Proportional only (‘P’). Reverse Action Pressure Controller fitted with a wax filled type sensor as used in the direct operated valves.

Proportional and Integral (‘P+I’). Mercury in steel sensor housed in stainless steel pocket and connected to the controller measuring unit by a capillary tube.

Proportional, Integral and Derivative (‘P+I+D’). This system may use a controller having an electro-mechanical transducer which converts an electric input signal of 4-20 mA or 0-20 mA; 1-5V Dc or RT 100 resistance to a pneumatic signal which is then relayed to the valve actuator.