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Field inspection manual—volumetric measuring devices

Part 4, section 22—Special instructions—prover wetting and drainage


This procedure applies when using measures, open provers and vapour displacement provers.


Provers (and test measures) are calibrated to deliver, meaning that when they are filled to their nominal capacity, the amount that could be drained from them is that nominal capacity. Wetting the prover eliminates the loss that would occur from test liquid clinging to dry internal surfaces.

Legislative references

Not Applicable.


Fill the prover to its nominal capacity.

Empty it and allow to drain for the appropriate time as listed below or on the Certificate of Calibration issued for that particular prover.

Unless otherwise stated on a Certificate of Calibration, the following drain times are applicable.

Nominal value (litres, or equivalent in gallons) drain time

5 litres or less: 10 seconds

over 5 up to and including 20 litres: 20 seconds

over 20 up to and including 500 litres: 1 minutes

over 500 up to and including 5 000 litres: 2 minutes

over 5 000 up to and including 15 000 litres: 3 minutes

over 15 000 up to and including 30 000 litres: 4 minutes

over 30 000 litres: 5 minutes

Drain time starts when product flow turns from a continuous stream to a drip.

Note: When a prover is used on viscous products, such as heavy oils, liquid cattle feed, etc., inspectors should consult the Volumetric Specialist for actions to take with regard to possible excessive clingage.

Note: Vapour displacement provers must be filled before beginning to perform tests. The purpose of filling the prover is to wet it as for open provers, and also to stabilise its temperature and pressure. The pressure and temperature of the liquid in the prover and in the supply tank must be brought close in order for the tests to be valid. Vapour displacement provers are not drained as open provers are. A zero liquid is established before the tests.


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