Basics of Differential Pressure transmitter
Nowadays, one of the most common instruments used to measure pressure in Industries is the differential pressure. It is also called DP or ΔP.Firstly, you should know about the Pressure. For more information, Check out our blog on Pressure
To know more about differential pressure transmitters we first have to know about differential pressure.
What is differential pressure?
As the name suggested it will measure the difference between two points; high Pressure and Low pressure. DP means differential pressure or ΔP suggests Delta (change in) pressure.
Differential pressure is commonly used in both domestic and industrial settings. During flow measurement, the DP measurement is required. It's also useful to know the level, density, viscosity, and temperature from time to time.
As we have said that DP measurement is necessary to know the flow rate. By measuring the difference in fluid pressure while the fluid flows through the pipe we can calculate the flow rate.
Flow measurement, in general, includes both primary and secondary elements. As the flow increases, the primary element produces a pressure difference. Orifice, Venturi, Pitot tube, and other primary elements are the most common.
What is a Differential Pressure Transmitter?
A differential pressure transmitter is a secondary element of the instrument. It measures the pressure produced by the primary element. It consists of two pressure sensing elements at the bottom and the electronics circuit on the top. The pressure difference between two points; marked as “high” and “low” is measured and then converts into an electrical signal 4- 20 mA. As it also may include digital communication like HAR, Profibus, or Modbus. The measurement of the pressure transmitter must not be affected by changes in fluid pressure or temperature. Or any other properties
What is the construction of the DP transmitter?
There are mainly three units in the differential pressure transmitter:
- Pressure sensing Elements
- Electronic Circuit
- 4-20mA Current Transmitter
As we discussed above there are two pressure sensing elements at the bottom of the transmitter are marked as High and Low. Commonly diaphragm is used as the pressure sensing element in the DP transmitter. It is located between the two pressure sensing element. The diaphragm will deflect as the pressure is applied between two pressure points. And this deflection is converted into an electrical signal using sensors such as differential capacitance, vibrating wire, or string gauges to the produced electrical signal that will be sent to the electronic housing where it is processed, converted, and amplified into a current or voltage output that can be read by the control system.
Electronic Circuit:
The electronic circuit is located at the top of the transmitter which is used to amplify the signal produced by the sensor. As the sensor converts the signal at a very low range voltage usually in millivolt it needs to be amplifier amplify and convert into 4-20 mA.4-20mA Current Transmitter:
The current output is commonly a 4 to 20 milliamp signal or in some cases a zero to five or zero to 10 volts signal. The electrical output generated by the DP transmitter electronic module is linear and proportional to the actually measured delta p. Depending on the range of measured pressure, a four milliamp signal would equal a measure delta P of zero 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.
The converted output of the Electronic housing is 4 to 20 milliamp or in some cases, zero to 5 volts or 10 volts. Depending upon the range of measurement a four milliamp signal is equal to the 0 Psi and a 20 Psi signal is equal to the maximum range. The electrical signal is directly displayed to PLC closed-loop controller, etc.
What is the operation of the Differential Pressure Transmitter?
The most common uses are for level and flow measurement. We'll look at how a DEP transmitter measures water flow in a pipe in this section. The primary element of a DEP transmitter is commonly used to measure flow by introducing a constriction in the pipe. When a restrictive element is used on the primary side, high and low pressure is created. More pressure drop is created as the flow increases. The secondary element will, as we recall, measure the difference in pressure caused by the primary element. The difference in pressure created by the primary element will be measured by the secondary element.
One side of the diaphragm receives pressure from the “high” port and the other side o the diaphragm receives pressure from the “Low” port.
When the pressure at the “H” port is more than the “L” port, then the output of the transmitter will go towards the 20mA. When the pressure at the “H” port and “L” port is similar, then the output of the transmitter will be at 4mA. When the pressure at the “H” port is less than the “L” port, then the output of the transmitter will go towards the 4mA.
When the pressure at the “H” port is more than the “L” port, then the output of the transmitter will go towards the 20mA. When the pressure at the “H” port and “L” port is similar, then the output of the transmitter will be at 4mA. When the pressure at the “H” port is less than the “L” port, then the output of the transmitter will go towards the 4mA.
These readings are sent to the electronics housing, where they are processed using the Bernoulli equation, which states that the pressure drop across a constriction is proportional to the square of the flow rate.
------------------------------------------------------------------------
0 Comments