ,they are limited in capacity to a nanogram ,they are limited in capacity to a few grams or milligrams.few grams or milligrams.

Weighing vessel itself will be very heavy.Weighing vessel itself will be very heavy. Weight of gas collected will be very less Weight of gas collected will be very less

compared to the weight of empty vessel.compared to the weight of empty vessel. Hence weighing gas flow standards are Hence weighing gas flow standards are

generally used for special applications.generally used for special applications.