Action of flow controllers, temperature controllers, level controllers and pressure controllers.

For level and pressure controllers where outlet flow is controlled, the controller OP will increase with increasing PV.

• For fail safe close valves (Air to open) like level controllers, the controller is direct acting
• For fail safe open valves (Air to close) like pressure controllers, the controller is reverse acting

For level and pressure controllers where inlet flow is controlled, the controller OP will decrease with increasing PV.

• For fail safe close valves (Air to open) like level controllers, the controller is reverse acting
• For fail safe open valves (Air to close) like pressure controllers, the controller is direct acting

For flow controllers, the controller OP will decrease with increasing PV (same as the case of level/pressure controller where inlet flow is controlled).

• For fail safe close valves (Air to open) like level controllers, the controller is reverse acting
• For fail safe open valves (Air to close) like pressure controllers, the controller is direct acting

Permissive, interlock - Difference

The PERMISSIVES, are some conditions that need to be satisfied before you can start the machine. For e.g. a compressor can be started only when there is sufficient suction pressure.

INTERLOCK: During the process, if a condition fails, interlocks are activated. For e.g. a pump shutdown interlock is activated when the level of drum goes low.

An interlock can also be a permissive, but the converse is not true. For the compressor example (in Permissives), if insufficient suction pressure is a permissive, the compressor will not shutdown, if there is insufficient suction pressure. It is just a condition for the system to start. But if insufficient suction pressure was an interlock, the system will shutdown, when ever the suction pressure goes below a fixed value. But once shutdown, all the permissives need to be satisfied before the system can start again.

More details here