Should the tank be closed and a gas or vapour exists on top of the liquid, the gas pressure must be compensated for. A change in the gas pressure will cause a change in transmitter output.
Moreover, the pressure exerted by the gas phase may be so high that the hydrostatic pressure of the liquid column becomes insignificant.
For example, the measured hydrostatic head in a boiler may be only three meters (30kPa) or so, whereas the steam pressure is typically 5 MPa.
Compensation can be achieved by applying the gas pressure to both high and low-pressure sides of the level transmitter.
This cover gas pressure is thus used as a back pressure (or reference pressure) on the LP side of the DP cell.
One can immediately see the need for the threevalve manifold to protect the DP cell against these pressures.