What is the working principle of pneumatic transmeter?

I think your are asking about the working principle of a pneumatic transmitter. (not transmeter) A pneumatic transmitter measures liquds or air temperatures over a given range-usually a 200 degree temperature range. The range, selected for its purpose could be -40 to +160 or 0 to 200 -as in the case of Honeywell or even 100 deg. ranges, as in the case of say-Johnson Controls. (0-100) Aside from temperature range, the other important criteria is the output of the transmitter. In the case of commercial controls, the correlation of 200 degree temperatures (which is 201 values, not 200) occurs over a pressure span/output of 3 to 15 pounds.(13 values) For example, in the case of a 200 degree transmitter of say 0 to 200 degrees, the zero temperature value output equates to a 3 psi signal. At 200 degrees, the top of the range, the transmitter would output a 15 psi signal. In other words every degree change up from zero degrees would account for a pressure increase (rounded)of .064 psi. (13 pounds divided by 201 degrees)=.0646766) Pneumatic transmitters are typically direct acting and are one-pipe constant bleed devices. Their working principle involves movement of a flapper a given distance in direct response to the sensed temperature acting upon it. As the temperature rises, the bleed port positions itself toward a closed position which allows less air to escape to atmosphere. Less air to atmosphere dictates more air building up in the air line. This value is then transmitted/measured by the controller-which is pneumatically attached to the transmitter.