Larger valves tend to be fitted with pneumatic actuators. The positioner is used to position the valve by filling the actuator with the corresponding amount of air, or by venting from the actuator. However, the air output capacity required to achieve fast positioning in larger actuators (with diaphragm areas of 1,400cm2 up to 2,800cm2) is simply too much for normal positioners and so accuracy can be compromised. Also, an added complication in these situations is that the positioner controls a volume booster which achieves a much higher air capacity than the positioner and consequently results in an overshoot on responding to small signal changes.
The Samson control valve positioner package volume boosters with different air output capacities overcomes this problem.