Differential Pressure Transmitter Explained

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Simply put, a differential pressure transmitter is a device that measures two opposing pressures in a pipe or vessel.

To understand how a differential pressure transmitter works, we need to know some principles of pressure.

The pressure is the result of force applied onto a unit of area.

In the case of fluid and gas, this area would be the inside of a pipe, tank, vessel or other sealed housing.

Since pressure is a physical quantity, it can be measured.

The pressure is measured in one of the following five units.
– Pascal
– Bar
– Standard atmosphere or atm
– Torr
– Pounds per square inch or psi

The most commonly used measurements of pressure in an industrial fluid and gas processing environment are bar and psi.
One bar is the equivalent of the earth’s atmosphere at mean sea level.

A pound per square inch is the force of one pound on an area of one square inch.

You will most likely use these units of measurement in your work as they are commonly used and almost every manufacturer of pressure measurement devices calibrate them to these units of measurement.

Delta-P or DP is the industry-standard term for the measured differential pressure.
Delta, meaning change in, and P meaning pressure.

In addition, most people will simply call a differential pressure sensor a DP sensor or transmitter.

A DP transmitter is a pressure-measuring gauge or an electrical device that uses two elements to measure the differences of pressure in a sealed container such as a pipe.

DP transmitters will have
– A primary element
– A secondary element
– An electronics housing

The primary element will produce a difference in pressure as the flow in the pipe increases.

Different types of primary elements include
– Orifice plates
– Venturi tubes
– Pitot tubes
– Flow nozzles
– Laminar flow elements
– Wedge elements

The secondary element will measure the difference of pressure produced by the primary element as accurately as possible.
The primary element is sometimes called the high side, and the secondary element is called the low side.

The elements in a DP transmitter will have sealed diaphragms and one of several ways to convert the pressure applied onto the diaphragms into an electrical signal.

Sensors such as differential capacitance, vibrating wire or strain gauges are used to produce the electrical signal that will be sent to the electronics housing, where it is processed, converted and amplified into a current or voltage output that can be read by the control system.

The current output is commonly a 4-20 milliamp signal, or in some cases a 0 to 5 or a 0 to 10 volt signal.

The electrical output generated by the DP transmitter electronic module is linear and proportional to the actual measured Delta-P.

Depending on the range of measured pressure, a 4 milliamp signal would equal a measured Delta-P of 0 psi and a 20 milliamp signal would equal the maximum measurable value. In this case, we’ll say that the maximum Delta-P is 100 psi.

One very common way of measuring flow using a DP transmitter is for the primary element to introduce a constriction in the pipe.

Using a restrictive element on the primary side will create high and low pressure.
When the flow increases, more pressure drop is created.

As we remember, the secondary element will measure the difference in pressure created by the primary element.
These measurements are sent to the electronics housing where the signal processing is done.

By using Bernoulli’s equation, which states that the pressure drop across the constriction is proportional to the square of the flow rate, the Delta-P is then converted into units of flow.

These units of flow can be gallons per minute or cubic meters per hour. Using DP transmitters as a method to measure flow works best on fluids that are relatively clean and of low viscosity.

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