Techniques using hydrostatic head

Several techniques of varying complexity are used for directly measuring the hydrostatic head exerted by a liquid in a vessel. These include  

Measurement of the level of a volatile liquid using a differential pressure cell 

If the liquid is not volatile, the condensing reservoir shown is not required and the high and low pressure connections on the DP cell are reversed. The DP cell is normally located below the lower measuring point (or tapping) in the vessel. Any lines connected to the DP cell which are liquid filled should be bled through the DP cell bleed port to make sure that there is no trapped vapour present. 

Hydrostatic head sensors are easily installed, contain no moving parts, and have good accuracy (approaching 0.5 per cent of full scale) and repeatability. 

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It should be noted that the frictional drop was calculated by subtracting the hydrostatic head and acceleration losses from the measured total pressure-drop where void data were lacking, a homogeneous flow model was assumed. This modification of X by use of the Froude number appears very similar to the technique used by Kosterin (K2, K3) for horizontal pipes, in which the equivalent of volume-fraction of gas flowing, with mixture Froude number as the correlating parameter. 

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