A three valve manifold serves to protect the capsule from being exposed to pressures above the acceptable range and also enables transmitter isolation from the loop of the process
In closed tank level measurement installations the pressure of the gas if presence of gas on top of the liquid will need to be taken account by the use of a reference connection from the level transmitter to the tank top. The valve used in pressure equalization is kept close when the process system is operating normally.
In the event that the transmitter is removed or put into service, it is necessary to ensure that the gas phase which is at the higher pressure is does not over-range the Differential Pressure DP capsule
