Differential pressure transmitters are versatile and level measurement is just one of the processes where DP transmitters are applicable. Even though there are specialized devices for these measurements today, many field engineers still prefer good old differential pressure transmitters.

A differential pressure transmitter is composed of a sensing cell (pressure sensor) and a transmitter. The cell has two pressure chambers separated by a diaphragm.

We call one chamber the high-pressure side and the other the low-pressure side, but we don’t have to take that literally. It just tells us the direction of the impact that pressure will have on the output signal.

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Deltabar S PMD75

Differential pressure transmitter with metal sensor for continuous measurement

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PMD75

## Setting up a differential pressure transmitter for level measurement

To do a DP transmitter level calculation (DP stands for differential pressure), the meter setup will determine the math needed to find the level in the tank. We 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 in-compressible capillary liquids will provide the correct pressure measurement.

Then to set up the transmitter. we shall 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, we 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:

For the process parameters:

And the differential pressure:

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

If you need help choosing a pressure transmitter for your application, take a look at our new pressure smart assistant.

To know more about DP transmitter level calculation, you can get in touch with our engineers and we will be happy to help.