Beginner's guide to Differential Pressure Transmitters

This article is a straightforward and informal guide with illustrations aimed at helping beginners to understand Differential Pressure Transmitters principles.

1. What is pressure?

Pressure is the force per unit area. Pressure is expressed as P=F/A, F=force and A= area of application of force. Pressure is described in SI unit as pascal. The other units of pressure are bars, millibars, atmospheres and millimeters of mercury (mm Hg).

Let’s take a look at some examples around us. Think of a chamber filled with gas. The countless atoms and molecules of gas will constantly rebound on the walls of the chamber and exert pressure on it. But, you do not have to measure the pressure on the wall. You can measure pressure inside the chamber along any plane. The thrust of air on the surface of the earth is proportional to the weight of air. This explains why the pressure will reduce at higher altitude. But when you go underwater deeper and deeper into the sea, the pressure exerted on your body will go on increasing.

In case of fluid, it is different. Fluid is incompressible and so, when pressure is exerted at a certain point in a volume of fluid, it gets equally distributed throughout the volume of the fluid. This is Pascal’s law. Pascal's law states that increase in pressure at any point in an enclosed fluid will give rise to equal increase in the entire fluid. The concept of hydraulic jack is governed by Pascal’s law. The pressure exerted at one point in the oil of a hydraulic jack is transmitted to other points through the incompressible fluid oil.

Fig-1 What is pressure

2. What are the types of measurements being performed for pressure?

There are 3 types of pressure measurements:

a) Absolute
b) Gauge
c) Differential

a) Absolute Pressure is measured with reference to Vacuum.
This is explained in Fig.2. The abbreviations used are Pascals Absolute (PAA) or Pounds Per Square Inch Absolute (PSIA).

Fig-2 Absolute Pressure Measurement

Tip	You can see our range of Absolute Pressure Transmitters here.

b) Gauge Pressure is measured with reference to ambient atmospheric pressure (please see Fg.3).
In Gauge Pressure, Pascals Gauge (PAG) or Pound per Square Inch Gauge (PSIG) abbreviations are used.

DP-Article-Fig-3

Tip	You can see our range of Gauge Pressure Transmitters here.

c) Differential Pressure is measured with reference to a specific reference pressure (please see Fg.4).
In Differential Pressure, Pascals Differential (PAD) or Pound Per Square Inch Differential (PSID) abbreviations are used.

DP-Article-Fig-4

The fundamental of differential pressure measurement is established.

3. Differential pressure measurement is largely used for various process control measurements.

It is used to measure flow, level and even temperature in industrial processes.

Flow rate measurement (Fig-5): Flow rate is complex and it is difficult to measure the quantity. There is no principle to measure flow directly. The reduction of pressure in a fluid flow is measured by differential pressure transmitter. We will be able to find the flow rate by extracting the square root of the differential pressure.

Differential pressure flow meters have a primary and a secondary element. The primary element in the form of orifice plate, venturi, and flow nozzle will produce a change in the kinetic energy of the flow. This change in kinetic energy will create differential pressure. The secondary unit will measure this differential pressure and provide an electrical signal for transmission to a remote process control instrument. See Fig-5.

DP-Article-Fig-5

Now we have to find out how this measured signal can be transmitted to remote distance for onward use by process indicators and controllers. We will focus our attention to Differential Pressure Transmitter theory and application.

4. What is a Differential Pressure Transmitter?

The most common and useful industrial pressure measuring instrument is the differential pressure transmitter. This equipment will sense the difference in pressure between two ports and produce an output signal with reference to a calibrated pressure range.

DP Transmitter Fig-6

The industrial differential pressure transmitters are made of two housings (See Fig-6). Pressure sensing element is housed in the bottom half, and the electronics are housed at the top half. It will have two pressure ports marked as “High” and “Low”. It is not compulsory that the high port will be always at high pressure and the low port always at low pressure. This labeling has its relation to the effect of the port on the output signal. This point is clarified in Fig -7 (Please see Fig 7).

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At this stage, we will try to learn the internal construction of the transmitter.

5. Differential pressure transmitter construction:

A differential pressure transmitter has three functional parts.

1) Direct Pressure sensing element (located in the lower housing).
The majority of industrial DP Transmitters are fitted with diaphragm as the pressure sensing element. This diaphragm is a mechanical device. It is placed in between the two pressure inlet ports. The diaphragm will be deflected by the applied pressure.

This is clarified in Fig-7. This deflection is converted into an electrical signal. This is normally done by the sensors. The commonly used sensors are (a) Strain Gauge (b) Differential Capacitance (c) Vibrating wire. The sensor output is proportional to the applied pressure.

DP-Article-Fig-7

2) Electronic Unit: The electrical signal generated at the lower chamber by the sensor is in the range of milli-volt only.
This signal is to be amplified to 0-5V or 0-10V range or is to be converted to 4-20mA for onward transmission to a remote instrument. This upper housing is the Transmitter portion of the DP Transmitter which houses the Electronic Unit. See Fig-7 for further clarifications.

3) 2-Wire 4-20mA Current Transmitter:
A DC output current is generated which is directly proportional to the pressure range of the Differential Pressure Transmitter. The lower range is 4mA, and the upper range is 20mA. This controlled current output is not affected by load impedance variation and supply voltage fluctuations. This 4-20mA output is superimposed with digital communications of BRAIN or HART FSK protocol.

6. Industrial applications of Differential Pressure Transmitters:

There are unlimited industrial applications of Differential Pressure Transmitters.

Water and effluent treatment plants. It is largely used to monitor filters in these plants.
It is used to monitor Sprinkler Systems.
Remote sensing of Heating Systems for Steam or Hot Water.
Pressure drops across valves can be monitored.
Pump control Monitoring.

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This article has covered the basic aspects of Differential Pressure Transmitters. You can also have a look at our range of Differential Pressure Transmitters or contact us if you have a specific application that you would like to discuss.