Primary standards for gas flow metering & calibration

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Primary standards for gas flow metering & calibration

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Primary standards for gas flow metering & calibration

  1. 1. Primary standards for gas flow metering & calibration NIDHIN MANOHAR
  2. 2. calibration  Correlation of readings of any equipment with a standard of known uncertainty
  3. 3. Why calibration of Flowmeters Initial calibration by manufacturers  To confirm uncertainty by users  Improvement of uncertainty  After major repairs  Joint calibration under dispute  Legal/contractual obligations 
  4. 4. Calibration Methods Weighing Methods  Comparison Method using Reference flowmeters  Estimation using geometrical parameters 
  5. 5. Basic types of flow standards Primary standards  Secondary standards  Working standards 
  6. 6. Primary standards A flow measuring system in which the flow of a fluid is established to within a known degree of uncertainty by the application of “First principles” of Physics.  Eg :Gravimetric system  Basic Parameters:Length,Mass,Time 
  7. 7. Primary parameters in gas flow Length  Mass  Time  Volume 
  8. 8.  Though Volume is a derived quantity,it is considered as a primary measure in gas flow measurements.  Eg : Provers
  9. 9.  Mass of a quantity of gas is relatively small when compared to liquids.Hence it is easier to accurately determine the volume rather than it’s mass.
  10. 10.  Common primary methodsfor gas flow are Volume- Time based systems  Mass flowrate:Vol.flowrate * Density
  11. 11. Reference standards (gas flow) A flowmeter having good long term repeatability but for which an experimental calibration is required.  Eg: Sonic nozzles  Turbine flowmeter  PD meters etc 
  12. 12. Traceability  Traceability is the method of establishing the calibration chain with National Standards.
  13. 13. Uncertainty  An estimate characterizing the range of values within which the true value of a measurand lies
  14. 14. Pulley Cross bar Strips Resistance temperature detector Limit switch Bell Support column RTD indicator Digiruler display Tank Digiruler Scale Digiruler sensor head Blower S.S. pipe Data Acquisition System Medium : Air Range : 0 to 40 m3/h Uncertainty : ±0.2% Make : SIM Brunt, Italy Solenoid valve Ball valve Solenoid valve Pressure connector BELL PROVER
  15. 15. Pulley Cross bar Strips Resistance temperature detector Support column P - Pressure Bell Limit switch t - Test T - Temperature Rh - Relative humidity V1 - Flow control valve RTD indicator V2 - On/Off valve Digiruler display Tank Digiruler Scale Digiruler sensor head Data Acquisition System Blower S.S. pipe V2 Pt V1 Tt Flexible hose Diaphragm gas meter (meter under calibration) Solenoid valve Ball valve Pressure connector Solenoid valve DIAPHRAGM GAS METER UNDER CALIBRATION To atmosphere Rh
  16. 16. Nitrogen at 4 bar (valve operation) CALIFLOW A200 Mass flow controller 50000 sccm Pressure regulator Mass flowmeter calibrator (Test meter) 5000 sccm Temperature 500 sccm 50 sccm Power supply Computer Signal conditioner A200 interface Nitrogen cylinder Cylinder pressure Schematic of calibration setup Key board Printer
  17. 17. Soap film burettes It contains an accurately calibrated burette of Known volume  At the bottom of the burette is a flexible bulb filled with soap solution  At the bottom there is a gas inlet tube  Time taken by the soap film to travel a known volume of burette is noted  Estimate flow rate (V/t) 
  18. 18. Application of SFB’s Very small flow rates  Especially for leakage flow measurements  Suitable for small operating pressures,then only soap film will be stable & film will not break.  Only for air/gas applications. 
  19. 19. Calibration of SFB To be done by gravimetric methods using distilled water of known properties & precision balances.  Accuracy of SFB’s :0.1-0.2 % 
  20. 20. P - Pressure T - Temperature RTD - Resistance Temperature Detector T1 to T12 T,P BFV - Pneumatically actuated butterfly valve RTD with indicator RTD / Pressure on rear side FS - Flow straightener T10 Rh- Relative humidity T9 Compact capacitance gauge (Dual gauge) T8 P Multifunction pressure indicator T7 Po Multifunction pressure indicator T6 P1,P2 T11,T12 P3 Gate valve (25NB) T5 To T4 T3 Nozzle FS T2 Rh Critical Flow Venturi BFV (100NB) T1 Nozzle assembly Vacuum pump Volume Vessel PVTt facility for calibration of Critical Flow Venturi Nozzles
  21. 21. PVT t Method Volume of tank :Approx: 2m3  Sonic Nozzles upto 90 m3/hr can be calibrated in this facility.  Larger collection time ensures greater accuracy.  Ensure sonic flow during collection.  It is a primary method of calibration. 
  22. 22. Gravimetric gas flow standard A system that weighs the collected gas measures the mass flow rate without having to perform density corrections  Direct weighing systems are more accurate than volumetric systems  But due to the relatively low density of most gases,very large quantities of gas must be collectedfor an accurate mass measurement. 
  23. 23. Schematic of Primary Standard Gravimetric System
  24. 24. Weighing balances Although there are electronic electronic balances that can detect microgram or nanogram ,they are limited in capacity to a few grams or milligrams.  Weighing vessel itself will be very heavy.  Weight of gas collected will be very less compared to the weight of empty vessel.  Hence weighing gas flow standards are generally used for special applications. 

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