What is custody transfer uncertainty?

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.

What is an acceptable custody transfer uncertainty?

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.

Custody Transfer Uncertainty Calculator

Meter Configuration

Meter Type

Flow Rate

MSCF/d

Gas Price

$/MMBTU

Heating Value

BTU/SCF

Primary Measurement Uncertainties

Differential Pressure (DP) Uncertainty

Static Pressure Uncertainty

Temperature Uncertainty

Secondary Measurement Uncertainties

Gas Composition Uncertainty

Calibration Uncertainty

Installation Effect

Coverage Factor (k)

k=2 for 95% confidence, k=3 for 99.7%

Understanding Measurement Uncertainty

Combined Uncertainty
Individual instrument uncertainties are combined using the root-sum-square (RSS) method per ISO 5168, assuming independent error sources.

Expanded Uncertainty:
U = k × u_c where k is the coverage factor (typically 2 for 95% confidence level per ISO/GUM).

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Formula

U = k × √(∑ u_i²)

U = Expanded uncertainty (%)

k = Coverage factor (2 for 95%)

u_i = Individual standard uncertainties

RSS = Root-sum-square combination

Standards & References

AGA Report No. 3
Orifice Metering of Natural Gas
AGA Report No. 7
Measurement of Gas by Turbine Meters
AGA Report No. 9
Measurement of Gas by Multipath Ultrasonic Meters
ISO 5168
Measurement of Fluid Flow — Estimation of Uncertainty
ISO/IEC Guide 98-3 (GUM)
Guide to the Expression of Uncertainty in Measurement

Engineering Notes

Coverage factor: k=2 provides approximately 95% confidence interval for normally distributed errors
Typical targets: Custody transfer stations aim for <0.5% expanded uncertainty
Financial impact: 0.1% uncertainty on a 100 MMSCF/d station at $3/MMBTU is ~$111k/year
Calibration: Regular calibration reduces systematic uncertainty. Follow AGA schedules.

Custody Transfer Uncertainty Calculator

Meter Configuration

Primary Measurement Uncertainties

Secondary Measurement Uncertainties

Understanding Measurement Uncertainty

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Results

Formula

Standards & References

Engineering Notes

Related Calculators

Meter Proving

Gas Density

Supercompressibility

Orifice Flow