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Ronald A Buchanan, president of Madison, examines the science of selecting a liquid level float switch.

When liquid level switches are used in high-reliability applications, such as aircraft, submarines, nuclear power plants and medical equipment, it is essential that they operate with precision and dependability.

Therefore, great care must be taken in choosing the best switch to fit each specific application.

With many choices available in terms of switch style, size and material, knowledge of several scientific principles related to fluids is valuable during the float switch selection process.

One of the most common level-sensing switches used in industrial applications is the float switch.

It consists of a float holding a magnet and a dry reed switch encapsulated within a stem.

As the float follows the level of liquid in the vessel, the magnet also moves up and down.

The magnet activates the reed switch when they are both at the same level, providing an ‘on’ or ‘off’ signal.

The signal can be conditioned to activate alarms and controllers or complete a circuit that turns on pumps to fill a tank.

Before specifying a float switch, the customer must know the fluid being monitored.

Characteristics of the liquid, such as its turbulence, specific gravity, temperature and viscosity usually dictate essential switch characteristics.

The liquid could also be a strong acid or base, which would influence material selection.

In industrial applications, turbulence can interfere with proper float switch operation.

Turbulence can be caused by a vibrating tank, mixer agitation, boiling liquids or surging of liquid while the tank is being filled.

A slosh shield can be used as a very low-cost and reliable solution.

It isolates the switch from fluid motion and has holes to let air and water flow in and out.

Buoyancy, the tendency of a body to float or rise when submerged in a fluid, comes into play when a float switch is almost the same size as the vessel.

The float will displace the liquid, so less liquid could actually be in the vessel than would be indicated by the switch.

Miniature and sub-miniature switches minimise this possibility in small vessels.

Specific gravity is the ratio of the density of a material to the density of water.

Materials that are lighter than water (having a specific gravity less than 1.0) will float in water, while those with specific gravities greater than 1.0 will sink in water.

Water is by no means the only fluid with which float switches are used.

Therefore, it is essential to know not only the specific gravity of the float, but also that of the fluid used in the application.

The fluid’s specific gravity must be greater than that of the float at the application’s maximum temperature.

Specific gravities of floats used for liquid level sensing can range from 0.45 to 0.85, depending on size and the material they are made of.

Knowing a bit about specific gravity also makes it possible for engineers to design floats that differentiate between two liquids.

Such floats, called interface floats, sink in one liquid and float in another.

A typical application would be one in which a tank holds both oil and water, but the user is only concerned with the water level.

The specific gravity of the oil would be between 0.8-0.9 and that of water is 1.0.

So a float with the specific gravity of 0.95 would sink in the oil and float in the water.

Buna-n and polypropylene full-size floats can be modified to meet this demand.

Temperature, both maximum and minimum, must also be considered.

For example, 316 stainless steel is ideal for high-temperature applications up to 300C.

On the other hand, polypropylene should only be used where it will be subjected to temperatures of 105C or lower.

Other commonly used switch materials (and their corresponding maximum temperatures) include; brass (130C); buna-n, or nitrile, (120C); PBT, or polybutylene terephthalate, (120C); polysulfone, or PSU, (148C); kynar (105degC); and Teflon PTFE (260C).

As to the other end of the temperature range, buna-N, also called nitrile, can withstand the lowest temperatures, down to -50C.

Viscosity is a measure of the resistance to flow of a substance.

Viscosity can vary greatly from liquid to liquid; for instance, water has low viscosity while vegetable oil has high viscosity.

Fluids with high viscosity do not flow readily, so floats used in viscous liquids should have a rounded shape, to eliminate places fluid could pool.

Switch operation can also be affected by a vessel’s composition or components.

Since a float switch consists of a magnet-containing float and the float’s travel in the fluid magnetically actuates the reed switch, a vessel containing or made from magnetic materials can influence the switch’s performance.

Care must be taken that switches are not mounted too close to any coupling, fitting or tank wall whose properties could interfere with them.

Customers should also take into consideration how the switch will be mounted.

Are the vessel walls strong enough to support the switch? Is the vessel made of a material compatible with the switch? And what are the long-term effects of material build-up on the switch? This discussion of the scientific principles involved in the operation of liquid level float switches is by no means exhaustive.

I hope it has given you a basic understanding of the many considerations that go into the design of our products.

I also anticipate that this knowledge will help you to choose a liquid level float switch that provides reliable and long-lasting switch operation in each of your applications.

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