Differential pressure transmitters (also called DP transmitters) are among the most versatile measuring instruments. In addition to pressure, they are used to measure level and flow in many industries, on a wide variety of fluids even at high temperatures. The article here explains how differential pressure transmitters work and how to use them in various applications.

What is a differential pressure transmitter?

Even though there are specialized devices for these measurements today, many field engineers still stick to the good old differential pressure transmitter. 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.

To know more about differential pressure transmitters, you can read our article on electronic differential pressure tranmitters

The differential pressure transmitter’s working principle

So imagine we have a process with -20 bar to 20 bar and no pressure in either chamber. At that point, the signal will read 12 milliamps (mA), representing 0 bar, 50 percent of the range. If we apply pressure on the high-pressure side of the differential pressure transmitter, then the value will rise towards 20 mA, giving you a positive reading. On the other hand, if we apply pressure to the low-pressure side, then that drives the signal towards 4 mA and negative reading.

We can find several options for converting the pressure in the sensing cell to an electronic signal – piezoresistive (strain gauge), piezoelectric, resonant, and capacitive methods all will get us there.

Differential pressure transmitters can measure the pressure difference in a sampling chamber divided by a diaphragm. The sensitive transmitter will register the shape of the diaphragm and record changes due to the pressure difference on both sides of the sampling chamber.

Differential pressure transmitter applications

Pressure measurement using DP transmitters

DP transmitters differ from other pressure transmitters in that they have reference pressure systems. The reference pressure will depend on the type of pressure that the application needs.

Gauge pressure measurement

For gauge pressure measurement, the high end of the sampling connects to the vessel or pipe, with the low end exposed to atmospheric pressure. Thus, the DP transmitter will measure values relative to atmospheric pressure.

Absolute pressure measurement

For absolute pressure measurement, the low end of the DP transmitter is in a vacuum. Thus, the transmitter will measure the atmospheric pressure at atmospheric conditions.

This article will tell you more about the difference between absolute, gauge, and differential pressure.

Vacuum measurement

For vacuum measurement, the low end of the DP transmitter connects to the vacuum vessel, with the high end exposed to atmospheric pressure. In this case, the greater the vacuum on the vessel, the greater the response of the transmitter.

Level measurement using a differential pressure transmitter

For both open and closed vessels and tanks, differential pressure transmitters can measure the pressure of the fluid in the vessel as well as the pressure head above the level. In this case, the differential pressure transmitter can determine the level of the tank at any given moment using the differential pressure.

The relationship between pressure and level: P = ρ*g*h

ρ = density of the liquid

g = acceleration due to gravity

h = height of the column of liquid

Differential pressure transmitters should only measure fluids with densities that don’t change with temperature variation since this will disturb the precision of your measurement.

Deltabar M PMD55

Differential pressure transmitter with metal sensor for measuring pressure differences

Flow measurement using a differential pressure transmitter

In differential pressure transmitter flow measurement, we install a primary element in the measuring line. This primary element provides a mechanical restriction that will change the flow inside the pipe. The change in cross-section and laws of continuity cause the speed of the fluid after the cross-section to increase. At the same time, the static pressure at that point decreases.

Differential pressure transmitters can measure the gradient of pressure before and after the primary element and thus measure the fluid velocity, mass, and volume flow. The Bernoulli equation describes the relationship between the velocity of a fluid and its pressure. If the velocity of the fluid increases, the drop in pressure will increase as well. For this reason, the process parameters will influence the type of primary element needed.

Many companies use the orifice plate for a wide range of industrial applications. Nevertheless, we must use different primary element designs, depending on whether we want to avoid a high-pressure drop or have fluid mixed with solids that may damage the mechanical restriction.

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