Custody Transfer, Explained
Custody transfer is where measurement becomes money. This guide walks the quantity chain from an observed tank reading to net standard barrels and metric tons, with a worked example.
Custody transfer is any handover where product changes hands, or where responsibility for it moves from one party to another: shore tank to ship, pipeline to tank, terminal to truck. At that moment quantity and quality are measured to a standard both sides accept, because the figures become the basis for payment, duties, freight and any loss claim. A fraction of a percent on a large cargo is real money.
Measurement is never just reading a level. Oil expands and contracts with temperature, carries water and sediment, and sits in tanks whose capacity tables were themselves measured. The industry handles this with a fixed sequence of volumes and corrections set out in the API Manual of Petroleum Measurement Standards (API MPMS) and its international equivalents. Learn the sequence once and every inspection certificate becomes readable.
The quantity chain
Every certificate of quantity climbs the same ladder. You start with everything physically in the tank at the observed temperature and strip away, step by step, whatever is not saleable product at standard conditions. The abbreviations below appear on almost every gauge report and inspection certificate in the trade.
| Step | What it means | How it is derived |
|---|---|---|
| TOV | Total observed volume | Read from the tank capacity table at the gauged level, at observed temperature |
| FW | Free water | Water layer at the tank bottom, found with water-finding paste or an interface probe |
| GOV | Gross observed volume | TOV minus free water, adjusted for shell temperature and any floating-roof correction |
| VCF | Volume correction factor | From API MPMS Chapter 11.1, using density or API gravity and product temperature |
| GSV | Gross standard volume | GOV multiplied by VCF, giving volume at 60°F or 15°C |
| S&W | Sediment and water | Water and solids suspended in the product, found by laboratory test on a representative sample |
| NSV | Net standard volume | GSV multiplied by (1 minus S&W percent) |
API gravity and the VCF in plain terms
API gravity is an inverse density scale: API = 141.5 / SG - 131.5, where SG is specific gravity at 60°F. Fresh water is 10 °API, and the higher the number, the lighter the oil. Outside the US, density at 15°C in kg/m³ does the same job. Either value, together with temperature, is the key to the volume correction.
The VCF converts a volume measured at observed temperature into the volume it would occupy at standard temperature. Warm oil has expanded, so its VCF is below 1.0; cold oil has contracted, so its VCF is above 1.0. Because heavier and lighter liquids expand at different rates, the API MPMS tables come in families: 6A and 54A for crude oils, 6B and 54B for refined products.
Worked example: from tank to metric tons
The figures below are hypothetical and rounded for illustration. Shell-temperature and floating-roof corrections are left out to keep the arithmetic clear. Assume a crude oil of 32.0 °API at 60°F, gauged at an average product temperature of 86°F, with 0.30% S&W by laboratory test, and a weight factor taken for weight in air.
| Line | Calculation | Result |
|---|---|---|
| TOV | From the tank capacity table at the gauged level | 500,000 bbl |
| Free water | Interface reading at the tank bottom | 1,200 bbl |
| GOV | 500,000 - 1,200 | 498,800 bbl |
| VCF (illustrative) | Table 6A, 32.0 °API at 86°F | 0.9881 |
| GSV at 60°F | 498,800 x 0.9881 | 492,864 bbl |
| NSV | 492,864 x (1 - 0.0030) | 491,385 bbl |
| Weight factor (illustrative) | Metric tons per barrel in air at 32.0 °API | 0.1373 |
| Metric tons in air | 491,385 x 0.1373 | 67,467 MT |
Weight in air versus weight in vacuum
Metric tons on a commercial certificate are often stated as weight in air, the figure a weighing in air would register. Weight in vacuum is slightly higher because it removes the effect of air buoyancy; for a medium crude the gap is a little over one-tenth of one percent. Both are legitimate, but they must never be mixed. Compare an in-air load-port figure with an in-vacuum discharge figure and you will find a loss that does not exist.
Outturn versus B/L, and where losses hide
The bill of lading (B/L) quantity is usually based on shore-tank or meter figures at the load port. Outturn is the quantity measured into shore tanks at discharge. The two rarely agree exactly. Clingage on tank and ship walls, ROB (cargo remaining on board after discharge), line-fill differences, temperature errors and the accuracy of each capacity table all play a part.
Inspectors reconcile the figures with a voyage analysis: shore figures at both ends, ship figures after loading and before discharge, OBQ (on-board quantity before loading) and ROB, and the vessel experience factor (VEF), which compares the ship's past measurements with shore figures. Sales contracts commonly name a loss tolerance and state whether payment is on B/L or outturn quantity.
- Same reference temperature at both ends: 60°F or 15°C
- Same table family and edition for the VCF
- Same S&W method and the same treatment of free water
- Line condition, full or empty, confirmed before and after transfer
- Weight in air compared with weight in air, never with weight in vacuum
How it works
- 1Line and tank check
Our terminal team and the appointed inspector confirm line condition and seal the valves that must not move.
- 2Opening gauge
Once the tank has settled, level, temperature and free water are measured and samples are drawn.
- 3Transfer
Product moves under continuous ATG monitoring and, where the contract specifies one, a proved custody meter.
- 4Closing gauge
The same measurements are repeated on the same tank, by the same method, under the same conditions.
- 5Certificates
The inspector issues quantity and quality certificates, which we reconcile against our records and the ship's figures.
Frequently asked questions
Who appoints the independent inspector?
The sale contract usually decides. Often buyer and seller appoint one inspector jointly and share the cost; sometimes each appoints its own. We give appointed inspectors access to tanks, lines, samples and our gauging records, and our terminal staff witness their measurements.
Why does my NSV differ from the shipper's figure?
Check the basis first: reference temperature, table family, S&W method and the treatment of free water. Most disagreements come from comparing figures prepared on different bases, or from weight in air set against weight in vacuum, rather than from product actually going missing.
Is a meter more accurate than tank gauging?
A well-maintained meter that is proved regularly generally gives lower measurement uncertainty than gauging a large tank, because a small level error is multiplied across a wide tank diameter. Many contracts still default to shore-tank figures, so the contract decides which governs.
What loss tolerance is normal?
There is no single industry figure. Tolerances depend on the product, voyage length, measurement method and the parties' bargaining positions, and they are written into the sale contract or the storage agreement. What matters is that the clause states the measurement basis, the tolerance and who pays the inspector.
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Read moreQuestions about a figure on your certificate?
Our terminal team will walk through it with you line by line, from TOV to metric tons.