Beginner's guide to Differential Pressure TransmittersThis 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 experienced when a force is applied on an area. This means that we can increase the pressure by either increasing the force or by reducing the area. We also tend to think of force in terms of weight (this is only true, if gravity is constant). As an example, the force produced by 1lb of weight due to gravity acting on 1 inch x 1 inch (1sq inch) would produce a pressure of 1lb/sq inch, this is often written as 1 Psi (1 pound per square inch). If the same 1lb was applying a force to only half the area, we would actually say the pressure was equal to 2 Psi. In our day to day experience we may pump our car tyres to 26 Psi. We also experience pressure in the form of weather (or atmospheric pressure). This could be considered the force being appleid by the atmosphere on our heads. As the atmospheric pressure changes, so does the weather. High pressure usually referes to clear sunny days, while low pressure produces cloudy ones. If we refer back to our car tyres, two pressures are acting on the wall of the tyre. The pressure of the atmoshere on the outside of the tyre and the pressure we read on the gauge when we pumped the tyre up.
2. What are the different types of pressure measurement?Using the above example, we can illustrate 3 types of pressure measurement. a) Gauge Pressure  (the pressure in the tyre) or (difference between absolute pressure and atmosheric pressure) But they are all related to each other
a) Gauge Pressure is measured with reference to atmospheric pressure (please see Fg.3). Pressure measurement that measures the difference between atmospheric pressure and the tyre is called Gauge Pressure. To avoid confusion we normally add "g" to the units of measurement eg. Pound per Square Inch Gauge (Psig). All pressure gauges, sensors, transducers and transmitters that measure gauge pressure actually measure the difference between atmospheric pressure and the pressure to be measured as shown in fig3. If you would like to know more about this subject why not contact one of our technical team.
b) Absolute Pressure is measured with reference to Vacuum. So an absolute pressure is equal to gauge pressure and atmospheric pressure. If you would like to know more about this subject why not contact one of our technical team.
c) Differential Pressure is not measured with reference to a specific reference pressure (see Fg.4). Unlike Gauge or Absolute pressure transmitters, Differential Pressure Transmitters do not attempt to fix the reference. Importantly an increase in differential can be the result of increasing one of the pressures or decreasing the other. An increase in differential pressure would occur if P1 became smaller OR if P2 became larger. In a similar way, a decrease in differential pressure would occur if P1 became larger OR if P2 became smaller. The differential pressure measurement is not concerned whether the lower of the two pressures is at a vacuum, atmospheric or some other pressure. It is only interested in the difference between the two. Asshown in Fig4
The fundamental of differential pressure measurement is established. Not all Differntial Pressure transmitters, gauges, sensors, and transducers that measure differential pressure actually measure the difference between two pressure as shown in fig4. Some devices attempt to measure two gauge pressures and then mathematically calculate the difference. This method in our opinion is flawed, as it contains the uncertainty of four pressure measurements. If you would like to know more about this subject why not contact one of our technical team. 3. Where is Differential Pressure (DP) Measurement used?Differential pressure measurement is largely used in domestic and industrial applications. It is often the basis of other measurements such as flow, level, density, viscosity and even temperature. The most common being level and flow. DP Flow rate measurement (Fig5) is one of the most common applications for differential pressure transmitters. By measuring the difference in fluid pressure while the fluid flows through a pipe it is possible to calculate the flow rate. Differential pressure flow meters have a primary and a secondary element. Generally speaking , the primary element is designed to produce a difference in pressure as the flow increases. There are many different types of primary element, the most common being the orifice plate, venturi, flow nozzle and pitot tube. The secondary element of the flow meter is the differential pressure transmitter. It is designed to measure the differential pressure produced by the primary element as accurately as possible. In particular it is important that the differential pressure measurement is not affected by changes in the fluid pressure, temperature or other properties such as ambient temperature. A good dp transmitter will ensure that the differential pressure is measured accurately independent of other changing parameters and will reliably transmit a signal to represent the differential pressure. In the case of a dp flow transmitter the output signal may also include square root extraction. Although it is common these days for this function to be carried our in a flow computer of Dcs system. The output signal from and industrial DP transmitter is likely to be 420mA, but it may also include digital communications such as HART, Profibusm Fieldbus, Modbus 485 RTU or one of many other communication protocols. The objective being to provide an electrical signal for transmission to a remote process control instrument. See Fig5. Now we have discovered what differential pressue is and how it is used we can now focus our attention on the Differential Pressure Transmitter itself 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. The industrial differential pressure transmitters are made of two housings (See Fig6). 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).
At this stage, we will try to explain 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). This is clarified in Fig7. 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. 2) Electronic Unit: The electrical signal generated at the lower chamber by the sensor is in the range of millivolt only. 3) 2Wire 420mA Current Transmitter: 6. Industrial applications of Differential Pressure Transmitters:There are unlimited industrial applications of Differential Pressure Transmitters.
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. See also


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