Ullage report and sounding tables: from tape to volume

A sounding table is the calibration table of one tank: it turns a measured liquid level into a volume. The surveyor measures the sounding from the bottom up or the ullage from the top down, corrects the reading for trim and list, and reads the volume from the table. The ullage report records those readings tank by tank.

Sounding and ullage: two ways to measure one level

Ullage is the free space above the cargo; sounding (or dipping) is the height of the liquid. Either can measure a tank. On seagoing ships ullage is the usual method: a ship trims and lists, and a correction is easier to apply to the free space from a fixed point at the tank top than to a tape lowered all the way down. On shore tanks and tank barges sounding is the usual method and the more precise one. Many newer terminals use tank radar or an automatic gauging system that reports the level directly, which the surveyor checks together with the terminal.

FOSFA's Code of Practice for Member Superintendents (revised November 2022) defines both. A dip, or sounding, is the depth from the liquid surface to the calibrated zero at the tank bottom. Ullage is the space above the liquid, measured from the surface to a fixed datum at the tank top. The total height, also called the reference height, is the calibrated distance from the bottom datum to that reference point. At one gauge point, sounding = reference height − ullage.

For shore tanks the code prefers a dip with a steel tape and dip weight, with ullage as the alternative. For ship's tanks it recommends ullaging with a steel tape and ullage rule from a marked datum; for free water it notes the added problem aboard of dipping to a known bottom. The NIOP Trading Rules (effective 17 April 2025) leave the choice to the calibration table: gauge by outage (ullage) or innage (sounding) as the table prescribes, at the points it names, and note the reference height before an innage gauge so the tape goes down to the exact height. How the same quantity is measured in a shore tank, from calibration to the ship/shore difference, is covered in shore tank measurement and tank gauging.

Ballast tanks are sounded through their sounding pipes. The UNECE draught survey code (1992) requires a calibrated steel tape and water-finding paste and no entries such as "full" or "empty", and at exposed berths recommends at least three soundings (or a float), averaged.

What a sounding table is, and why it fits one tank only

The table is entered with the level at the gauge point and returns the volume at that level, with the ship on even keel. It is derived from that tank's geometry, including whatever takes up space inside it, and from one reference point; used for another tank or hatch, it gives a wrong volume. The UNECE code calls them the ship's tank sounding tables, NIOP calibration tables. FOSFA's code lists calibration data and ullage references among the information obtained from the ship.

FOSFA's code notes that a ship's calibration is generally based on the builder's drawings, so its accuracy may be limited. NIOP requires each storage tank to be calibrated by a recognised authority, with a certified table, at construction or placement, and to be recalibrated if the tank shows a history of differences. Differences between ship's and shore figures over a series of voyages are the subject of the vessel experience factor.

Trim and list corrections to the sounding table

A liquid surface stays level while the tank tilts with the ship. With trim by the stern, a gauge hatch aft of the middle of the tank shows a smaller ullage than the tank would show on even keel, and a hatch forward of the middle a larger one. List works the same way across the tank. The table assumes even keel, so the reading is corrected first. FOSFA's code has draughts, list and trim noted at the time of ullaging and the ship's figures adjusted; NIOP puts the corrections on the gauge reading.

Trim corrections come from tables supplied with the tank tables. The UNECE code has the surveyor record whether they apply to the soundings or to the volumes, since tables do it either way; apply the correction as, and with the sign, the table gives. For a draught survey the code also sets limits: a list of no more than half a degree, and a trim within the range of the tank tables' trim corrections, failing which the surveyor stops until the ship is re-trimmed. For ballast tanks the code adds that a small quantity at a large trim can form a wedge that does not cover the whole bottom; without suitable tables, its volume must be worked out geometrically.

Worked example: interpolating in a sounding table

The tank and all figures are invented for this example and come from no existing ship. Tank 3 port, reference height 14.200 m, even-keel ullage table (extract):

Ullage (m)	Volume (m³)
1.30	1,932.050
1.40	1,915.780
1.50	1,899.410
1.60	1,883.010

1. Observed ullage at the gauge point: 1.432 m. List 0.0°, trim 1.00 m by the stern.
2. The gauge hatch lies aft of the tank's middle, so the reading is too small; the tank's trim table gives +0.028 m at this trim. Corrected ullage = 1.432 + 0.028 = 1.460 m, which equals a sounding of 14.200 − 1.460 = 12.740 m.
3. 1.460 m lies between the 1.40 and 1.50 lines. Fraction = (1.460 − 1.40) / 0.10 = 0.6.
4. Volume = 1,915.780 + 0.6 × (1,899.410 − 1,915.780) = 1,915.780 − 9.822 = 1,905.958 m³.
5. Temperatures at top, middle and bottom: 37.1, 36.4 and 36.0 °C, average 36.5 °C. NIOP takes the average of at least three levels as the temperature of the oil.

Rounding rule: ullages to the millimetre, volumes to three decimals, rounded half up at the end of each step, with the interpolation itself computed unrounded. Linear interpolation assumes the tank's horizontal cross-section is constant between the two lines; the UNECE code prescribes the same method for reading the ship's hydrostatic tables.

In this band one millimetre of ullage is 0.164 m³. Leaving out the trim correction gives 1,910.542 m³: 4.584 m³, or 0.24%, too much.

The 1,905.958 m³ is an observed volume at 36.5 °C, not yet a contract quantity. Correcting it to 15 °C and turning it into tonnes continues, with these figures, in volume correction factor and density at 15 °C. For palm oil, see palm oil density.

What goes into an ullage report

Based on FOSFA's annexes, NIOP's rules and the UNECE forms, an ullage report holds:

• Ship's condition at the time of gauging: draughts forward and aft, trim and list, with date and time.
• Per tank: reference height, observed ullage or sounding, trim and list correction, corrected reading and volume from the table. UNECE Form D1 uses similar columns for ballast.
• Free water, measured with water-detecting paste, treated paper or electronic means. FOSFA warns that the boundary between free water, emulsion and oil is often indistinct.
• Temperature per tank, averaged over several levels.
• After discharge: tanks inspected empty, with anything above a normal tare recorded with the ship's officer (NIOP).
• Who measured and who witnessed: under NIOP, the surveyors gauge ship's tanks jointly, witnessed by the ship's officer on duty.

FOSFA's annex for discharge reports adds the weight on board that corresponds to the ullages, corrected for trim and list.

Based on the working practice of our sector specialist: 25 years in the port, 20 of them as a cargo surveyor. Seagoing ships are almost always measured by ullage, the free space above the cargo; shore tanks and tank barges by sounding, the height of the liquid, which is more precise.

Write down the trim and list with each reading, not once for the whole ship: without them an ullage cannot be corrected or checked afterwards. That record is at the heart of what an independent cargo surveyor does. At discharge, the outturn report sets the quantity received ashore against the bill of lading weight.

FOSS

FOSS is software for inspection companies and the port, with a sector specialist at the table. Readings are entered once, offline on board if need be, and nine report types come from that input. The outturn per receiving medium is set against the bill of lading weight, with a protest flag when it is more than 0.5% short.