Pressure switch
A pressure switch is a form of
Another type of pressure switch detects mechanical force; for example, a pressure-sensitive mat is used to automatically open doors on commercial buildings. Such sensors are also used in security alarm applications such as pressure sensitive floors.
Construction and types
A pressure switch for sensing fluid pressure contains a capsule, bellows,
The pressure switch may be adjustable, by moving the contacts or adjusting tension in a counterbalance spring. Industrial pressure switches may have a calibrated scale and pointer to show the set point of the switch. A pressure switch will have a hysteresis, that is, a differential range around its setpoint, known as the switch's deadband, inside which small changes of pressure do not influence the state of the contacts. Some types allow adjustment of the differential.[1]
The pressure-sensing element of a pressure switch may be arranged to respond to the difference of two pressures. Such switches are useful when the difference is significant, for example, to detect a clogged filter in a water supply system. The switches must be designed to respond only to the difference and not to false-operate for changes in the common mode pressure.
The contacts of the pressure switch may be rated a few tenths of an ampere to around 15 amperes, with smaller ratings found on more sensitive switches. Often a pressure switch will operate a relay or other control device, but some types can directly control small electric motors or other loads.
Since the internal parts of the switch are exposed to the process fluid, they must be chosen to balance strength and life expectancy against compatibility with process fluids. For example, rubber diaphragms are commonly used in contact with water, but would quickly degrade if used in a system containing mineral oil.
Switches designed for use in
An electronic pressure switch incorporates some variety of
Pneumatic
Uses of pneumatic pressure switches include:
- Switch a household well water pump automatically when water is drawn from the pressure tank.
- Switching off an electrically driven gas compressor when a set pressure is achieved in the reservoir
- Switching off a gas compressor, whenever there is no feed in the suction stage.
- in-cell charge control in a battery
- Switching on an alarm light in the cockpit of an aircraft if cabin pressure (based on altitude) is critically low.
- Air filled hoses that activate switches when vehicles drive over them. Common for counting traffic and at gas stations.
Hydraulic
Hydraulic pressure switches have various uses in
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. A differential pressure switch may be installed across a filter element to sense increased pressure drop, indicating the need for filter cleaning or replacement.
Examples
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Mechanical Pressure Switch with integrated connector
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Mechanical Pressure Switch with Plug-in connector
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Electrical pressure switch with LED display
Pressure sensitive mat
A pressure sensitive mat provides a contact signal when force is applied anywhere within the area of the mat. Some mats provide a single signal, while others can resolve the position of the applied force within the mat. Pressure sensitive mats can be used to operate electrically operated doors, or as part of an interlock system to ensure machine operators are clear of dangerous areas of a process before it operates. Pressure sensitive mats can be used to detect persons walking over a particular point, as part of a security alarm system or to count attendance, or for other purposes.
See also
- Dynamic pressure
- List of sensors
- Pressure sensor
References
- ISBN 1420064029pages 790-793