Pneumatic DP type transmitter require air supply to operate, no power require whatsoever. This was one of the most commonly used type of transmitter and still in use.
A DP sensor is a diaphragm capsule in which two metallic sulcate diaphragms are used and the inside area is filled with silicon oil which is shown in figure. Used oil isolate process fluid and can very based on application.
A force bar is pivoted in the diaphragm capsule and with respect to change in pressure the force bar moves towards low pressure side.
The movement of force bar is utilized to adjust the position of flapper nozzle mechanism. A constant 20 psi air supply is given to air relay and is connected to the nozzle. Increase in gap between flapper and nozzle will decrease output pressure.
It provide output between 3-15 psi.
For calibration it contain zero and span screw by adjusting them we can calibrate transmitter. Here is in figure 13 is zero adjustment and 7 is span adjustment.
1. Input supply 20 PSI
2. Output supply 3 – 15 PSI
3. Air relay
7. Span adjusting screw
8. Connecting link
9. Force bar
10. Differential pressure capsule
11. Drain plug
12. vent valve
13. Zero screw
15. Air to feedback bellow