Custody Transfer Measurement

The API MPMS chapters we work to

Each chapter answers one question: how much does the tank hold at each level, how deep is the liquid, how much passed the meter, how hot was it, what is in it and what does that make at standard conditions. A quantity is only as good as its weakest step, so temperature and sampling get the same discipline as the gauge reading.

Automatic or manual gauging

An automatic tank gauge (ATG), usually radar or servo, reads level continuously and feeds inventory systems and level alarms. Manual gauging with a calibrated tape and bob, under API MPMS Chapter 3.1A, remains the reference method, and many contracts still require it at custody transfer. We use ATGs for operations and check them against manual gauges on a set schedule.

Gauges are taken as innage, the depth of liquid above the datum plate, or ullage, the empty space from the reference point down to the liquid surface. Free water at the bottom is found with water-finding paste or an interface probe. Readings are repeated until they agree within the tolerance the standard sets.

From gauge to net standard volume

Petroleum expands as it warms, so an observed volume means little until it is corrected to standard temperature: 60 °F in US practice, 15 °C in most other markets. API MPMS Chapter 12.1 sets the sequence shown in the steps below. For clean products, gross and net standard volume are usually the same; crude is invoiced on net standard volume, after sediment and water are deducted.

Barrels and metric tons

US trade quotes barrels; much international trade quotes metric tons. The conversion depends on density, so each cargo is converted from its own measured density through the standard tables, never from a rule of thumb. The averages below are for planning only. For example, 50,000 MT of Jet A-1 is roughly 394,000 barrels at a typical jet density.

Meters, provers and independent inspection

Pipeline and rack movements are measured by custody meters under API MPMS Chapter 5 and proved under Chapter 4 to set a meter factor. Proving frequency and acceptance limits are agreed by contract. Automatic samplers under Chapter 8.2 take a flow-proportional sample through the whole transfer, so the sample represents the whole parcel.

Either party may appoint an independent inspection company, of the kind buyers commonly use such as SGS, Intertek or Bureau Veritas, to witness or perform gauging, sampling and calculation. We give the appointed inspector access, calibration tables, meter and prover records and retained samples. Where shore and ship figures differ, the vessel experience factor under Chapter 17.9 helps explain the gap.