How Does a Flow Switch Work?

  • Published December 23, 2019
  • Sensors
  • by Proteusind

Flow switches are mechanical devices used in controlling the flow of air, steam, or liquid. A flow switch operates by conveying a trip motion (relay, reed switch, paddle) to another machine within the system, usually a pump. The trip signal will indicate to the pump to turn on or turn off.  It is necessary to provide damage protection and circuit cooling or adjust flow rates that are too high or too low.

Flow switches can serve numerous functions and also get used in a variety of applications. If one needs to monitor the flow rate or to protect the pump, then the flow switch can serve your purpose. The flow sensor will send an electrical signal to PLC or any other electronic controller.

Definition of Flow

Flow refers to the velocity or physical movement of gas, steam, or liquid within a pipe that triggers the actuation of the flow switch. When there is no flow present, the velocity either drops or completely stops; in either case, the switch will revert to its original position. A complete stoppage or decrease in velocity gets indicated when there is no flow. Also, the stoppage allows the switch to return to its original position.

Working of the flow switch 

Firstly the connected paddles activate the flow switches. As water and gas passes, the connected paddle gets displaced. The switch will automatically lock the circuit off when the substance’s volume will be too high and too low. It is done to prevent any damage to the system.

Flow Switch Uses

A liquid flow switch is applicable in the chlorination of a swimming pool, hot water heating, air conditioning, liquid transfer systems, fire sprinkler systems, and industrial laser cooling systems.

Water flow switches are implemented in water sprinkler systems. In the case of a fire, for example, a flow switch can trigger an electrical fire alarm device to turn on.

Airflow switches are used to cleanroom filter systems, exhaust ventilation, and air treatment systems.

Flow switches are used in many other applications, such as:

  • Water treatment systems
  • Additive or blending systems
  • Air supply systems
  • Duct type heating

Also, you can read All You Need to Know About the Vortex Type Flow Meter

More Flow switch uses

  • The flow switch triggers an alarm when the supply gets off.
  • When there is insufficient flow, the electric motor will shut down.
  • The flow switch also gets used in air conditioning and ventilator units.
  • In the central heating systems, the flow switch gets used.
  • In the chlorination systems fitted in swimming pools, flow switches can be used.

Working of Sensors

One should know about the following factors to select the best sensor. The applications in which flow switches are used are as follows: in coolant systems, pump protection, and lubrication systems.

Services provided by flow switches

Flow switches can provide their services in all kinds of liquids and gases and large pipes. The types of fluids which are monitored by the 100 series flow switches are described here.

What are some of the functions of a flow switch?

Flow switches perform precise functions based on set requirements. They are capable of stopping a motor when there is no flow as well as triggering the same motor to run once the flow starts. It can sound an alarm when the flow arrests, and it can turn off the alarm when the flow rate reaches an appropriate level.

Since flow switches play such an integral role in many industries, there are a few important considerations.

Measuring flow rate accurately is crucial. This is inferred by calculating the change in dynamic energy or velocity; the velocity, in turn, is dependent upon the pressure differential that forces the liquid through the conduit or pipe. Since the cross-sectional area of a pipe is constant and known, the velocity is a good indication of the flow rate. Calculating the flow rate must be done as it assists in setting flow switches to a pre-determined close function.

In closing, here are some types of flow switches to consider for your specific use.

Vane-Operated Flow Switch

A vane-operated flow switch relies on fluid pushing against an internal paddle. As the flow drops to preset levels, it triggers an internal magnetic mechanical and coupling switch actuation lever which essentially trips the switch.

Variable Area Flow Switch

This device has an internal piston that activates the switch. The flow enters a port and adds pressure to the magnetic poppet; once this pressure hits the preset level, the switch will turn off.

Ultrasonic (Doppler) Flow Switch

Attached to the outside of a pipe, ultrasonic sensors send signals when it detects a change in the flow.

Model available 

At present, the flow switch model is available to us.

100 series 

In flow ok position models, reliability cannot jam as it gets assured by the active design. There has been the transition from non-RoHS compliant to RoHS compliant. The material of the rotor also got changed from a nylon-based to PPS based.

  • 100 series flow switches come with a five-year warranty
  • comprise a flow rate of 0.2-227 LPM.
  • The flow switch has an adjustable trip point. 
  • It cannot jam in flow ok position.


There are numerous other benefits of flow switches. Besides this, various models are also available in flow switches, which can serve your variety of purposes. One should not underestimate the significance of flow switches.

Here are a few more topics that you shouldn’t miss:
What is a liquid flow meter & what type of application is it used for?
What Is Water Flow Meter And What Type Of Application Is Suited For It?
What Is The Principle Of Operation Of Thermal Control Flow Meters?

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