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A flow switch is a device used to monitor the flow rate and pressure of liquids, air or other gaseous media through a duct, system or loop.

These switches or sensors can be used to monitor flow over a given period of time, or set up to continuously monitor total flow. Technically, a flow meter or indicator may not necessarily be a true flow ‘switch’ - if the device only registers and displays information, it’s more properly called a meter or indicator.

In most cases, though, they’ll be able to trigger actions or stop/start other components elsewhere in the system - pumps, for example - in order to adjust flow rate and pressure to the desired and programmed levels. This is then a true flow switch.

A flow switch is a device used to monitor the flow rate and pressure of liquids, air or other gaseous media through a duct, system or loop.

These switches or sensors can be used to monitor flow over a given period of time, or set up to continuously monitor total flow. Technically, a flow meter or indicator may not necessarily be a true flow ‘switch’ - if the device only registers and displays information, it’s more properly called a meter or indicator.

In most cases, though, they’ll be able to trigger actions or stop/start other components elsewhere in the system - pumps, for example - in order to adjust flow rate and pressure to the desired and programmed levels. This is then a true flow switch.

To understand how a flow switch works and what it does, it’s helpful to outline the core components that make up a typical switch or sensor of this kind.

Many types will include a paddle or magnetic trigger of some sort (the primary device), which is connected to a circuit and placed in the channel through which liquid or gas is passing. This paddle is displaced or rotated by whatever substance is flowing by, and sends a signal reading back to a secondary component known as the transducer.

The transducer takes this raw signal from the paddle and passes it on to a transmitter in a readable format. The transmitter, in turn, measures this reading against a predefined set of parameters and performs whatever signal or action is required to adjust the behaviour of components and mechanisms elsewhere.

In this way, flow switches are able to monitor, report and control the flow rate of liquid or gas through a specific part of a system, or through the entire system, ensuring flow stays within those pre-set parameters. If the rate exceeds or drops below what’s required, it can instantly trigger a series of actions such as activating an alarm, powering on a pump, diverting flow, shutting off parts of the system. Which actions a switch performs will depend entirely on the type of switch and what it’s been designed for.

Not all flow switches will have a physical paddle as such. Ultrasonic and other non-intrusive versions are also fairly commonplace, which work by bouncing a signal back off the medium being monitored. This is particularly useful in cases where the medium is contaminated, physically damaging or otherwise hazardous, or where it’s helpful to have no moving parts exposed to constant wear and tear.

Rotaflow or Variable Area Flow Sensors are more simple devices that use a tapered tube with a float (or sometimes an electromagnet) to measure, restrict or allow the free passage or gas or liquid through. Various types of magnetic flow switches are available, and they can be very effective in a range of applications as long as the medium is somewhat conductive.

Because there are so many different kinds of flow switch, as we’ll see in subsequent sections, there’s also a very wide range of flow switch installation methods and techniques. When looking for information on how to wire a flow switch, most important thing is to have a clear understanding of which type of switch you’re using and for what purpose.

There are some basic rules of thumb that apply to most flow switch installation methods, however. Broadly speaking, these include:

• Installing flow switches on a straight section of pipe or duct
• Ensuring there’s a good length of straight pipe both in front of and behind the switch - ideally, the equivalent of 10x the pipe’s diameter
• Avoid fitting or wiring a flow switch near to bends, other fittings, valves, drains, narrower or wider sections of pipe, and any other features that might cause obvious fluctuations in flow rate through this area

Just as the exact methodology for installing a flow switch will depend on what kind of switch you’re using and where, the same is true for testing the healthy function of any device already installed. To know how to test a flow switch, you’ll need to know precisely what it is you’re testing for.

In many cases, you’ll be able to confirm a flow switch is working properly simply by observing it in action and checking that the correct responses are triggered when flow starts or stops. In other cases, you may need to use an ohmmeter to measure the resistance of the switch's electronics and confirm that there’s a completed circuit (or ‘continuity’) being registered when the switch is activated.

If either of the above steps don’t produce the required result, it may be time to replace or repair the switch. For certain types and locations of flow switch - including those attached to safety equipment such as fire sprinklers in public places - it’s typically a legal obligation to have them tested regularly.

There are numerous different types of flow switches available, and each will take a subtly - but importantly - different approach to measuring linear, nonlinear, volumetric or mass flow rate of liquids or gasses. Before ordering or installing a flow switch, it’s extremely important to have a clear understanding of which specific type you need for the exact application you have in mind.

No single variety or brand of flow switch will perform any/all flow switch tasks to a suitable standard in any given scenario, so you’ll need to know what sort you need for the precise function it’s intended to perform.

To know how a flow switch is used, you first need to know what task the switch is intended to perform in the specific system you’re talking about. The vast majority are intended to operate fully automatically unless damaged or degraded, so knowing how to use a flow switch is often more a case of knowing how to tell when it’s not functioning as it should. That will depend entirely on the role it plays in a given system.

Flow Switches on boilers

Flow switches on boilers can be found in both water and gas pipelines. For water channels, they’re generally installed to protect the boiler from operating in low/no flow states. Gas boiler flow switches are mainly used to prevent the boiler from firing before any build-up of gasses has been vented by fans via the flue.

Flow Switches for pumps

A pump flow switch is usually installed to prevent the pump from running dry in the event of a failure in the water supply. As water pumps can quickly overheat or become damaged by continued running in low/no flow situations, flow switches for pumps often function to shut down the power to the pump temporarily when it isn’t being supplied with enough liquid.

Flow Switches for shower pumps

A shower pump flow switch helps maintain water pressure being sent up through the piping and out of the shower head. They can be found on both hot and cold supply channels, and are generally mechanical.

Demand for water to the shower head causes the switch to activate and this, in turn, completes a circuit which tells the pump to begin operating. Over time, shower pump flow switches can degrade due to limescale, heat and other issues, and may eventually need replacing.

Flow Switches for heat pumps

A heat pump flow switch is often found in swimming pools and spas, where a fairly large heat pump is required to bring the water up to the desired temperature once it’s switched on, and recognises a demand for both warmth and water flow. The flow switch, in this case, works to stop the pump from operating when there’s an interruption in the water supply or when the pool is not in use.

Flow Switches for pool pumps

Pool pump flow switches perform very similarly to the function outlined above for heat pumps - the switch prevents the pool pump from continuing to run in low/no flow situations, which can cause serious damage to the component, and when the pool isn’t being used.

Flow Switches for sump pumps

Sump pump switches are typically installed to ensure that the water from a drainage system won’t overflow. Again, running a non-submersible pump dry will quickly damage it, so the switch will also help protect the workings of the pump by shutting it off if the water supply fails or the flow rate falls too low.

Flow Switches for hot tubs and jacuzzis

A hot tub flow switch also protects the pump components in spas, jacuzzis and hot tubs from overheating if the flow rate is too low. In the event of a water flow failure or an air pocket forming in the pump, the switch will open to prevent a meltdown of the pump or heating elements.

High temperature and pressure Flow Switches

High temperature flow switches are commonly used in industrial or manufacturing systems, where a continuous flow of liquids or gases sometimes needs to be maintained at temperatures up to 350 Celsius and above.

High pressure flow switches are designed and built ruggedly for use in similarly challenging applications where they’re likely to be subjected to far greater forces and flow rates from various media than would normally be found in household systems.

Sanitary and hygienic Flow Switches

Sanitary flow switches are often necessary in industries where hygiene is especially important - this might include food production, pharmaceuticals, medical and laboratory environments, or in industrial research and development where it’s important to keep the risk of contamination to a minimum.

They’re usually made from stainless steel and sometimes have completely sealed-off inner chambers housing turbine or magnetic switches.

Chiller Flow Switches

Chiller flow switches are usually installed to protect against low/no flow scenarios which can lead to freeze damage. They’re often found on industrial, commercial or institutional air-cooling systems, as well as chilling many different kinds of hot-running equipment including machine tools, medical imaging units, food and beverage production systems and more. Chiller flow switches can be either air or liquid types, depending on the compression system a given cooler is designed to use.

As well as identifying the correct type of flow switch for the position and application you need, it’s also important to select the right size and voltage of switch to work properly in the duct or system you’re installing to. You can usually filter online searches for flow switch sizes and by voltage, making it easier to find the product you need.

1 inch flow switches

1-inch flow switches of all kinds are intended for use in pipes or ducting of 1-inch diameter.

2 inch flow switches

2-inch flow switches of all kinds are intended for use in pipes or ducting of 2-inch diameter.

4 inch flow switches

4-inch flow switches of all kinds are intended for use in pipes or ducting of 2-inch diameter.

24 volt flow switches

24-volt flow switches are commonly found across a wide range of switch types and sizes, including paddle, rotary and electromagnetic flow switches for both gas and liquid applications.