A convenient means of measuring liquid level, where there is a considerable change in level employs conventional industrial instruments which is actuated by changes in hydrostatic pressure head of the liquid as the level changes.
This head is the “weight” of liquid above a reference level or datum line. Head is often expressed in terms of pressure or level height.
Measurement of pressure due to liquid head can be translated to level height above the datum line by the following basic relationship:
where h=height or level
P=pressure due to hydrostatic head
Ρ=density of the liquid
g=acceleration due to gravity
For the readings to be accurate the density have to be constant. The accuracy will be affected for e.g., temperature variations is sufficient to cause changes in the density of the liquid.