A full dry leg installation with three-valve manifold is as shown. If the gas phase is condensable, say steam, condensate will form in the low-pressure impulse line resulting in a column of liquid, which exerts extra pressure on the low-pressure side of the transmitter.
A technique to solve this problem is to add a knockout pot below the transmitter in the low-pressure side. Periodic draining of the condensate in the knockout pot will ensure that the impulse line is free of liquid.
Phigh=Pgas + S.H
The effect of the gas pressure is cancelled and only the pressure due to the hydrostatic head of the liquid is sensed. When the low-pressure impulse line is connected directly to the gas phase above the liquid level, it is called a dry leg.
In practice, a dry leg is seldom used because frequent maintenance is required. One example of a dry leg application is the measurement of liquid poison level in the poison injection tank, where the gas phase is non-condensable medium. In most closed tank applications, a wet leg level measurement system in used.