How Does A Float Switch Work?


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A float switch contains an internal mechanical switch that opens or closes an electrical circuit. The float switch moves up and down with the liquid level, detecting the amount of liquid in a tank, and sending an alarm when the level becomes too high or low. The electrical circuit is what turns the float switch on and off, similar to a water pump. 

Float switches are used to detect the level of a liquid in a tank. The switch floats on top of the surface and acts as a mechanical switch, going up or down depending on the liquid level. They are often used to control pump devices, valves, and alarms in various residential and industrial applications. Float switches are a popular choice for controlling liquid levels, because they are very reliable, cost-effective, and they can be used in different liquids. 

How Do Float Switches Work?

To put it simply, a float switch is a mechanical switch that floats on the surface of a liquid. The float switch moves vertically up and down as the liquid level changes. 

The mechanical switch inside the float opens or closes depending on the counterweight and pre-set trigger, allowing an electrical current to pass through to the device. The sensor probes inside the float switch are what trigger the pump to fill or stop filling the tank or container with liquid.  

To understand this further, we can take a look at the different components of a float switch. Float switches are fairly simple pieces of equipment, consisting of a hollow body and an internal switch containing electrodes. 

The hollow body is what makes the float switch float on top of the water, and the internal switch acts as a sensor.

Internal switches vary depending on which type of float switch you have, but the most common is a reed switch (mechanically operated switches), which also contains a magnet inside. Even though there are different types of float switches, they all work on the same principle, they open or close depending on the liquid level.

When the magnet is close to the reed switch, an electrical current can flow through, and close the circuit, signaling the pump to stop pumping liquid into the tank.

Therefore, when the magnet moves away from the reed switch, no electrical current can flow through the float switch, opening the circuit and signaling the pump to turn on and pump more liquid into the tank. The level of the liquid in the tank or container is what controls the angle of the float switch, directly controlling the magnet’s position inside the switch. 

It is important to note that float switches have a maximum switching current and voltage, so be sure to read the manufacturer’s guidelines to not exceed this specification. 

It is also essential to correctly wire your float switch and set the preset liquid level for the type of application you are working with. When you install a vertical float switch, it will either have a fixed point or an external counterweight. 

External Counterweight

If your float switch has an external counterweight, it will have set angles that it operates at. The angle is determined by the cable length from the counterweight, to the float switch. When deciding which cable length, check the flow switch requirements, as they all have a minimum and maximum cable length to operate. 

How To Wire A Float Switch?

The wiring is extremely important for a float switch to work properly because they rely on the electrical signal to open or close the circuit. 

The most challenging part is figuring out whether you need a normally open or normally closed float switch. Most float switches come with a normally open or normally closed operation. However, some float switches are user reversible, so always check which operation the float switch has at the time of purchase. 

How Do “Normally Open” & “Normally Closed” Float Switches Work?

Normally Open: When the float switch is set to “normally open” it will open when it is in a down position and close when it is in an up position. This means when the liquid rises to the pre-set level in the tank or container, the float switch will close. Normally, open float switches are used when liquid levels are known to get too low.

Normally Closed: A normally closed float switch is the opposite. So, when the float switch is positioned down the circuit will be closed, and open when it is in an up position. Therefore, the float switch opens when the liquid rises to the pre-set level inside the tank or container. These types of float switches are used when low-level alarms are required.

Types Of Float Switches

Almost all float switches function using a magnetic system to open and close the switch. There are two main types of float switches: stem-mounted and cable-suspended float switches. 

Both stem-mounted and cable-suspended can experience failures if the float switch is poorly maintained, it becomes degraded, or the incorrect configuration is used.

Float Switch Applications

As float switches come in a variety of types and designs, they can be used for a wide variety of applications. The most common applications include the following:

  • Sump and sewage pits: Float switches are used to detect and operate pumps to prevent the water from rising into the pits.
  • Potable water, surface water, wastewater, and sewage systems: Plastic float switches are used for highly corrosive applications.
  • Refrigerators and air conditioning units to control water levels.
  • Beverage and food industries to control the in and outflow of liquids: Stainless-steel float switches are used for food or drinking water applications.
  • Industrial washers to monitor the water level. 

Summing Up, Float Switches

There are many ways you can monitor the amount of liquid in a tank or container, but float switches will provide you with high accuracy, and the simplicity and low cost of a float switch beat a pressure sensor any day. 

A float switch monitors the level of liquid in a tank by using a float, magnet, and reed switch system that automatically closes and opens when the water level changes. 

If you have any questions regarding float switches or are unsure which float switch is for you, please do not hesitate to contact our world-class team at Atlas Scientific.

Float Switches

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