In situations where the vapors of the process fluid tend to condense at standard ambient temperatures or happen to be corrosive and unsteady, the reference leg can not be kept dry. It must be then filled with an inert liquid to produce a wet reference leg.
For a wet reference leg, it is always suggested that the chosen filling liquid must have a low thermal expansion rate. In case of wet reference legs following two factors should be carefully noted:
The specific gravity of the filling liquid and the height of the reference column are required to be correctly evaluated. Besides, the differential pressure cell ought to be lowered corresponding to the hydrostatic head of the same column.
A sight flow indicator should be mounted above the wet leg in order to get visual indication of the height of the reference leg.
A typical closed tank wet reference leg is shown in the figure below.