Explain pressure actuated switch with neat constructional diagram.
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Pressure actuated switch: A pressure actuated switch is a device designed to monitor a process pressure and provide an output when a set pressure (setpoint) is reached. A pressure actuated switch does this by applying the process pressure to a diaphragm or piston to generate a force which is compared to that of a precompressed range spring. A pressure actuated switch is used to detect the presence of fluid pressure. Most pressure switches se diaphragm or below as the sensing element. The movement of this sensing element is used to actuate one or more switch contacts to indicate as alarm or initiate a control action.

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The basic parts of a typical pressure actuated switch are shown in the schematic diagram.


 i) Micro-switch ii) Insulated trip button iii) Operating pin iv) Trip setting nut v) Range spring vi) Operating piston vii) Switch case or housing

Operating Principle: The inlet pressure is applied to the bottom of the operating piston. This piston is forced upwards by the inlet pressure against the range spring. The tension of the range spring can be adjusted so that it is compressed at a certain pressure or setpoint. When this pressure is reached, the operating pin will hit the trip button on the micro-switch and change it over. The normally open contacts (NO to C) will become closed and the normally closed contacts (NC to O) will open. The pressure at which the micro-switch changes over is set by adjusting the trip setting nut. This nut adjusts the tension of the range spring (e.g. if the nt is turned clockwise the trip pressure will be higher). 

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