Head Pressure for Differential Pressure Level Transmitter

The force applied to the sensor diaphragm is rho*g*h, but g is assumed to be a constant for the application of DP in liquid level, so the g factor is left out of the equation for practical purposes. It's a reasonable assumption by the vendors that NASA won't be using DP for level measurement in the space station.

In many cases, the DP transmitter isn't even configured to display true elevation level units. Typically, the xmtr displays level as a percentage or level in standard pressure units of inches, feet or mm or m of water column. The rho density factor/SG is typically used by the receiver/indicator/controller/HMI to get true elevation values because it's a linear scaling factor applied to the interpretation of the analog 4-20mA. Sounds like your DP indicator has the functionality to do the arithmetic. Foundation Fieldbus or Profibus or just really smart 4-20?

There is a g factor compensation for a dead weight tester, depending on the local gravity where the dwt is installed. But dead weight testers are primary standards, not industrial tank level measurements.

The medium's Specific Gravity (SG) is used for the rho density factor in the head pressure level calculation because

- the DP measurement is ordinarily in pressure units of water column

- SG is a ratio that uses the mass of water as its reference

The properties of water become the common reference for both the measurement and the density compensation.

SG is a 'dimensionless' ratio value so no engineering units for it are included in the calculation. SG for water is 1.000: a medium lighter than water less than 1; a medium denser than water,greater than 1.

If the medium were a mixture or blend that has no published SG, the SG can be determined empirically by making a head pressure measurement at a level that can be determined by observation (stick/tape/bypass) and then ratioing the measured head pressure value to the observed level value.

For example, the DP reads the medium level at 3.432m but the level in the bypass gauge is observed to be 3.876m (assuming the zero points are the same for both).

The SG is 3.432/3.876 = 0.885

Important: When the head pressure measurement (in water column units) is converted to true level, the head pressure measurement is divided by the SG.

Using the values from the example, the DP measurement, 3.432m wc, is divided by the SG 0.885 to get true level, 3.876m

3.432m/0.885 = 3.876m