Custody transfer is the point in the petroleum supply chain where ownership of oil changes from one party to another—for example, from a producer to a pipeline operator, or from a terminal to a tanker truck. At these transfer points, accurate and tamper-proof measurement is essential because the financial value of the transaction is directly tied to the measured quantity. Flowmeters used in custody transfer applications must meet strict performance standards defined by industry bodies such as the American Petroleum Institute (API), the International Organization of Legal Metrology (OIML), and national weights-and-measures authorities.
Custody transfer flowmeters are typically required to achieve a very high level of accuracy, often within ±0.1% to ±0.25% of the actual flow quantity. To achieve this, they are carefully selected, installed, and calibrated according to approved procedures. Common technologies used in oil custody transfer include Coriolis mass flowmeters, turbine flowmeters, positive displacement meters, and ultrasonic flowmeters, depending on the product and flow conditions. Proving systems—known as pipe provers or compact provers—are used in the field to verify the meter's accuracy on a regular basis against a certified volume reference.
In addition to meter accuracy, custody transfer systems include proving connections, temperature and pressure compensation, flow computers, and audit trail logging to ensure the integrity of every transaction. Flow computers apply corrections for oil density changes due to temperature and pressure, converting measured volumes to standard conditions (typically 15°C or 60°F) as required by trading contracts. The entire metering system is typically subject to regulatory inspection and approval by government weights-and-measures authorities. Proper design, installation, and ongoing verification of custody transfer metering systems are essential to fair commerce and regulatory compliance in the oil industry.