How a Pressure Switch Works

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What is a Pressure Switch?
This is a device designed to monitor a process pressure and provide an output when a set pressure (setpoint) is reached. A pressure switch does this by applying the process pressure to a diaphragm or piston to generate a force which is compared to that of a pre-compressed range spring.

A pressure switch is used to detect the presence of fluid pressure. Most pressure switches use a diaphragm or bellow as the sensing element. The movement of this sensing element is used to
actuate one or more switch contacts to indicate an alarm or initiate a control action.

Pressure switches have different designs with different sensing elements. One of the most common is the one with diaphragms or bellows as the sensing elements. The one I will discuss here uses a piston as the pressure sensing element. In any case, the operating principle for this piston type is the same with a diaphragm or bellow type pressure switch.
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Basic Parts of a Pressure Switch:
The basic parts of a typical pressure switch are shown in the schematic diagram below:

Pressure Switch Diagram
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A sectional view of the pressure switch showing all the basic parts of the switch is shown above.
Also shown below is a pictorial view of the pressure switch:

The following basic parts can be identified on the sectional view of the pressure switch:
  • Micro-switch
  • Insulated trip button
  • Operating pin
  • Trip setting nut
  • Range spring
  • Operating piston
  • Switch case or housing

Micro-switch
The micro-switch is used to make or break an electrical circuit when the pressure switch operates. The micro-switch in the sectional view is a single-pole double-throw (S.P.D.T.) switch. This switch is made up of one normally close contact (NC) and one normally open contact(NO). When the pressure switch actuates, the NO contact become close and the NC becomes open.

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Micro-switches with gold contacts are normally used on low voltage, low current applications (i.e. on circuits that are intrinsically safe). For higher voltages/currents silver contacts are used.

Insulated Trip Button
This button causes the NO and NC contacts to switch when the pressure switch actuates

Operating Pin
As shown in the sectional view of the pressure switch above, the operating pin is attached to the operating piston. When the piston actuates as a result of inlet pressure changes, the operating pin either moves up and make contact with the trip button or moves down and breaks contact with the trip button



Trip Setting Nut
The trip setting nut(could also be called a range screw) is used to adjust the setpoint of the pressure switch. It does this by changing the amount of compression on the range spring. The more the range spring is compressed by the trip setting nut, the higher the pressure set point for the switch. The lesser the compression, the lower the setpoint for the pressure switch

Range Spring
This is a pre-compressed spring and the force generated by this determines the pressure at which the switch operates.

Operating Piston
This is the part of the pressure switch in contact with the process. The process pressure acting upon the area of the operating piston generates the force that opposes that of the range spring. The range of the switch is a function of the area of the operating piston and the rate of the range spring (measured in lbf/inch, N/mm etc).

Switch Case or Housing
The enclosure containing the micro-switch and other accessories of the pressure switch is called the switch case or housing. For a pressure switch giving out an electrical output one or more tapped connections are provided to allow the cable to be brought into the housing via a suitable gland. For switch with a pneumatic output two or more bulkhead connections are provided for the output connections. Switch housings are usually available in either aluminium or stainless steel.

Operating Principle of a Pressure Switch
As shown in the sectional view of the pressure switch above, 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 C) 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 nut is turned clockwise the trip pressure will be higher). 

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