Differential pressure applications

Flow Level Pressure Q&A

In which kind of applications can I use a DP transmitter?

Expert’s answer: Differential pressure applications

You can use differential pressure transmitters to measure level, flow, and pressure in most industries. These differential pressure applications operate with a wide variety of fluids even at high temperatures, pressures, flow rates, and velocities.

For instance, DP 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. The chamber has a high and low end for the high and low pressures of the process.

Schematics of DP transmitter sampling chamber courtesy of Rosemount

Let’s have an overview and examples on how DP transmitters can measure different process parameters.

Pressure measurement

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.

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 the atmospheric pressure.

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

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

For closed vessels and tanks, DP transmitters can measure the pressure of the fluid in the vessel as well as the pressure head above the level. In this case, the DP 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

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

Flow measurement

In DP flow measurement, you 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.

DP 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, you must use different primary element designs, depending on whether you want to avoid a high-pressure drop or have fluid mixed with solids that may damage the mechanical restriction.

The video below may help you understand the concepts of DP flow measurement:

Want more information on differential pressure applications? Here’s a list of differential pressure flow manufacturers:

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Related tags: Absolute pressure Differential of pressure DP flow transmitter Emerson Automation Solutions Endress+Hauser Flow gauge pressure hydrostatic level level level application level measurement Pressure Yokogawa
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