Flow Switches

A flow switch are flowmeter devices used to measure the flow rate and pressure of liquid, gas or vapour that passes through them. They measure flow rates over a set time, or they can continually measure the medium passed through. They operate by sending signals to another component elsewhere in the system (e.g. pumps) telling it to stop/start in order to adjust flow rate and pressure to the desired and programmed levels.

Common types of flow switches

  • Water: uses a paddle (physical or ultrasonic), for water pumps and sprinklers.

  • Gas and air: HVAC and ducting, paddle air, boilers, inline air flow switches

  • Thermal dispersion: involves a sensor probe, allows the rate of gas or liquid flow to be calculated according to how quickly heat is carried away by the molecules flowing past.

  • Oil: can operate at higher temperatures with robust seals and housing.

  • Ultrasonic, Magnetic: emits an ultrasonic signal to the medium, this is then bounced back and converted by the primary device. Commonly used in dirty applications such as wastewater

  • Mechanical: need to be tested regularly as continual movements of parts will lead to wear and tear.

  • PVC: basic type ideal where a quick visual confirmation of flow/no flow status is needed. Designed for cleaning and maintenance.

  • Industrial: larger more robust so can withstand higher volumes, pressure and flow rates, designed for heavily contaminated or hazardous materials.

  • Pneumatic: for applications where there is a necessary pressure differential between different areas of ducting.

  • Adjustable: quickly and easily recalibrate the desired flow settings

  • Rotary: functions as a basic turbine, ideal for low flow liquid monitoring.

Some basic rules to install a flow switch:

  • Install on a straight section of pipe or duct

  • Make sure there is a good length of straight pipe in front and behind the switch

  • Avoid fitting or wiring near to bends, other fittings, valves, drains, narrower or wider sections of pipe

Each type of flow switch has its own specific applications and installation constraints. The way to select the right one is to think about the application as a guide, not the technology. Starting with the application, will help to select the technology based on accuracy, cost, durability and reliability, rather than trying to make the technology fit the application you have.

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