Custody transfer - also called fiscal measurement - is the point in an oil or gas operation where product changes ownership and money changes hands based on the measured quantity and quality. Because both parties invoice off the same number, custody transfer measurement is held to far tighter accuracy, calibration, and audit standards than ordinary process measurement.
Custody Transfer in one line: Custody transfer is the metering point where ownership of oil or gas passes between two parties, so the measurement must meet strict, auditable accuracy and proving standards because it directly determines what is bought and sold.
On a process line, a flow reading that is a percent or two off simply means a slightly imperfect control loop. At a custody transfer point, the same error is money - it is the basis for an invoice, a royalty payment, and a tax liability. A small systematic bias multiplied by millions of barrels or MMBtu per year becomes a large financial dispute, so custody meters are specified, installed, calibrated, and proved to recognized standards, and the whole chain is documented for audit.
Custody transfer is distinct from allocation measurement. Allocation meters divide a commingled stream among contributors and can tolerate more uncertainty because errors net out across parties. A custody meter defines the total that gets divided, so it must be the most accurate and best-controlled measurement in the facility.
Liquid custody transfer follows the API Manual of Petroleum Measurement Standards (API MPMS), which governs meter proving, temperature and pressure correction, sampling, and calculation of net standard volume. Gas custody transfer follows AGA reports - AGA 3 for orifice meters, AGA 7 for turbine meters, AGA 9 for ultrasonic meters, and AGA 8 for compressibility - plus API 21.1 for the flow computer's calculation and audit requirements.
Typical custody points include a LACT unit at a crude lease, an orifice or ultrasonic meter run at a gas gathering interconnect, a truck-loading rack, and a pipeline interconnect between operators. Every one relies on a flow computer to apply the standard corrections, store historical calculation parameters, and produce a tamper-evident audit trail. Those flow computers commonly expose their totals over Modbus, so a SCADA platform can trend custody volumes and flag a meter that has stopped proving cleanly.
Custody transfer measures the total quantity changing ownership between two parties and must be highly accurate because it drives invoices. Allocation measurement then divides a commingled total among multiple contributors and can tolerate more uncertainty, since allocation errors net out across the parties sharing the stream.
There is no single figure - it is set by contract and by the governing standard for the meter type. In practice custody meters are held to tight uncertainty budgets (often well under one percent) and, crucially, must be regularly proved or calibrated and fully documented, because provability and audit trail matter as much as the headline accuracy number.
Effectively yes for modern installations. Standards like API 21.1 and AGA 3 require live temperature and pressure correction, historical logging of calculation parameters, and a tamper-evident audit trail - all of which a flow computer provides. SCADA can then read those corrected, standards-compliant totals over Modbus for trending and alarms.
Safety & engineering notice. This article is general educational information, not site-specific engineering, safety, or legal advice, and it does not reflect any particular facility. Standards and regulations (for example OSHA, API, IEC, ISO, NFPA, NIST, and NERC CIP requirements) change and vary by edition, jurisdiction, and application. SCADA and remote monitoring cannot verify physical isolation, atmosphere, lockout/tagout, permit status, or a safe go/no-go decision. Qualified personnel must perform site-specific engineering, hazard analysis, and safety review, and confirm current requirements with the authority having jurisdiction, before acting.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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