Float switches

What float-based measuring systems for level monitoring are there?

Floats play an important role in float-based measuring systems for level monitoring, for example of liquids. These systems are often used in combination with pumps and comprise different types of instruments. They include float switches, level sensors, level switches and bypass level indicators.
Float switches are special switches that are used for point-based limit level detection in various liquids. They use floats to precisely determine the level inside of a tank. In particular, they are used for controlling pumps and can be used to monitor one or several fill levels. Because they are simple, universally applicable and exceptionally reliable, float switches have a very wide range of applications. Depending on accuracy requirements, level sensors and level switches are available with reed contacts, reed chains or magnetostrictive measuring systems. Bypass level indicators, similar to level switches, are mounted on the side of the tank and are often combined with a magnetic indication for visual level indication.

What does a float switch consist of?

Level measurement technology is based on the buoyancy of a hollow body, the so-called float. Due to its low density and buoyancy, the float is raised when the level of the liquid rises and lowered when the level of the liquid falls. A float-based measurement technology uses the movement generated by the level change to measure the level. The switch point, which is often realised using a reed contact, plays a key role when it comes to determining the accurate level. Modern float switches have a very sophisticated design. The size of the float switch also plays an important role for the use in various application areas and has to be carefully selected. In its simplest form, a float switch is made up of the following components:

• A hollow float with an enclosed magnet
• A guide tube for guiding the float
• Adjusting collars to limit the movement of the float on the tube
• A reed contact located inside

How does a float switch work?

A reed contact, also known as a reed switch, which is, for example, used in pumps, involves contact leaves that are located inside the hermetically encapsulated glass body. These contact leaves move towards or away from each other due to a magnetic field. If a float switch is used with a reed switch with the switching function of normally open and a magnetic field is applied, the contact leaves move towards each other. Current can then flow through the contact of the leaves and a switching signal can be detected. If a float switch with the switching function of normally closed is being used, the contact leaves move away from each other and the circuit is interrupted as soon as a magnetic field is applied. Another option is the use of a change-over contact, where three contact leaves are used within the glass body. Three contact leaves allow one normally closed and one normally open switch to be used simultaneously in each operating state.
To generate the desired switching signal, i.e. to close or open the contact leaves, a magnetic field must be applied. Adjusting collars are used to limit the movement of the float at the desired position and are fitted by the manufacturer. Thus, when the level rises to the defined level, the defined switching signal can follow.

How do I find a suitable float switch?

In order to select the suitable float switch or level switch for different liquids, especially if they are used in combination with pumps, certain requirements should be defined:

• Number of switch contacts / switching outputs
• Position and function of every switching output
• Guide tube length
• Electrical connection (e.g. PVC cable outlet)
• Process connection
• Material (stainless steel, plastic, …)
• Size to ensure that the float switch is suitable for the respective application