To do a DP transmitter level calculation (DP stands for differential pressure), your meter setup will determine the math needed to find the level in the tank. You can find it using the Pascal equation for hydrostatic pressure: P = ρ*g*h

ρ = density of the liquid

g = acceleration due to gravity

h = height of the column of liquid

Leg lines filled with incompressible capillary liquids will provide the correct pressure measurement.

DP transmitter
Level measurement configuration courtesy of Endress+Hauser

Then to set up the transmitter you need the following data:

  • Minimum and maximum (empty and full) states of the tank
  • Distance between nozzles
  • Density of the process fluid
  • Density of the capillary fluid
  • Pressure head (atmospheric pressure for open tanks)

So, given these values, you can find the differential pressure in the empty state and full state. Also, keep in mind the hydrostatic pressure from the capillary fluid.

DP transmitter level calculation

Let’s do a simple calculation:

Example sent by the customer
Example sent by the customer

For the process parameters:

screen-shot-2016-12-23-at-13-25-12

 

 

 

 

 

 

And the differential pressure:

screen-shot-2016-12-23-at-13-25-19

And now you have a minimum pressure of -98 millibars. Thus you should have the pressure cell above 100. Furthermore, you probably should choose 250 or 500, depending on what you can find on the market.

Want more information? If so, here’s a list of companies with DP transmitters:

Other resources on DP transmitter level calculation

Offset and slope for differential pressure transmitter
How do you do the offset and slope adjustment when installing and calibrating a Rosemount differential pressure transmitter?

Bubbler system for differential pressure transmitter
Can we use a differential pressure transmitter for a buried tank, which we need the level measurement on?

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