A mechanical switch consists of a spring and a diaphragm to control at what pressure the micro-switch is triggered. The spring acts as an opposing force to the inlet pressure and preset pressure or pretension is adjusted with a set screw or knob. The spring’s pretension depends on the pressure at which the switch makes contact. The switch comes/resets to its original state when the pressure drops.
When the preset pressure is reached and the switch is actuated, it will send an electrical signal for which no auxiliary power is needed. The difference between the switch point and reset point is called hysteresis and is expressed as a percentage of the switch point value.
There are three different types of contacts in these pressure switches: normally open (NO), normally closed (NC), and changeover (SPDT) contacts. For both NO and NC operations changeover contacts were used. The mechanical pressure switch is better than an electronic pressure switch suited for handling high voltages. For an increase or decrease in pressure, it should be used to make a contact change.
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Instrumentation Basics 