How to interpolate a tank gauge
Tank capacity tables (strapping tables) list volume at fixed steps, usually every inch or every centimetre. A gauge like 24 ft 3 5/16 in falls between two entries, so the volume is found by linear interpolation between them.
Worked example, feet and inches
Observed gauge 24-3-5/16. The table gives 15,230.47 bbl at 24-3 and 15,283.12 bbl at 24-4.
Factor = 5/16 ÷ 16/16 = 0.3125
Volume = 15,230.47 + (15,283.12 − 15,230.47) × 0.3125 = 15,246.92 bbl
The same gauge in decimal form is 24 × 12 + 3 + 0.3125 = 291.3125 in, or 24.2760 ft, or 7,399.3 mm.
Worked example, metric
Observed gauge 7,437 mm. The table gives 2,418.605 m³ at 7,430 mm and 2,421.889 m³ at 7,440 mm.
Factor = 7 mm ÷ 10 mm = 0.7
Volume = 2,418.605 + (2,421.889 − 2,418.605) × 0.7 = 2,420.904 m³
From TOV to net standard volume
The quantity tab follows the order in API MPMS Chapter 12.1.1 for upright cylindrical tanks.
GOV = [(TOV − FW) × CTSh] ± FRA
GSV = GOV × VCF
NSV = GSV × (1 − S&W% ÷ 100)
CTSh corrects the steel shell for temperature: CTSh = 1 + 2αΔT + α²ΔT², where ΔT is the shell temperature minus the table's base temperature. For an uninsulated tank the shell temperature is (7 × liquid temp + ambient temp) ÷ 8. Mild carbon steel uses α = 0.0000062 per °F (0.0000112 per °C).
VCF brings the oil to 60 °F or 15 °C using the API MPMS 11.1 (1980) equations behind Tables 6A, 6B, 54A and 54B: VCF = exp[−αΔT(1 + 0.8αΔT)], where α depends on the product's base density. The 2004 edition of the standard can differ in the fifth decimal, so confirm critical custody transfer figures against the edition named in your contract.
Weight in air uses an air buoyancy of 1.1 kg/m³: metric tons = volume at standard temperature × (density − 1.1) ÷ 1000.
Questions inspectors ask
Why is a bare number read as sixteenths?
On a phone keyboard the slash is hard to reach. Typing 24 3 5 gives 24 ft 3 5/16 in, which is how most tapes are read.
Should the upper and lower volumes come from the same table page?
Yes. Use adjacent entries from the same tank's certified capacity table. If your table is in 1/4″ or 1/8″ steps, change the table interval so the brackets match.
Which commodity table should I pick?
A for crude oils, B for refined products (gasoline, jet, diesel, fuel oils). B picks the product group from the density automatically. Use C when a lab gives you a specific thermal expansion coefficient, for example lube oils or chemicals.
Does anything leave my device?
No. All calculations run in your browser, and your last inputs are kept only in this browser.