The national standard is used to allow traceability back to the SI base unit "metre". The cubic metre of natural gas measured here provides the state-defined basic unit of measurement for high-pressure natural gas. For more information on the national standard, please go to the PTB website.
The national standard serves as a basis for regular calibrations of the pigsar™ working standards. These calibrations are performed using so-called secondary standards (DN100 G250 turbine flowmeters), which are calibrated directly with the piston prover up to pressures of p = 55 bar and flow rates of Qb = 480 m³/h. The extended measurement uncertainty of the piston prover is below 0.01 %, while the measurement uncertainty of the secondary standard is around 0.1 %.
The piston prover and the secondary standard are integrated into the pigsar™ piping to allow the working standards to be easily calibrated at regular intervals. This is one of the reasons for the very low total measurement uncertainty of pigsar™.
The working and transfer standards of other test facilities can also be calibrated against the national standard. Depending on the actual scope, this will either be done directly or indirectly against the secondary standards or the working standards of pigsar™.
Critical nozzles are flow devices used to stabilise the gas flow through the standards while also serving as reference standards. pigsar™ has a total of 8 flow nozzle meter runs which can accommodate a range of different nozzle elements.
Each individual nozzle element is calibrated either directly against the piston prover or against the secondary standards. They are also calibrated with air on PTB’s highly accurate air test facilities. Critical nozzles can also be used to calibrate other test facilities or to compare different test facilities. The diagram shows such an intercomparison with two other test facilities.
More about PTB, Fachbereich 1.43, can be found here.
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