Density Differences in Level Measurement

Level can be determined by head pressure, the force of gravity on the mass of liquid, which produces pressure.

A typical unit of measurement is inches of water column, that is, the pressure exerted by the height of a column of water above the measurement point. Commercial head pressure level transmitters have to use some standard pressure engineering units, one of those standard engineering units in the US is inches of water. Others are psi or bar.

But the head pressure of a liquid of a different density than water will vary from the pressure developed by the same column height of water in proportion to the density differences.

Specific gravity is expressed as a ratio of the density of a liquid to the density of water. Hence, the level of any liquid can be expressed by measuring the head pressure in units of inches of water and dividing by the specific gravity of the liquid to get the actual level.
Level reading (inches of water) / Specific gravity = actual level

In practice, measurement is typically made by the level insturment in standard engineering units, like inches of water, and then corrected with the specific gravity factor to get the proper actual level;the correction typcially done in a read-out device. 

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