When absolute pressure is to be sensed with bellows elements, it normally involves two bellows, one for measuring and the other for reference. The compensating (reference) element is fully evacuated and sealed, while the sensing element is connected to the process.
Motion Balance
An increase in process pressure will cause the measuring bellows to extend, which results in an increase of readout through the motion balance mechanism. If the process pressure is constant but the barometric pressure changes, the force will be exerted equally on the outside of both bellows, causing no change in the readout. These motion balance absolute pressure sensors are inaccurate to ±1% of full scale and, can give a reasonably reliable measurement of absolute pressures.
Force Balance
The force balance mechanism is available as a pneumatic or electronic transmitter, but not as a direct indicator because there is no motion to drive a pointer. The pressure being sensed is applied to the inside of the bellows within the capsule. The space on the outside of the bellows is evacuated, thus providing a zero absolute pressure reference. The process pressure exerts a force on the capsule that is applied to the lower end of the force bar. Due to the force balance nature of the unit, the force bar is constantly balanced; the sensing bellows do not move if the pressure detected is within the range of the instrument. If the range of the capsule is exceeded, the bellows tend to extend.
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Instrumentation Basics 