For the control of gases and liquids in a process, reproducibility is more important than absolute accuracy. But when it comes to the delivery of a certain quantity of product for which there is a charge, good reproducibility is no longer enough. Then one also wants excellent accuracy. The receiving party wants sufficient product for its money. While the selling party wants to get paid enough for the product delivered. Thus, the clearing measurement must be accurate and indicate the true value as best as possible.
Therefore, for transfer measurements (also called calibration or custody transfer flow measurements) accurate instruments are chosen, which are always calibrated. Calibration is the determination of the measurement error of an instrument against a reference. This reference is considered "the truth." This reference is also usually calibrated against a "higher reference". Everything in this calibration chain must be traceable, so that it can be shown that the deviation from the measurement is actually that large. This traceability is monitored by notified bodies. VSL is the notified body in the Netherlands. VSL is the National Metrology Institute of the Netherlands and has been appointed by the Minister of Economic Affairs and Climate Change as an independent and impartial organization to manage and develop the national primary measurement standards. In practice this means that companies that perform calibrations are certified by VSL, whereby both the calibration installation and the method followed are included in the certification.
There are two forms of calibration: "as received" or "as left". In the former, the deviation from the reference is determined. This deviation is documented in the calibration certificate.
In a calibration "as left", the deviation of the instrument from the reference is also determined, but then the instrument is adjusted so that the measurement is within the specification of the instrument. Finally, the flowmeter is again compared to the reference and the final deviation of the adjusted instrument is documented in the calibration certificate.
There are many methods to calibrate the a flowmeter. It depends on the accuracy of the instrument to be specified and, of course, on the flowmeter itself. "Ordinary" liquid flowmeters are sometimes compared to a "master meter". Calibration-worthy liquid flowmeters, on the other hand, are usually calibrated with a certified volume vessel or scale. In the latter case, correction is made for liquid density. And when Coriolis mass flowmeters are calibrated, the scale methodology naturally comes into play. Gas flowmeters are usually compared to accurate turbine flowmeters where pressure and temperature offsets are very important. Gas flowmeters for small flow rates are often calibrated with a primary or secondary standard. Such a primary standard simply consists of a cylinder with piston. There are switching points on the cylinder and the piston passes a precisely known volume between the first and second switching points.
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