source inlet port 42 that accepts pressurized gas from a fluid source not shown the positioner 34 may have an outlet port 43 that is fluidly coupled to an inlet port 44 of the actuator diaphragm casing 22 the actuator diaphragm casing may have an outlet vent 45 for venting the non pressurized side of the diaphragm casing 40 while the embodiment of fig2 illustrates that the vent outlet port 45 may be coupled to the positioner 34 it may be common that vent 45 is open to ambient atmosphere the positioner 34 may generally operate to control the amount and timing of applied gas to the diaphragm casing 40 from a pressurized source such as a compressed air source not shown as gas is applied to the inlet port 44 of the actuator the pressure in the actuator chamber e g chamber 46 may exert a force on the actuator rod 35 that is proportional to the applied pressure and the effective area of the actuator diaphragm 36 thus as the diaphragm 36 is displaced the volume of a lower chamber 47 may source or sink fluid through outlet port 45 while fig2 illustrates a pneumatic actuator embodiment employing a plurality of springs 37 it should be noted that some pneumatic actuators may use only a single spring such embodiments may be illustrated in fig3 a and 3b in fig3 a and 3b a single spring 48 and 49 is used to bias a diaphragm 51 and 53 respectively.