What is Flow Switch : Working & Its Applications

In an industrial system, monitoring the measurement of flow rate is necessary for controlling fluid movement safely. So, flow monitoring and controlling is necessary for all industrial processing. Here the flow is the movement of gases, liquids & steam throughout a processing system. The measurement of flow rate is mainly used to measure the volume of fluid process media in a particular cross-sectional region per unit of time. So this article discusses an overview of a flow switch and its working with applications.

What is Flow Switch?

A mechanical switch that is used to monitor the flow rate & liquid pressure throughout a duct, loop, or system is known as a flow switch. This switch is also known as a flow sensor or flow indicator. The main function of the flow switch is to monitor liquid, gas, or steam flow in a particular time period to monitor total flow continuously. The flow switch symbol is shown below.

Flow Switch Symbol
Flow Switch Symbol


The flow switch is constructed with a paddle system, permanent magnet, reed contact, and a second magnet.

Flow Switch Construction
Flow Switch Construction

Paddle System

The liquid flow within the pipeline pushes the paddle of the switch to move in the flow of fluid direction. The required fluid flow to activate the paddle mainly depends on the size of the line.

Permanent Magnet

At the top of the paddle, a permanent magnet is attached to operate a reed switch to provide a potential-free output.

Reed Contact

A reed contact is arranged outside of the fluid flow above the magnet.

Second Magnet

A second magnet has an opposite polarity that is used to generate a reset force.

The paddle system moves when it approaches contact with the liquid flow which is to be monitored. The permanent magnet will change its position in relation to the contact of the reed switch. The contact of this switch will be ON or OFF based on the type of contact.

As soon as the liquid flow is disrupted, the paddle in the switch will return to its actual position. This change within the contact is mainly used to specify the required output flow signal. These switches are mainly used to guard a pump once the air, liquid, or gas flow throughout the particular line is required to be monitored.

Flow Switch Working Principle

The working principle of a flow switch mainly depends on the type of media. This switch is mainly used to monitor the pressure & flow rate of fluids, air, or gases throughout a system.

To know how this switch functions, it is useful to outline the core components that make up a typical flow switch circuit. Generally, many types of flow switches are available with a magnetic trigger or a paddle (primary device). So this paddle is connected to a circuit always and after that, it is placed in the lane wherever the liquid flows. Once the gases or fluids are supplied, the paddle turns and then transmits the signal to the secondary device like a transducer. This transducer will receive the signal and passes it to the transmitter in a readable format. After that, the transmitter will measure the readings.

Once the flow rate of liquid reaches a set-point of the switch then it can either close or open the circuit by turning a pump ON/OFF or by generating an alarm. The configuration of these switches can be done either NO (Normally Open) or NC (Normally Closed) which is known as the default condition of the switch. When the switch is in NO condition, the circuit is OFF (Open). Similarly, when the switch is in NC condition, then the circuit is ON (Closed).

Types of Flow Switch

These switches are categorized into four types gas flow, liquid flow, volumetric flow, and velocity flow switches which are discussed below.

Liquid flow switches

These switches are used to measure water, chemicals, slurries & lubricants. So these are used in a wide range of industry applications. Automated industrial systems that process liquid media mainly depend on these switches to ensure secure & best flow rates.

Liquid Flow Switch
Liquid Flow Switch

Gas Flow Switches

Gas flow switches are devices that are mainly used for measuring gas in terms of velocity. These may be sensor systems or plain sensors with displays. The important specifications of these switches are the flow rate to be measured.The device contains a rotating vane, which measures the amount of fluid passing through the flow switch. The vane operates a set of contacts that complete an electrical circuit when the flow reaches a predetermined rate. These switches are also used to measure the flow of air and vapors. These switches are normally used in HVAC applications. The technologies of these switches differ widely.


Gas Flow Switch
Gas Flow Switch

Volumetric Flow Switches

These switches are mainly used in measuring the liquids or gases flow. This measurement can be done depending on the volume for each unit time. So these types of switches measure the moving volume flow or quantity of fluid in terms of a unit of volume for each time. Generally, the volume flow is used to describe & measure pumping speeds.

Volumetric Flow Switch
Volumetric Flow Switch

Velocity flow switches

These types of switches are mainly used for measuring the flow speed of moving media. So this measurement can be done in terms of velocity or feet for each minute.

Velocity Flow
Velocity Flow

Some other Types of Flow Switches

There are different kinds of flow switches available in the market based on the requirements which are discussed below.

Paddle or Flapper Type Flow Switch:-

The flow switch which includes a paddle or flapper device is known as a paddle switch. In this switch, the paddle can be connected with the flat surface and the flow of direction can be faced throughout a hinge. A permanent magnet can be attached to this end and a reed contact is connected above this magnet at the outside of the fluid flow.

Paddle Flow Switch
Paddle Flow Switch

These switches are applicable in a variety of chemical industries for monitoring the liquid flow within the pipelines. The features of the paddle flow switch mainly include; high performance, strong construction, compact design, and its functional life is longer.

Diaphragm Flow Switch

Diaphragm flow switches are similar to DP or Differential pressure switches but the main difference is that there is a space that allows liquid to flow throughout the transition between the inlet & the outlet port.

Diaphragm Flow Switch
Diaphragm Flow Switch

The flow of liquid inside the switch body usually takes a zigzag route once this is completed. So this effect will result in a significant drop in pressure, which occurs whenever a liquid flows throughout the flow switch.

Shuttle Type Flow Switch

Shuttle-type flow switches mainly operate due to the velocity or differential pressure of the fluid on a disk based on the concept of a moving force.

Shuttle Type
Shuttle Type

A shuttle is connected at the base of the switch and a magnet is connected above the disk. The upper part of the shuttle body like the spindle is working against a gravitational force.

Once the liquid flow increases to the actuation area then the shuttle is displaced. This shuttle controls the reed switch in the unit stem once displaced through the flow of fluid; the shuttle comes back once the liquid flow reduces.

Solid-State Flow Switch or Thermal Flow Switch

These switches operate on the heat transfer principle. A type of solid-state flow switch like a thermal dispersion flow switch includes two temperature sensors. One sensor is used to measure the media temperature where the switch is submerged and the second sensor can be arranged close to a fixed heating element.

Thermal Flow Switch
Thermal Flow Switch

When the media flow rate is increased then it has the effect of cooling the heated sensor. So the flow rate of fluid is higher the cooling is more. The temperature reduction between the two sensors specifies that the liquid flow rate exceeds the adjustable setpoint. Once the liquid flow rate decreases, then cooling is less which causes an increase in the temperature differential.

Piston Flow Switch

A piston flow switch has a permanent magnet that is arranged in the liquid flow path of the unit housing. Once it is displaced through the pressure differential from liquid flow, the piston activates a hermetically sealed reed switch magnetically in the unit.

Piston Flow Switch
Piston Flow Switch

The base diameter of piston metering exactly sets the actuation point by changing bypass clearance. When the reed switch is actuated then it is used to operate indicators and remote alarms or it may be incorporated into automatic system controls.

Flow Switch Circuit Diagram

The circuit diagram of the flow switch circuit is shown below. This circuit is used to control the flow of fluid. We know that a flow switch can be arranged within a pipe so that once fluid or air supplies against a paddle which is a part of the device the switch can be activated. This switch either opens or closes a set of electrical contacts where these contacts are connected to strengthen motor starter coils, indicating lights or relays. Generally, this switch includes both normally open (NO) & normally closed (NC) electrical contacts.

Flow Switch Circuit Diagram
Flow Switch Circuit Diagram

In the above flow switch circuit diagram, once the contact of this switch closes with enough liquid/ air movement, then coil ‘M’ can be energized then the motor gets started. Here, a flow switch is used with a single-phase circuit. So when the flow of liquid occurs then the motor will be turned ON. Similarly, if there is no liquid flow then the motor stops working. Here, Overload heaters in the above circuit are used to protect motors from overcurrent.

Difference between Flow Switch and Tamper Switch

The main difference between a flow switch and a tamper switch include the following.

Flow Switch

Tamper Switch

The flow switch is also called a flow indicator. The tamper switch is also called a supervisory Switch.
The main function of a flow switch is to detect and monitor the flow rate of process media like liquids, gases, and steam. The main function of a tamper switch is to detect once a sprinkler valve has been partly to completely closed.
Flow switch weight is 569 grams. Tamper switch weight is 2 lbs.
The construction of this switch can be done by a hinged or spring-mounted paddle. The construction of this tamper switch can be done by actuating unit, generally a lever or cable with a resting position

Advantages and Disadvantages

The advantages of flow switch include the following.

  • These switches are very reliable devices for determining flow rates.
  • These switches offer accuracy, versatility, and economy.
  • They provide high accuracy.

The disadvantages of flow switch include the following.

  • Gas inclusions can cause errors.
  • Lower conductivity.
  • Installation is expensive and required maintenance.
  • It is vulnerable to rust from water-based liquids.
  • Its design is complex.
  • Poor zero stability.
  • Fluids with low density cannot be measured.

Applications of Flow Switch

The applications of flow switch include the following.

  • These switches are mainly used for determining flow rates.
  • These switches play a key role in fluid flow the detection & fan speed measurement.
  • This switch protects a heating element in a central heating system from being strengthened before the flow of air from the blower is recognized.
  • These switches are applicable in the protection of pumps, monitoring of safety spray nozzle, testing of oil well system, monitoring of relief valve, and drain line flow.
  • These switches are mainly used for monitoring & controlling the fluid flow rate in an industrial process system.
  • These types of switches are used in pump staging, failure of pump or valve, flow protection, and detection of flow or blockage.
  • These switches are normally used in tank storage systems and in reservoirs to maintain the tanks at the designated level.
  • Large flow switches are used in large & commercial buildings for HVAC systems.

How to install a flow switch?

Flow switches are typically installed in the piping system of an industrial application. They are usually cylindrical or rectangular in shape and have two ports on either side of the device. These ports allow for easy installation of the switch into any piping system. The ports must be connected to two separate pipes that carry different liquids so that they can be used to detect which liquid has reached its destination first when they arrive at their respective ends of the piping system at the same time. When this happens, an electrical current will pass through one port while not passing through another port due to differences between fluids’ viscosities (thickness). This allows for detection of which fluid arrived first so that it can then be switched off or turned on depending on whether it was desired for them to arrive simultaneously or not.”

What kind of fluid does a flow switch detect?

A flow switch detects any type of liquid, whether it’s a clear liquid like water or an opaque (turbid) liquid like sand.

Thus, this is all about an overview of a flow switch – working with applications. These switches are used to monitor the flow throughout a channel & transmit a trip signal to different devices within the system like pumps. There are some parameters that need to consider while selecting this switch like media type, the diameter of the pipe, range of media temperature, and operating pressure. These switches are used in air supply systems, duct type heating, water treatment systems and blending or additive systems, etc. Here is a question for you, what is a reed switch?