AGA 3/7/9 & ISO 5168 Measurement Uncertainty
k=2 for 95% confidence, k=3 for 99.7%
Understand measurement uncertainty principles, ISO GUM methodology, AGA report requirements, and financial impact analysis
Custody transfer uncertainty is the combined measurement error of all instruments in a metering station at the point where gas or liquid ownership changes hands. It is calculated per ISO 5168 or AGA Report methodology by combining individual instrument uncertainties using root-sum-square (RSS) method, then expanded to 95% confidence level using a coverage factor of 2.
For natural gas custody transfer, typical contractual limits are 0.5-1.0% expanded uncertainty at 95% confidence. AGA Report No. 3 (orifice) achieves about 0.5-1.0%, AGA 7 (turbine) about 0.25-0.5%, and AGA 9 (ultrasonic) about 0.1-0.3%. Lower uncertainty directly reduces financial exposure for both buyer and seller.