This technique obtains a level indication indirectly by monitoring the pressure exerted by the height of the liquid in the vessel.
The pressure at the base of a vessel containing liquid is directly proportional to the height of the liquid in the vessel. This is termed hydrostatic pressure.
As the level in the vessel rises, the pressure exerted by the liquid at the base of the vessel will increase linearly. Mathematically,
P=S.H
where
P=Pressure ( Pa), S=Weight density of the liquid ( N/m3)= ρ g
H=Height of liquid column ( m), ρ=Density (kg/m3)
g=acceleration due to gravity ( 9.81 m/s2)
DP capsules are the most commonly used devices to measure the pressure at the base of a tank. The level of liquid inside a tank can be determined from the pressure reading if the weight density of the liquid is constant. Use a pressure capsule that has a sensitivity range that closely matches the anticipated pressure of the measured liquid.