Dry leg method:
A DP transmitter is connected to the closed tank: the low pressure tap is the pressure of the gas above the liquid in the upper art of the tank.
The pressure of the gas is also applied to the high pressure tap at the same time. Hence, when taking the pressure differential, it cancels out and so does not affect the transmitter output.
If condensation from the gas in the upper part of the tank collects inside the tapping tube, the low pressure tapping pressure in the tube will change and the output of the d/p transmitter will be affected. To avoid this, the condensation is collected in a drain pot.
Liquid-level measurement: Closed tank –- Dry Leg
The following relationship exists:
High pressure tap pressure, PH = ρ1g ( H+h1)+PG
Low pressure tap pressure, PL=PG
Pressure differential, PH-PL= ρ1g (H+h1)
PG = the pressure of the gas in the upper part of the tank
ρ1=density of the liquid
H=distance between the surface and the minimum liquid level
h1=distance between the minimum liquid level and the pressure detector.