What is Pressure Switch

A pressure switch is a form of switch that operates electrical contacts when its input reaches a certain set fluid pressure.Switches can be designed to make contact on rising or falling pressure.Pressure switches are widely used in industry to automatically supervise and control systems that use pressurized fluids.Another type of pressure switch detects mechanical force; for example,pressure-sensitive pads are used to automatically open doors in commercial buildings.These sensors are also used in security alarm applications such as pressure sensitive floors.

Structure and type 

Pressure switches used to sense fluid pressure consist of a capsule,bellows,Bourdon tube,diaphragm,or piston element that deforms or displaces proportionally to the applied pressure.The resulting motion is applied directly or via an amplifying rod to a set of switch contacts.Since the pressure may change slowly but the contacts should act quickly,some kind of over-center mechanism (such as a miniature snap action switch) is used to ensure fast action of the contacts.A sensitive pressure switch uses a mercury switch mounted on a Bourdon tube; the moving weight of the mercury provides a useful eccentricity characteristic.Pressure switches can be adjusted by moving contacts or adjusting the tension in the balance spring.Industrial pressure switches may have a calibrated scale and needle to show the set point of the switch.Pressure switches suffer from hysteresis,a differential range around their set point called the switch's deadband,within which small changes in pressure do not affect the state of the contacts.Some types allow adjusting for variance.The pressure sensing element of the pressure switch may be arranged to respond to the difference of the two pressures.Such switches are useful when the difference is large,for example,to detect if a filter in a water supply system is clogged.Switches must be designed to respond only to differentials and not to misoperate due to changes in common-mode pressure.Pressure switch contacts may be rated from a few tenths of an amp to around 15 amps,with less ratings on more sensitive switches.Pressure switches typically operate relays or other control devices,but some types can directly control small motors or other loads.Since the internal parts of the switch are exposed to the process fluid,they must be selected for a balance of strength and life expectancy as well as compatibility with the process fluid.For example,rubber diaphragms are typically used in contact with water,but will degrade rapidly if used in systems containing mineral oil.Switches designed for use in hazardous areas where flammable gases are present have an enclosure that prevents arcing at the contacts from igniting the surrounding gas.Switch housings may also need to be weatherproof,corrosion resistant,or submersible.An electronic pressure switch consists of a variety of pressure sensors (strain gauges,capacitive elements,or other) and internal circuitry to compare the measured pressure to a set point.These devices can provide improved repeatability,accuracy and precision compared to mechanical switches.

Pneumatic

Uses for pneumatic pressure switches include:

• Automatic switching of domestic well water pumps when water is drawn from the pressure tank.

• Turns off the electric gas compressor when the set pressure is reached in the air tank

• Shut down the gas compressor as long as there is no feed in the suction phase.

•  In-battery charge control

• If the cabin pressure (based on altitude) is extremely low, turn on the warning lights in the cockpit of the aircraft.

• The inflation hose activates the switch when a vehicle passes by. Commonly used in traffic statistics and gas stations.

Hydraulic

Hydraulic pressure switches are used in automobiles for a variety of purposes,for example,to warn when engine oil pressure falls below a safe level,or to control automatic transmission torque converter lockup.Prior to the 1960s,pressure switches were used in hydraulic brake circuits to control power to the brake lights;more recent cars use a switch that is directly actuated by the brake pedal.In a dust control system (baghouse) a pressure switch is installed on the header which will sound an alarm when the air pressure in the header falls below the necessary value.A differential pressure switch can be mounted on the filter element to detect an increase in pressure drop,indicating the need for filter cleaning or replacement.