Charles Kolstad
Author: Charles Kolstad
Charles Kolstad has been working at Tameson since 2017 and is from the United States of America. He has his Mechanical Engineering degree from the University of St. Thomas in Minnesota, USA. He works remotely while traveling throughout Europe, Asia, and the Americas. However, he does visit Tameson’s HQ from time to time to meet the new members of the team and to work in the office.

Float switches are mechanical switching devices used to detect the level of liquids in reservoirs, tanks, or containers before taking appropriate action. They float on top of the liquid surface, rising and falling as the fluid level changes. These switches are designed to actuate external machines or devices such as alarms, pumps, and valves. Float switches are cost-effective, flexible, and reliable. Let’s learn more below.

Float Switch

Working Principle of Float Switches

A float switch rests on the liquid surface and moves vertically as the liquid level changes. The switch is designed to either close or open. This allows an electrical signal to flow through it and into a connected device such as a pump, a valve, or an alarm. The design of a float switch is simple. It consists of a sensor or an internal switch and a hollow body that allows it to float. A switch used with most of these devices is a reed switch. It also contains a magnet enclosed inside the body.

Switching Mechanism 

When the magnet sits close to the switch, an electric field is created, which allows the current to flow. As the magnet moves further away from the switch, the circuit becomes open since there’s no electric field, hence no current induced. The position of the magnet from the reed switch is determined by the liquid level. Similarly, each switch has its current and voltage ratings.

For vertical switch installation, a preset level or an external counterweight determines the position that the switch goes from open to closed states and vice versa. One example is a float switch installation designed to stop water inflow at a high point in a water reservoir. With this type, the floating body will rise as the water levels increase. This causes the magnet to move closer to the reed switch, closing the switch.

As soon as the circuit is complete, an electric signal travels to a water pump and turns it off such that no more water is pumped into the tank. When the water level drops, the float switch falls, and the magnet moves away from the switch, opening the circuit. This signals the pump to turn on and begin pumping water into the reservoir. The float switch wiring and the preset levels determine the working mechanism and application of the float switch.

External Counterweight

Float switches with external counterweights have preset angles at which they operate. The cable length between the float switch and the counterweight creates an angle with the water level. For this reason, these switches have minimum and maximum cable length requirements to ensure optimal operation.

Float Switch with Counterweight

Float Switch Wiring

Float switching wiring will always vary based on the switches’ closed or open positions. The two common float switch configurations are normally closed and normally open. With normally closed (NC) float switches, the circuit is always open in the up position and closed in the down position. In other words, the switch will open as the liquid level rises to the preset level but will close as the water level drops.

On the other hand, the normally open (NO) float switch has an open electrical circuit in the down position but closed in the up position. That means rising water levels will close the circuit once it reaches the preset level, while dropping water levels will open the circuit.

Types of Float Switches 

There are several types of float switches, but the most common ones are the KPM Series, MPM Series, and JPM Series. Here’s how these three types differ.

  • KPM Series – it consists of some of the highly reliable float switches suitable for a broad range of liquids. They open or close the electric circuit at a ±45° angle. That means the cable length after the counterweight determines the fluid level at which the switch will operate. KPM Series float switches are commonly used to control clean water.

  • MPM Series – float switches in this series have a double liquid-proof chamber that offers more protection on the inside. Like the KPM Series, they require a counterweight and operate at a ±45° angle. They are commonly used in sewage water applications due to their large size and shape.

  • JPM Series – these are tilt level switches that don’t require a counterweight due to their heavy-weight design. They are free from irregularity and will rotate around when submerged. JPM Series float switches are often used in pairs to enhance two functions, i.e., emptying and filling. They are either NO or NC and ideal for industrial wastewaters and turbulent water.


Float switches are designed to fall or rise with the liquid level and, in turn, control an external device such as a pump, an alarm, or a valve. There are several types and designs of float switches, but their working principle is rather similar. Some of the common applications of float switches are in the industrial washers beverage industry for filling and emptying of beverage tanks, water tanks, and sewage systems.

Before choosing a float switch, always understand its control features and float switches working principles. You also want to compare these aspects with your system requirements and check for any incompatibilities. If it’s your first time buying a float switch, you may consider enlisting the help of a trusted, third-party professional.