Capacitive Level SensorsFloat switches and other liquid 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 flow of liquid in or out of the vessel, as with sump pumps or fuel level gauges, liquid level sensors can be found in a variety of different forms being used for countless different applications. Here’s a primer of some of the most common varieties of liquid level sensors, and how they work.

Vertical Reed Float Switches

Reed float switches are some of the most prevalent level sensors in common use. Reed switches open or closes an electrical circuit based on certain external conditions; in a float switch, this means liquid levels. Reed float switches usually hang vertically in the liquid medium, and 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.

Horizontal Float Switches

A visual diagram illustration how a horizontal float switch works.While vertical reed switches hang or are mounted above the liquid surface or at the bottom of the tank, horizontal float switches 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.

Ultrasonic Liquid Level Sensors

Ultrasonic liquid level sensors work by emitting pulses of sound at the liquid surface. The sensor then measures the liquid level by recording and calculating the time it took the sound wave to bounce back. The lower the liquid level, the farther the sound travels and the longer the process takes.

Optical Liquid Level Sensors

Optical level sensors work by sending out a beam in infrared light from a specially designed prism and measuring the amount of light that returns. When the liquid rises to cross the path of the light, it interrupts the beam and the sensor receives less light, triggering the switch.

Hydrostatic Liquid Level Sensors

Hydrostatic sensors work by measure the hydrostatic pressure (also called head pressure) of the liquid. Hydrostatic pressure is the amount of pressure exerted by the liquid at a given point, due to the force of gravity. When properly calibrated, hydrostatic sensors measure the height of the liquid medium by taking into account its density and the force of gravity.  Downhole submersible liquid level sensors are common examples of hydrostatic switches.


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