Selecting a liquidlevel float switch

Ronald A. Buchanan
Vice President
Madison Co.
Branford, Conn.

Liquid-level switches used in aircraft, nuclear-power plants, medical equipment, and other critical applications must be precise and reliable. For that to happen, design engineers must know the application and options to choose just the right switch. One of the most common level-sensing switches 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're 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. Float switches come in many styles, sizes, and materials.

Before specifying a float switch, the engineers 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 floatswitch operation, and even cause premature failure of both the switch and relay due to chattering. Turbulence can be caused by a vibrating tank, mixer agitation, or liquid swirling around while the tank is filled. A slosh shield is low-cost solution. It isolates the switch from fluid motion and has holes to let air and water flow in and out.

Buoyancy comes into play when a float switch is almost the same size as the vessel. The float will displace liquid, so less liquid could actually be in the vessel than would be indicated by the switch. Miniature and subminiature switches minimize this possibility in small vessels.

Water is by no means the only fluid in which float switches are used. Thus, the engineer should know the specific gravity of the float and liquid. The fluid's specific gravity must be greater than that of the float at the application's maximum temperature. Specific gravities of floats range from 0.45 to 0.85, depending upon size and the material they're made of.

Knowing a bit about specific gravity lets engineers 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 oil and water, but you're only concerned with the water level. The specific gravity of the oil is between 0.8 and 0.9 and that of water is 1.0. So a float with the specific gravity of 0.95. would sink in oil and float in water. Buna-N and polypropylene full-size floats could be modified to meet this demand.

Temperature, both maximum and minimum, must also be considered. For example, 316 stainless steel is ideal for applications with temperatures up to 300°C. On the other hand, polypropylene should only be used when temperatures will be 105°or lower. Other common switch materials (and corresponding maximum temperatures) include; brass (130°C); Buna-N, or nitrile (120°C); PBT, or polybutylene terephthalate (120°C); polysulfone, or PSU (148°C); Kynar (105°C); and Teflon, PTFE (260°C). As to the other end of the temperature range, Buna-N withstands the lowest temperatures, down to 50°C.

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.

Installing a float switch in a vessel containing or made of magnetic materials can affect switch operation. Care must be taken that switches are not mounted too close to any coupling, fitting, or tank wall with properties that could interfere with them.

Engineers 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?