Tank Sounding Explained: Sounding Tables and Flowmeter Gaps

Tank sounding is the oldest and still the most common way to find out how much fuel is in a vessel's tanks: lower a tape into the tank, read the depth of the liquid, and look up the matching volume in the tank's sounding table. It is simple, it needs no power, and every engineer knows how to do it. It is also why sounded figures and flowmeter figures so often disagree, and understanding tank sounding explains most of that gap.

How tank sounding works

A sounding is a depth measurement. The engineer lowers a graduated tape with a weighted bob down the tank's sounding pipe until the bob touches the bottom or the striking plate, then pulls it up and reads where the fuel wet the tape. That reading is the sounding: the height of liquid above the tank's reference point.

Some tanks are measured the other way round. An ullage is the distance from a reference point at the top of the tank down to the liquid surface. Both methods end in the same place: a height that has to be converted into a volume.

Many vessels also have remote level gauges, but the manual sounding usually remains the reference that people fall back on when numbers are disputed.

What a sounding table is

A sounding table, also called a tank calibration table, lists the volume in a tank for each sounding height. Tanks on a vessel are rarely simple boxes. They follow the shape of the hull, wrap around frames and narrow toward the bottom, so the same centimetre of depth can hold very different volumes at different heights. The table captures that shape.

Most tables are prepared for the vessel on an even keel. When the vessel is trimmed by the head or stern, or listing to one side, the liquid surface is no longer parallel to the tank bottom, and the sounding pipe reads a height that does not match the table's assumption. For that reason many vessels also carry trim and list correction tables.

To turn a sounding into a usable quantity, the usual steps are:

1. Take the sounding and note trim, list and fuel temperature. 2. Apply trim and list corrections to find the corrected sounding or volume. 3. Read the observed volume from the sounding table. 4. Correct the volume from the observed temperature to a standard reference temperature. 5. Multiply by density if the quantity is needed in mass.

Why sounding and flowmeter figures differ

When a sounding and a flowmeter disagree, it is tempting to assume one of them is wrong. More often, they measure different things under different conditions.

Trim and list. A sounding taken without the right trim and list correction can be off by more than people expect, especially in tanks with a sloping or narrow bottom.

Temperature and density. Fuel expands as it warms. A sounding tells you a volume at the tank's temperature. A flowmeter may report volume at line temperature, or mass. Unless both are brought to the same reference, they will never match, even when nothing is missing.

Manual reading. A tape read in poor light, on a moving deck, or in a hurry introduces error. Foam or air in the fuel can also make the surface hard to read and inflate the apparent level.

Timing. A sounding is a snapshot. Fuel needs time to settle after a bunker or a transfer, and fuel sitting in pipes, filters and day tanks does not appear in a storage tank sounding.

The table itself. Sounding tables are only as accurate as the survey behind them, and modifications to tanks or pipes over the years are not always reflected in the table.

The meter itself. Flowmeters need correct installation and calibration too. A meter that has drifted will produce a clean, confident, wrong number.

Getting better soundings

• Sound at a consistent time and after the fuel has settled.
• Record trim, list and temperature with every sounding, not just the tape reading.
• Use the correction tables every time, not only when numbers look odd.
• Use water-finding paste where water in the tank is a concern.
• Have a second person check readings during bunkering.
• Keep the sounding table in the same unit and reference as the rest of your fuel records.

Using sounding and flowmeter together

Sounding and flowmeters are not competitors. A flowmeter records the flow continuously, including rate, density and temperature. A sounding confirms what is physically in the tank. Each one is a check on the other.

MANTRAZ is built around that idea. During bunkering it measures flow rate, density, temperature, mass and volume through the delivery, and afterward the bunkering log sets the target quantity against the MANTRAZ flowmeter total and the tank sounding before and after, with the discrepancy in KL and percent. Corrections are recorded as adjustments with a reason, such as meter calibration or spillage, along with who made them and when. On the engine side, main engine fuel is measured by flowmeters on the supply and return lines, with fuel density and temperature shown alongside.

If you want to see flowmeter and sounding figures compared on data similar to yours, request a MANTRAZ demo.