They contain an infrared light-emitting diode (LED) and a light receiver. Light from the LED reflects from a prism forming the tip of the device back into the light detector. With no liquid present, most LED light reflects within the prism to the receiver. When liquid immerses the prism, light refracts out into the liquid so that little or no LED output reaches the light detector. The receiver actuates electronic switching circuitry to signal this condition. The circuitry then operates an external alarm or control circuit.

Many limitations associated with electro-optic switches center on reflectivity. These devices may get confused when in proximity to shiny surfaces. Similarly, they do not work well sensing reflective media such as milk. And they are not recommended for use in liquid that crystallizes or which leaves a solid residue.

These switches typically sense liquid level to within ±1 mm. They operate from 4 to 20 mA at 5 or 12 Vdc with no load. Most of this power is consumed by the LED. All their components are solid state. Switch outputs are compatible with TTL and CMOS circuitry. Open collector outputs generally can sink 40 mA at up to 30 Vdc. Switches that are of a more recent design tend to be physically smaller than older versions mainly because they can make greater use of compact surface-mount circuitry.

The typical mounting configuration for these devices is on the side of a holding tank. But special models are available that incorporate an extension tube that positions the sensor tip up to 15 in. away from the mounting body. This option is used for sensing fluid level at some distance from the top of a tank.

Switch housings can be either plastic or metal alloy. Plastic housings tend to be Polyethersulfone or Isoplast, both of which handle temperatures above 200°F. Models made of Teflon work in corrosive liquids. Stainless or carbon-steel versions target heavy-duty applications such as off-road vehicles. These incorporate prisms made of fused glass rather than the usual plastic.

Gems Sensors Inc. provided information for this column, www.gemssensors.com.