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How Does A Differential Pressure Transmitter Work?

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  • How Does A Differential Pressure Transmitter Work?
  • December 7, 2018

How Does A Differential Pressure Transmitter Work?

The pressure in an enclosed system can be measured against various reference values; as atmospheric or absolute pressure, gauge pressure or as differential pressure. Differential pressure is defined as the difference in value in pressure between two distinct points, connected to the two pressure ports in the pressure sensors. It is of critical importance across a wide range of process-controlled mechanisms and applications, primarily in the process industry.

What is a Differential Pressure Transmitter?

A differential pressure transmitter is an instrument that measures the difference in pressure at two specified points in any enclosed systems. The sensors come with two pressure ports, which are connected to the desired measurement points. The difference in pressure between the two points is calculated and displayed digitally or as the signal output.

How Differential Pressure Transmitter Works

A differential pressure transmitter has primarily three working parts – the piezoresistive pressure sensing element, the electronic unit and a two wired 4-20mA transmitter.

The direct pressure sensing element shapes like a diaphragm and is located in the lower part of the apparatus. The device is placed in such a way that the diaphragm lies in between the two pressure points whose differential pressure is to be measured. As the pressure is applied, the diaphragm is deflected and the waves of deflection are converted into electrical signals. This entire process is conducted through sensors. The typical sensors used are vibrating wires, strain gauge or differential capacitance. The capacity of the sensor device is in proportion to the pressure to be applied.

The function of the electronic unit is to amplify the electric signals generated by the sensors. Typically, the signals are emitted in milli-volts per volts. This tiny measurement signal requires amplification by internal or external amplifier into the readable ranges by the displaying unit, generally in 0-10V or 4-20mA. These amplified signals are then forwarded to the transmitter unit.

The transmitter unit is located on the upper part of the device. Once the amplified signals are received by the transmitter, a DC current is generated, which is in direct proportion to the pressure applied on the two pressure points. The current generated is between 4mA and 20mA. This transmitter output is superimposed with a digital mechanism that gives the final reading in the difference in pressure between the two points.

Several applications of Differential Pressure Transmitters

Typically, differential pressure transmitters are highly resistant to corrosion and moisture. The sensing diaphragm is made from stainless steel that is inherently resistant to corrosion, moisture or any type of gas absorption from the surroundings. For measurement in marine environment, the diaphragm can be constructed from Inconel (a nickel, chromium and iron alloy) for protection in salt water.

Differential pressure transmitters are also commonly used in the pharmaceutical and food industry, particularly in the laboratory and clean room settings. These applications typically require a low differential pressure transmitter that is capable of measuring as little as 10Pa. These type of pressure transmitters can only handle dry gas and cannot be used in the environment where humidity level is high.

Another application of these sensors is in the level measurement or filter and pump monitoring in the industrial environment. For these types of applications, users will be looking for high accuracy sensors and capability of the sensors to interface with the industrial computer. Some high-end differential pressure transmitters have built-in amplifier for converting the analogue into digital signals. Alternatively, a signal conditioner/converter can be used to connect the sensors to the industrial PC.

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