In this unit, the air supply passes through a restriction before being applied to the top of the relay diaphragm and the nozzle. An increase in process pressure tends to straighten out the spiral, which causes the flapper to move closer to the nozzle. This increases the nozzle back pressure sensed by the relay diaphragm, which will move down, opening up the air supply to the output. The increased output pressure is felt by the feedback bellows and restores the flapper to its throttlingposition. For each value of process pressure there is a corresponding definite flapper position and output pressure, hence, the name motion balance. The inaccuracy of these motion balance pressure transmitters is generally placed at ± 0.5%. As shown in Figure 5.4g, they will read the process pressure even if the air supply is lost.