Level Calculation for Diaphragm Seal Transmitter

Another installation method for level measurement using Differential Pressure level transmitter is by utilizing the diaphragm seal plus capillary tube. To determine the calibrated range for Differential Pressure level transmitter, the density of the filling fluid within the capillary must be taken into account in the calculation since the fill fluid also creates hydrostatic force exerting both DP level transmitter ports.
The density of fill fluid must be known, and the information could be obtained from manufacturer specification.
An example below describes the calculation in determining the calibrated range for Differential Pressure level transmitter with a capillary tube.

Simple Differential Pressure Level Calculation

Diaphragm Seal DP Transmitter Level Measurement

Sample Process Data

Process fluid density = 0.96 g/cm3
Fill fluid density = 1.05 g/cm3
100% from Process Connection (h) = 4 m = 4,000mm
Height Difference between Process Connection (h2) = 5 m = 5,000mm

When level at 0% = 4mA (Left Image)

∆P at 0% level
∆P = P1 – P2
P1 = 0 mmH2O
P2 = 1.05 x 5,000 = 5,250 mmH2O

Therefore Range at 0% is
P1 – P2 = 0 – 5,250 = -5,250 mmH2O

When level at 100% = 20mA (Right Image)

∆P at 100% level
∆P = P1 – P2
P1 = 0.96 x 4000 = 3,840 mmH2O
P2 = 1.05 x 5,000 = 5,250 mmH2O

Therefore ∆P at 100% is
P1 – P2 = 3,840 – 5,250 = -1,410 mmH2O

Scaling to set to Differential Pressure Transmitter
0% to 100% = -5,250 ~ -1,410 mmH2O

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