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Flow Switch : What is it ?
Listed on Nov 30,2018
By admin
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.

The flow switch consists of a paddle system. which has a permanent magnet located at its upper end. A reed contact is positioned outside the flow above this magnet. A second, magnet 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 changes its position in relation to the reed switch contact. 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 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.

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.


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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