What Type of Prover Water Draw is Best?

Volumetric or Gravimetric?

Since inception, the API Petroleum Measurement Manuals were written to design and define test solutions to reduce the uncertainty and improve the accuracy of all petroleum measurements. As technologies evolve, so do the written standards.
The continuous growth and use of Unidirectional Captive Displacement Provers has been influential in the adoption of these changes. These Provers have smaller displaced volumes, hence the generic name Small Volume Provers (SVPs).

When using volume for water draw certification of an SVP, can sizes (from 5 gallons to 130 gallons displaced volumes used for 1,000 BPH to 28,500 BPH flow rates respectively) are much smaller than previously required for ball type displacement provers. The uncertainty in the measurement of volume varies with the size of the can. The smaller the can size, the greater the uncertainty of the measured volume. Additional uncertainties can stem from reading the calibrated cylinder, the temperature and other variables required to correct to the actual volume measured.

With the advent of new high-resolution electronic weigh scales, and defined gravimetric calibration procedures, field gravimetric water draw calibration on SVP’s have become common in the industry. The uncertainty of this measurement is fixed and defined by a scale and is influenced by the buoyancy due to pressure in altitude changes.

When comparing gravimetric to volumetric water draws, results show a dramatic improvement (decrease) in uncertainty when the gravimetric method is used. This decreased uncertainty becomes more pronounced as the Prover volumes get smaller.

Manufacturers of SVP’s have recognized the importance of accurate volume calibrations and employed the gravimetric method in their final acceptance testing for years.

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