Mechanical Switches

Mechanical switches, pressure or vacuum, employ a spring-biased diaphragm as a sensing mechanism. Electrical-output versions include a standard SPDT snap-action switch that can be wired normally open (NO) or normally closed (NC) as the application requires. 

Pneumatic output versions include a three-way servo-piloted valve and must be ordered specifically as NO or NC output. The servo-pilot greatly reduces the valve actuating force so it can be operated by a small diaphragm. However, being a bleed-type device, it leaks a small amount of compressed air to make it work (this is not a defect).