Float Switches Guide

What is a Float Switch?

A float switch is a type of sensor that detects the level of liquid in a container and actuates a set of contacts as the liquid level rises or falls. The levels at which the switch contacts will open or close depend on where you set up the float in your tank and the differential distance.

The differential is the distance the float has to travel in order for the contacts to change from one state to another (e.g. open to close). For example, a float switch being used to keep an aquarium tank topped off may have a differential of less than 1 inch, while other switches being used in applications where a tank is being completely filled or emptied may have differentials as large as the height of the tank.

Many float switches use a tipping-point mechanism that produces a snap-action, so that the contacts will not move until the float reaches specific points in its range of motion. Other switches will use reed contacts that are actuated by magnets.

NOTE: You will want to isolate reed switches from magnetic fields generated by nearby equipment, such as motors or electromechanical relays. Otherwise, you will get undesired closing of the reed contacts and shorten the operating life of your switch.

Type of Liquid

This should always be the first thing to specify as it will determine other factors like the enclosure type and specific gravity rating of the float. The types of liquid can vary widely from water level control (e.g. in aquariums or swimming pools) to heavy-duty sewage applications.

Enclosure Material and Construction

You will need to find switches with an enclosure (and wire insulation) that can handle being submerged in a given liquid.

Normally-Open (N.O.) and Normally-Closed (N.C.) Contacts

For single-pole, single-throw (SPST) switches with one N.O. or N.C. contact, the “normal” position is when the float is sitting, “at rest,” at the bottom of the tank, and the float actuates the contacts as it rises.

SPST float switches that are normally-closed will conduct a current when the float is at the bottom of a tank, while normally-open switches will not.

If you want to fill a tank, then you will need a normally-closed switch to start a pump motor when the liquid level is low and to stop the pump once the tank is full. If you want to empty a tank, then you will need a switch with N.O. contacts to start a pump (e.g. sump pump) when the liquid level gets too high. (See the picture above for an example of a N.O. switch.)

However, SPST is not the only configuration possible. For example, the float switches we carry are single-pole, double-throw (SPDT) with one N.O. and one N.C. contact.

Temperature Rating

Some float switches can be used in tanks containing boiling water, while other switches have floats designed for tanks containing liquid nitrogen.

Specific Gravity (or Relative Density)

Specific gravity (SG) is a ratio comparing the density of one substance to the density of a reference substance, which is usually water or air. By this definition the SG of the reference substance will always be 1. Thus, if water is used as the reference, substances with specific gravities greater than 1 will sink, because they are more dense than water, while substances with specific gravities less than 1 will float.

Regarding float switches the question is, “Will the float work in a given liquid?” For example, the SG of the gasoline in your car is about 0.74, so even if a float will work in a tank of water it may sink if placed in a tank of gasoline. In other words, make sure that the SG of the float < SG of the liquid.

A float’s SG rating is especially important for applications involving various types of oil or alcohol, both of which are less dense than water (i.e. lower SGs).

Keep in mind:

- Density = Mass / Volume or mass per unit volume.
- Temperature and pressure are factored into specific gravity calculations because these two values will affect the volume of a substance.

- SG calculations are done by comparing equal volumes of the sample and reference substance.

- True SG is a ratio of densities (mass/volume), while apparent SG is a ratio of weights (mass*gravity / volume).
Since SG = density (of sample) / density (of reference), the units cancel, leaving just a number.

True SG = density of sample / density of reference
Apparent SG = weight of sample / weight of reference

For the SG of liquids, 1 atmosphere is the pressure at which measurements of substance densities are taken, or assumed to be taken, since the effects of small differences in pressure on most liquids is negligible. Thus, the pressure acting on the substances being measured is not as much of a concern as temperature of the substances.

Step 1

Remove the attached plastic lock ring from the counterweight.

Step 2

Clip plastic lock ring to the cord where you would like the counterweight positioned.

Step 3

Insert the cord in the conical part of the counterweight and over the lock ring.