A pressure switch is a form of switch

Switch

In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....

 that makes electrical contact when a certain set pressure

Pressure

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 has been reached on its input. This is used to provide on/off switching from a pneumatic or hydraulic source. The switch may be designed to make contact either on pressure rise or on pressure fall.

Pneumatic

Uses of pneumatic pressure switches include:

• Switching off an electrically driven gas compressor
  Gas compressor
  A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
  
   when a set pressure is achieved in the reservoir
  Pressure vessel
  A pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.The pressure differential is dangerous and many fatal accidents have occurred in the history of their development and operation. Consequently, their design,...
  
  
• Switching off a gas compressor, whenever there is no feed in the suction stage.
• in-cell charge control
  In-Cell Charge Control
  In-Cell Charge Control or I-C3 is a method for very rapid charging of a Nickel-metal hydride battery, patented by Rayovac. Batteries using this technology are commonly sold as "15 minute rechargeables"....
  
   in a battery
• Switching on/off an alarm light in the cockpit of an aircraft if cabin pressure (based on altitude) is critically low.

Hydraulic

Hydraulic pressure switches have various uses in automobile

Automobile

An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

s, for example:

• To switch on a warning light if engine oil pressure
  Oil pressure (internal combustion engine)
  Oil pressure is an important factor in the longevity of most internal combustion engines. With a forced lubrication system , oil is picked up by a positive displacement oil pump and forced through oil galleries into bearings, such as the main bearings, big ends and camshaft bearings...
  
   falls below a safe level
• To switch on brake lights automatically by detecting a rise in pressure in hydraulic brake
  Hydraulic brake
  The hydraulic brake is an arrangement of braking mechanism which uses brake fluid, typically containing ethylene glycol, to transfer pressure from the controlling unit, which is usually near the operator of the vehicle, to the actual brake mechanism, which is usually at or near the wheel of the...
  
  pipes
• In dust control systems (bag filter), a pressure switch is mounted on the header which will raise an alarm when air pressure in the header is less than necessary to gain or decline energy beyond the set value