Virtually all pneumatic instruments used for measuring process parameters such as pressure, flow, level and temperature use a flapper/nozzle sensor and pneumatic relay . In real pneumatic instruments, there are two basic schemes for utilizing the flapper/nozzle/pneumatic relay system, namely, the motion-balance and the force-balance systems.
|Force - balance principle in pneumatic systems|
As shown above, when an input pressure signal, is applied to a bellows which is opposed by a similar bellows for the reference signal, the differential force applied to the lever supported on an adjustable pivot is opposed by a spring/bellows combination. Adjacent to the bellows is a flapper/ nozzle sensor.
During operation, if the input signal exceeds the reference signal, the resultant force causes the force bar to rotate clockwise about the adjustable pivot so that the flapper moves closer to the nozzle, with the result that the pressure in the output bellows increases until equilibrium of the force bar is re-established. The change in output pressure is then proportional to the change in the measurement signal and a standard 3 – 15 psig signal is generated.
Practically 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 sensor or used directly to operate a mechanism such as an indicator, a recorder pen, or a switch. The measurements most widely used in the process industries are temperature, pressure, flow, level, and density