Virtually all pneumatic measuring systems depend on a primary element such as an orifice plate, Bourdon tube, etc., to convert the physical parameter to be measured into either a force or a displacement which, in turn, can be sensed by some form of flapper/nozzle system or used directly to operate a mechanism
Filled thermal systems are used almost exclusively in pneumatic systems for temperature measurement and control
Temperature measurement using pneumatic system:
Filled thermal systems are used almost exclusively in pneumatic systems for temperature measurement and control.
The sensing part of the system consists of a bulb connected to a spiral or helical Bourdon element or bellows through a fine capillary tube.
The increased volume of the enclosed fluid or its pressure is transmitted to the Bourdon element when the bulb temperature rises, which response to the change by moving its free end.
This movement can be used directly to position a recording pen or an indicating pointer, either of which can also be combined with a flapper/nozzle mechanism by a system of links and levers for pneumatic controller actuation.
When a bellows is used to terminate the capillary the change in bulb temperature is converted into a force which, in turn, can be used to actuate a force-balance mechanism.
In the motion-balance systems the free end of the Bourdon tube is connected via adjustable links to a lever pivoted as shown in above figure.