Differential pressure is sensed by a liquid-filled diaphragm “capsule,” which transmits force to a “force bar.” If the force bar moves out of position due to this applied force, a highly sensitive “baffle” and “nozzle” mechanism senses it and causes a pneumatic amplifier (called a “relay”) to send a different amount of air pressure to a bellows unit. The bellows presses against the “range bar” which pivots to counter-act the initial motion of the force bar. When the system returns to equilibrium, the air pressure inside the bellows will be a direct, linear representation of the process fluid pressure applied to the diaphragm capsule.