What is principal of pressure transmitter?

well brother i am sorry to say that i am not getting your question clearly..  first of all for pressure measurement most methods are based on displacement and force.if its about pressure measuring then we have different instruments for them like Manometer, Diaphragm, Bailos, or the Borden tubes,( C type, Helix type, Spiral Type) All these are for low pressure measurement , while for high pressure measurement in industry usually Stain Gauge. 

but if your concern is about pressure transmitter then you should know that for trasnsmitting you should know  pressure” has to be converted into an electrically quantifiable variable.  and then you can transmit it, and for that transmission you will require pressure transducers and sensors like  Ceramic Thick Film Sensor, LVDT (Linear Variable Differential Transformer) Sensor, Piezoresistive (Piezo) Senso, Thin Film Sensor

The Pressure transmitters can be based on below principal:

1. Capacitance Type  (flexible diaphragm)

2. Inductive Type ( inductive coil)

3. Peizo resistive type (Strain gauge)

The basic principle of diaphram type pressure transmitter is'piezoelectric'effect. the capability of a material to produce current from a given stress. this principle works in all the pts. the applied pressure is converted to ma by the diaphram.

converting measured pressure by certain method like burden tube or diaphragm to be measured in an electrical method in volts or mA

Mr Shahab explain must part.

There are many pressure measuring technique like the manometer and bourdon with others.

any how must transmitter now containing diaphragm. So the main point is to measure the deflection as strain. then convert this into 4-20 ma 

Pressure transmitters are working simply converting pressure acting on unit area to Mechanical moments . which is suitably calibrated to indicate applied pressure .

A pressure transmitter senses a pressure and outputs a proportional current signal. THe working principle varies. Some are resistive, some are capacitive, some are piezoelectric, all with an amplifier and filtering circuit attached. You need to consult your sensor documentation to check what’s it type and how it works.

Differential pressure theory