Half the height of a tank is almost never half its volume

A storage tank filled to half its height does not hold half its capacity. It sounds like an arithmetic mistake and it is not: it depends on the shape of the vessel, and in a horizontal tank the gap reaches twenty per cent.

It is one of those things everyone on the plant floor knows and almost no monitoring system applies. The result is an inventory that does not match the physical count, and a monthly argument between operations and accounting that nobody wins.

What the camera actually sees

A thermal camera does not measure level. It measures temperature, and finds the level as a consequence: the liquid and the vapour space above it hold heat differently, so the tank wall shows a temperature step exactly where the surface is.

That step is sharp when there is a thermal difference between product and ambient. Dawn and dusk are when it reads best. At midday, with the sun on the shell, it can vanish — and an honest system should say "I cannot see it" instead of inventing a number.

A system that always returns a figure is more dangerous than one that admits when it cannot see. The invented figure gets used to decide a shipment.

From height to volume

The conversion depends on the shape. In a vertical cylindrical tank it is linear: half the height is half the volume. In a horizontal one it is not, and that is the problem.

The cross-section of a horizontal cylinder is a circle. Filling the first ten per cent of height adds very little volume, because the circle is narrow at the bottom. The middle stretch adds a lot. The last ten per cent adds little again.

Cone-bottomed, spherical and irregular tanks have the same problem with different curves. That is why the industry uses strapping tables: a table that says, for each height, how many litres are inside. You build it once, by measuring, and it holds for that tank.

What to ask a vendor

• Does the system report height or volume? If it reports only height, someone is doing the conversion on a calculator.
• Does it accept the tank's strapping table, or does it assume a vertical cylinder?
• What does it do when it cannot resolve the surface? Does it say so, or repeat the last value as if it were new?
• Does the reading depend on the time of day? It should tell you.

Why we wrote this

Because in SEPTEM tank level reports volume, not height, and it accepts the vessel's strapping table. And because when the camera cannot resolve the surface, the screen says so.

It is not a flashier feature than the others. It is the difference between a number you can ship product on and one that only fills a dashboard.