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The overall uncertainty (total allowable total error in measurement) for a custody transfer oil metering system is

typically around +/- 0.25%. Gas orifice systems aim to be around +/-1.0%.

The calculations to arrive at the exact figures are usually quite complex, and take account of individual instrument and

measurement element errors, together with possible errors in their calibration caused by the uncertainty of the test equipment

used. Ambient and operational conditions, such as normal operational flow rate, temperature, andpressure also have to

be taken account of when calculating a total uncertainty budget.

We have on occasions seen some rather simplistic uncertainty calculations, even some from system vendors. HMS can review and check the uncertainty calculations for new metering systems. Or prepare them for existing metering systems, where perhaps the operators are looking at upgrade options.