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Working out how much cargo is actually in the tanks is one of the tanker cargo officer's core duties at every load and discharge. It is where money, custody transfer, and pollution accounting all meet: a mistake feeds straight into the Bill of Lading, the ship/shore figure dispute, and any claim of short-loading or contamination. This page walks through the chain from a raw gauge to a net standard volume, and the reconciliation tools — the Vessel Experience Factor, and OBQ/ROB — that surround it.
The officer gauges each tank — usually the ullage (the empty space above the liquid) rather than the sounding — along with the cargo temperature and, separately, the free water at the bottom (by water-finding paste or interface detector). The ullage is looked up in the tank's calibration table to give a volume, which is then corrected for the ship's trim and listat the moment of gauging using the table's trim/list correction columns. The result is the Total Observed Volume (TOV); deducting free water gives the Gross Observed Volume (GOV) of cargo.
Because oil expands with temperature, the observed volume is corrected to a standard reference temperature (15°C or 60°F) with a Volume Correction Factor (VCF)from the ASTM D1250 / API MPMS petroleum-measurement tables, using the product's density and observed temperature. This gives the Gross Standard Volume (GSV); subtracting sediment and water yields the Net Standard Volume (NSV) — the quantity of pure cargo. Multiplying by density (and a weight-correction factor for the buoyancy of air) converts volume to weight in air or in vacuum, the figures that appear on cargo documents.
Ship and shore figures almost never match exactly — different gauging, tables, and reference systems are used. The Vessel Experience Factor (VEF)is the running average of the ratio of the ship's total calculated volume to the shore Bill of Lading quantity across recent voyages. Applying it produces an expected reconciled quantity; a load or discharge that falls outside the expected tolerance is a signal to investigate and, if needed, issue a letter of protest before signing documents.
Two related measurements bracket the voyage. On-Board Quantity (OBQ) is the oil, water, and sediment already in the cargo tanks before loading; Remaining On Board (ROB) is what is left after discharge. Both are gauged and reported so the true quantity transferred can be established and any excessive retention (clingage, non-pumpable oil, or wax) is documented — see OBQ / ROB. Accurate OBQ/ROB protects the ship against inflated short-delivery claims.
Ullage is the empty distance from the liquid surface up to a fixed reference point at the tank top; sounding is the depth of liquid from the surface down to the tank bottom (or datum). Tankers generally work in ullage. Whichever is measured, it is converted to a volume using the tank's calibration table, then corrected for the ship's trim and list at the time of the gauge.
Oil expands and contracts with temperature, so a volume measured in the tank (the gross observed volume) has to be corrected to a standard reference temperature — 15°C or 60°F — to be comparable and to match the shore figure. The Volume Correction Factor (VCF), taken from the ASTM/API petroleum measurement tables using the product's density and temperature, converts observed volume to Gross Standard Volume; deducting free water and sediment gives Net Standard Volume.
Ship-measured and shore-measured quantities rarely agree exactly. The Vessel Experience Factor (VEF) is the historical average ratio of the ship's total calculated volume to the shore Bill of Lading figure over many voyages. Applying the VEF to a load figure gives an expected, reconciled quantity and helps flag an abnormal apparent loss or gain that warrants a letter of protest.