The Air Lock pressure static device (AL) is mounted on pneumatic actuators when they are to be isolated from control devices (positioners, electro-pneumatic distributors, etc.) whenever air pressure falls below operating safety point.
The pressure switch device AL has been designed to meet different air-lock systems, to guarantee a specific safety position of actuators, as required by the final control element. The Air-Lock device consists mainly in a pneumatic pressure switch with set point calibration. The hysteresis is 20 kPa (15 PSI) approximately. This item operates one or more 3-way synchronous commutators, depending on the function required. 
Switching device operates immediately, also in case of a gradual air-pressure decrease to the set value. The differential avoids a continuous, inopportune commutation when air-pressure swings around the set point