How do float switches and liquid level sensors work ?

Level sensors are devices designed to measure the depth, volume, or other metrics in a tank or vessel. Most commonly used to control or measure the level of liquid in the tank, liquid level sensors can be found in a variety of different forms and materials being used for different applications. Here’s a view of the most common versions and how they work.

Working principle

A float fitted with one or more magnets moves with the liquid and actuates, due to its magnetic field, a hermetically sealed reed contact located in the body of the float.


  • One moving part

  • The Reed contact is actuated by a magnetic field only : no contact so no wear

  • The Reed contact is completely isolated from the liquid so perfectly waterproof

The above advantages allow a safety use, repetitiveness, precision and minimum maintenance.

Vertical level sensors

Our vertical level sensors are available in plastic or stainless steel housing, making it possible to use them with various chemical substances and/or operating temperatures.

They come in either “normally open” or “normally closed” positions.

Both feature a glass reed contained within the float, with two metal prongs that open or close the circuit depending on whether or not they are touching. Magnets outside the glass reed rise and fall with the position of the float, connecting or disconnecting the metal prongs as needed.

Normally closed float switches feature a completed electrical circuit when the device is in its prone position. Rising liquid levels move the magnets such that the connection is severed and the electrical circuit disrupted. This can be useful in applications like emergency tank shutoffs.

In normally open switches, the magnets bring the metal prongs together to complete the electrical circuit, activating any attached electronics. High-level alarms, bilge, or other pumps are examples.

With some sensors, it is possible to invert function by reversing the float or using the sensor upside down.

Horizontal level sensors

While vertical level sensors are mounted above the liquid surface or at the bottom of the tank, horizontal level sensors are mounted to the side of the tank or vessel. The switch is mounted at a specified level, over or under which the switch should be activated or deactivated. The float is attached to the side mount on a sort of a hinge that lets it hang in the open air with the electrical circuit open; when the water rises high enough, the float goes with it, eventually closing the circuit or vice versa.

Some versions are available for external mounting.

Liquids compatibility

We have divided this liquid compatibility into 4  levels :

Level 1 :

  • Water only

Our solutions : PTF01070PTFA1015

Level 2 :

  • Compatible with acid : acetic, citric, formic, lactic, nitric diluted, phosphoric, sulphuric diluted ; soda; alcohols : ethanol, methanol, propanol ; glycol ; mineral oil ; water
  • Not compatible with the following solvents : chloroforme, methylene chloride, trichloroethylene, toluene ; hard acids.

Our solutions : PTFA1103 (vertical), PTFA1104 (vertical), PTFA3315 (horizontal), PTFA3415 (horizontal)

Level 3 :

  • Compatible with fuels, engine oil, kerosene, lubricaring oil, mineral oil, vegetal oil,
  • Not compatible with almost all acids, methylene chloride
  • Acceptable resistance to water.

Our solutions : PTFA1210 (vertical), PTFA0100 (horizontal), PTFA3115 (horizontal)

Level 4 :

  • Compatible with almost all the liquids except hard acids.

Our solution : PTFA2115 (vertical)

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