Flow Switch : What is it ?

Flow Switch is a mechanical switch that is switched on or off in response to the flow or non-flow of a fluid such as air or water. The switch typically operates through the use of a paddle which gets displaced due to the force of fluid moving past it.

Flow Switch

Flow Switch Working

Image Credits : omega

The flow switch consists of a paddle system (1) which has a permanent magnet (2) located at its upper end. A reed contact (3) is positioned outside the flow above this magnet. A second, magnet (4) with opposite polarity is used to create a reset force.

The paddle system is moved once it comes into contact with the flow which is to be monitored. The magnet (2) changes its position in relation to the reed switch contact (3). The contact opens/closes depending on the contact type.

As soon as the flow is interrupted, the paddle returns to its original position and the reed switch contact opens/closes depending on the contact type. This change in contact (NO to NC or NC to NO) will be used to indicate the required flow signal output.

Flow Switch Principle

Flow switch is a special electric device with a very simple design and of a small size. But despite of its simplicity, flow switch plays important role in the variety of different applications.

These devices are used with the purpose to protect a pump and when the flow of air, gas or liquid through the specific line is needed to be monitored.

Also the alarms can be triggered with the help of them when it is necessary. Switches may be specific to a type of application.

Also Read : Identify Two Pressure Switches Logic

Why Flow switch Shows wrong indication ?

The below are the possible causes for the false activation of flow switch :

  • Entrapped air or gas.
  • Turbulence in tubing or unit
  • Mechanical sensor assembly damaged.
  • Liquid is too viscous.
  • Not using the fittings supplied by the manufacturer.

Water Flow Switch Principle


These switches find application in the detection of fluid flow and measurement of fan speeds.

A flow switch might be used to protect a central heating system electric heating element from being energized before the air flow from the blower is established.

Flow switches might also be used to alarm if a ventilation fan in a hazardous location fails and air flow has stopped.

  • Pump protection
  • Safety spray nozzle monitoring
  • Cooling water or heat exchangers
  • Oil well system testing
  • Drain line flow
  • Relief valve monitoring

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5 thoughts on “Flow Switch : What is it ?”

  1. Excellent article on flow switch, I learned a lot of information. Flow switch we can also operate with pump. How flow switch plays a different role?


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