Pressure transmitter



 Pressure transmitter





Pressure transmitter: is the device that measure the pressure and send it to the controller as an Analogue signal or Continuous signal mostly 4 :20 mA


 


Pressure transmitter construction

  • Pressure port
  • Sensor
  • Signal conditioning electronic module
  • Electrical connection
Pressure port
Primary element of pressure measurement
  • Bourdon tube
  • Diaphragm
  • Capsule
  • Bellow


Sensor (Transducer)

Sensing element transform diaphragm deformation under pressure into electric signal (mV)

Sensing principle : 



  • Strain gauge
  • Capacitance
  • Potentiometer
  • LVDT "Linear Variable Differential Transformer"







Signal Conditioning "Transmitter"








Converts sensor signal into standard normalized electric output such as

  • 4:20 mA
  • 0:20 mA
  • 1:5 V
  • 0:10 V

Also capable of digital communication using HART protocol or Modbus protocol


Why is 4:20 mA is preferred over 0:20 mA Signal?  

Live zero in signal 4:20 mA detect wire break 

 

Why is 4:20 mA is preferred over 0:10 V Signal?  

Voltage signal has voltage drop across long distance and it is easily affected with external interference

4:20mA signal can travel up to 1Km with out losses and current signal less sensitive to electrical noise




 Disadvantages of 4:20mA











The current signal may induce magnetic fields (induced noise current ) and cross talk to the parallel cables

This can be eliminated using twisted wire cables

One pair of cable can only carry one process variable

 
Electrical Connection

Electrical wiring that is used to wire a pressure transmitter to PLC , DCS

  • 2-wires (24 VDC same for 4:20 mA)
  • 3-wires 
  • 4-wires

 


How to Select a Pressure Transmitter?

1. Type of pressure measured

  • Gauge pressure
  • Absolute pressure
  • Differential pressure

2. Confirm the pressure range
Under normal operation, the actual measurement pressure is selected as 125% of the measurement

3- The measured media

Clean dry air , Dry gas, Viscous liquid, etc.

4. Confirm the system’s maximum overload

5. Confirm the accuracy level

6. Confirm the operating temperature range

7. Confirm the pressure interface connection

8. Confirm power supply and output signal

9. IP ingress protection

11.Turndown Ratio

 

How to Calibrate pressure transmitter in lab?


  • Fix reference gauge and Instrument under calibration on the comparator.
  • Apply Test points (0 , 25% , 50% , 75% and 100% of span).
  • Apply the pressure slowly and take the readings.
  • Observe the Instrument under calibration reading compared to reference gauge.
  • At least 30 sec waiting to observe readings
  • Calculate measurement and final results based on observation and device accuracy
  • Make Zero trim and Upper range trim by HART communicator if needed

 


 


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