Pressure switches are actuated by a change in the pressure of a liquid or gas.  They activate electromechanical or solid-state switches upon reaching a specific pressure level. The most important parameter to consider is the pressure range that is needed to be switched at, or the range over which linear output is needed.  Choices for pressure switches measurement ranges include absolute, gauge, vacuum, differential, and sealed. Absolute switches are used where pressures are to be measured independently of the natural fluctuations in atmospheric pressure. The pressure of the media to be measured is compared against a reference pressure of absolute zero (absolute vacuum) in a sealed reference chamber.  Vacuum measurement switches measure vacuum pressure (negative pressure).  Differential pressure switches give the relative pressure between two points. If both operating pressures are the same, the measuring element cannot move and no pressure will be indicated. A differential pressure is indicated when one pressure is higher or lower. Low differential pressures can be measured directly in cases of high static pressures.  Sealed gauge pressure measurement is similar in concept to an Absolute Pressure switch, except that the pressure of the media to be measured is compared to standard atmospheric pressure (at sea-level).

Pressure switches can have an electro-mechanical or solid-state switch construction.  The normal state can be normally open (NO), or normally closed (NC).  Pressure switches can have single poles (SP), double poles (DP), and more than two poles.  The throw type on pressure switches can be single throw (ST) or double throw (DT).  The switch may be TTL compatible.  Electrical characteristics of pressure switches to consider include switched current, switched AC volts, and switched DC volts.

Pressure switches may be a sensor element or chip, a sensor or transducer, an instrument or meter, a gauge or indicator, a recorder or totalizer, and a controller.  A sensor element or chip denotes a "raw" device such as a strain gage, or one with no integral signal conditioning or packaging.  A sensor or transducer is a more complex device with packaging and/or signal conditioning that is powered and provides an output such a dc voltage, a 4-20mA current loop, etc.  An instrument or meter is a self-contained unit that provides an output such as a display locally at or near the device. Typically also includes signal processing and/or conditioning.  A gauge or indicator is a device that has a (usually analog) display and no electronic output such as a tension gage.  A recorder or totalizer is an instrument that records, totalizes, or tracks force measurement over time.  Includes simple datalogging capability or advanced features such as mathematical functions, graphing, etc.  

Sensor technologies for pressure switches include piston, diaphragm, strain gauge, semiconductor piezoresistive, piezoelectric, MEMs, variable capacitance, and vibrating element.  Other important parameters to consider include switch accuracy, the type of material to be measured, features such as temperature compensation and temperature output, and electrical output.  Important operating parameters to consider include operating temperature, maximum shock, and maximum vibration.

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