Level Measurement · Glossary

What is Ullage?

empty space above the liquidLevel Measurement

Ullage is the empty height or volume above the liquid in a tank, measured from the reference point down to the surface.

Ullage Explained

Top mounted gauges such as radar measure ullage directly; level (innage) is then reference height minus ullage. Ullage volume shows how much more product the tank can take.

In tank gauging, the reference height must be checked regularly, since tank bottoms and roofs move with load and temperature.

Ullage Formula

Level = H_ref − Ullage

H_ref	reference height from datum plate to gauge reference
Ullage	distance from reference to surface

Example: Reference height 12.00 m, ullage 3.25 m: level 8.75 m.

Calculators for Ullage

CalculatorTank Ullage CalculatorFree online calculator with formula and worked example.CalculatorRadar Level CalculatorFree online calculator with formula and worked example.CalculatorTank Strapping Table GeneratorFree online calculator with formula and worked example.

Frequently Asked Questions

What is Ullage?

Ullage is the empty height or volume above the liquid in a tank, measured from the reference point down to the surface.

What is innage?

The height of liquid measured upwards from the datum plate, which is the usual level.

Why keep ullage in a tank?

To leave room for thermal expansion and to stay below the overfill level.

Learn more on the blog

Basics of level measurement

A practical guide on instrumentationblog.in with more detail on ullage and how it is applied on real plants.

Read Full Article

More Level Measurement Terms

Level termDished HeadA dished head is the curved end of a vessel, such as a 2:1 ellipsoidal or torispherical head, that adds…Level termVessel Holdup TimeVessel holdup time is how long the volume between two levels lasts at a given flow, used to set alarm and…Level termRadiometric Level MeasurementRadiometric level measurement uses a gamma source and a detector outside the vessel; the liquid absorbs…Level termStilling WellA stilling well is a vertical perforated pipe inside a tank that calms the liquid surface for level…Level termLevel BridleA level bridle is an external vertical pipe connected to a vessel at two points, holding level…Level termSight GlassA sight glass is a glass gauge on a vessel that lets operators see the liquid level directly.

All level measurement terms · Full glossary A to Z