Guide to Hydraulic Flow Switches and Flow Sensors

Hydraulic Flow Switches & Levels
Hydraulic Flow Switches & Levels

What is a flow switch?

Flow switches are used in a variety of applications to monitor air or liquid flow. They perform numerous functions, including balancing the pressure of liquid or gas, protecting pumps and triggering system alarms.

Flow switches are devices that monitor the flow rate and pressure of liquids or air through a system. They’re also called flow sensors and flow indicators. As they monitor flow over a continuous or set period of time, they trigger actions such as starting or stopping other components in systems. This adjusts the flow rate and pressure to required levels for optimum performance.

How does a flow switch work?

Most flow switches come with a magnetic trigger or a paddle as the primary device that connects to a circuit. It’s placed in a channel containing liquid or gas. Via rotation, it sends a signal reading back to a transducer. The transducer takes the signal and passes it to a transmitter to be read. The reading is measured against a set of parameters before performing the required action to adjust components.

In doing so, flow switches monitor, control and report the flow rate of gas or liquid through specific areas of systems. When the rate drops or goes above what’s required it triggers actions, like alarms or the closing down of system parts. The actions performed depend on the type of switch and what it's designed for.

Other than paddles, some flow switches use ultrasonic and non-intrusive methods. They work by feeding a signal back off the device that’s monitored. This is particularly useful when systems are contaminated or hazardous. Variable Area Flow Sensors and Rotaflow are simple versions comprised of tapered tubes with floats.


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How to Install and test a flow switch

Different types of flow switches require various installation methods. However, the most common installation method is to install them on straight sections of pipe or duct. It’s important to ensure a good length of straight pipe in front of and behind the switch. Flow switches shouldn’t be installed near bends, other fittings, drains or valves.

To test a flow switch, it’s crucial to know what the test is for. Generally, a flow switch test is conducted by observing it in action and checking that the triggers are correct when flow starts and stops. An ohmmeter is also used to measure the resistance of the electronics. This confirms a complete circuit registered after activation. In the event of a system breakdown, flow switches may need replacing or repairing. It’s important to test them regularly.

Different types of flow switches

Different types of flow switches are available to measure volumetric or mass flow rate of gasses or liquids. Before installation, it’s important to understand which type is required for specific applications. Among flow switch brands and manufacturers, Honeywell Flow Switches and Dwyer Flow Switches are popular choices.

Flow switches for water and fluids

In general, flow switches use a physical paddle or an ultrasonic wired to the channel where liquids and water pass through. The movement speed of the paddle specifies the flow rate to the transducer. As a result, the transmitter turns the information into action.

Types of water flow switches

There are many varieties of water flow switches available for a range of applications. They include:

  • Flow switches for water pumps designed to make automatic adjustments to operating speed or volume transfer of an electric pump in a liquid system. After the overall flow rate of water is sensed, they send signals to pumps to lower or raise a system’s water pressure.
  • Flow switches for automatic fire sprinklers detect water passing through the sprinkler pipework. If water exceeds a predetermined flow rate, it indicates that the system has been triggered. This sends a signal to the fire department.
  • Magnetic water flow switches measure movement speed and volume by creating a magnetic field for the water to pass through. As it does, the information is read by an electric sensor. The sensor triggers an action via the reading. These are commonly found in wastewater systems.
  • Inline water flow switches are used as an integral part of systems overall. They’re often more expensive. However, they require shorter straight run sections to work effectively and automatically condition flow after installation.
  • Paddle water flow switches consistently measure flow rates via a physical paddle hanging in the water or liquid pipe. These are triggered at a variety of pressures and speeds to send signals.

Types of air flow switches

Air flow switches are used for systems such as duct heating, exhaust venting and air filtration systems. Common air flow switches include:

  • Ducting and HVAC air flow switches often use a paddle operation to signal when flow decreases or increases beyond the predetermined rate. In these systems, air flow switches perform actions like fan and filter monitoring.
  • Paddle air flow switches feature mechanical sensors that extend into the duct. Air or gasses move the paddle and trigger an action when required.
  • Air flow switches for boilers monitor the venting of waste gases to ensure they’re safely expelled before use. Ignition is only performed once boiler fans are spinning at the appropriate speed.
  • Inline air flow switches are installed as an integral part of a system. They perform multiple purposes and require less upkeep than other flow switches.

Paddle flow switches

A paddle flow switch is activated by pressure from liquid as it passes through the duct or channel it’s attached to. Most often, it’s made of a strip of plastic or metal that’s attached to a tensioned spring or magnets. A liquid with too much or too little force to move it to a second position activates an alarm response.

Thermal dispersion flow switches

As opposed to a mechanical paddle, a thermal dispersion flow switch contains no moving parts. It has a sensor probe that’s inserted into the middle of the flow in the system. Electrical wattage heats the probe, which calculates the rate of liquid or gas flow according to how fast the heat disperses.

Oil flow switches

Oil flow switches work in the same way that liquid flow switches work and come in paddle or thermal forms. They’re designed for use with oils and fuels. Therefore, they’re durable and operate at higher temperatures.

Ultrasonic and non-intrusive flow switches

Ultrasonic flow switches don’t penetrate the wall of a pipe, so they’re a non-intrusive method. An ultrasonic flow switch attaches via a clamp to the outside. It reads and responds to flow rate by sending a signal back and forth from sensors. Most often, they rely on aeration in liquid and are found in wastewater systems.

PVC flow switches

Economical and easy to install, PVS flow switches are generally used for very basic applications. They’re usually mechanical and feature one or more moving parts.

Industrial flow switches

Larger than many types, industrial flow switches withstand high pressures, volumes and flow rates. They’re designed to deal with dangerous and contaminated materials.

Calorimetric flow switches

A calorimetric flow switch uses two temperature sensors. One is heated and one monitors the temperature of the air or liquid in the pipe. The flow rate is calculated based on differences detected in temperature.

Pneumatic flow switches

A pneumatic flow switch is generally used to control the passage of air through valves when flow stops or starts. Most often, it substitutes an air valve for mechanical switches.

Adjustable flow switches

For the benefit of recalibrating flow settings without taking the unit apart, adjustable flow switches are useful. Generally, they’re adjusted with a flat-head screwdriver.

Rotary flow switches

Wheel-mounted within a system’s stream, a rotary flow switch functions as a turbine. As liquid passes over it, the switch registers flow rate. A choice of designs includes paddlewheel and piston.

Flow switches for pumps

Pump flow switches are installed to prevent pumps from drying out when there’s a system failure. Flow switches are designed for a wide variety of pumps, including for showers, heating systems, pools, hot tubs and drainage systems.


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A flow switch is used to monitor the pressure and flow rate of air or liquids in a system. When inconsistencies are detected, flow switches trigger alerts.

Flow switches work via a paddle or magnetic trigger connected to a pipe or duct with air or water flowing through it. They detect volume or pressure that’s either too high or low via inconsistencies. Upon doing so, a signal passes to a transmitter that reads it and initiates the required action of stopping, starting or moving components.

Flow switches are used across a wide range of products that require the monitoring of air or liquid flow. They include fire sprinklers, heating systems, electric motors and pool chlorination systems.