Switch Tips: Ultrasonic level switches

Oct. 21, 2004
Ultrasonic switches transmit a signal in the range of a few megahertz to a receiver across a gap.

Principle of operation

The signal attenuates severely when passing through air, but the presence of liquid greatly enhances transmission. Thus signal strength of the ultrasonic burst serves as the sensing mechanism. Upon receipt of a valid signal, solid-state electronics generate a data-enable condition that indicates liquid is present. This signal energizes a relay to provide an output condition.

The typical reason for using ultrasonic switches as opposed to simpler technology is to get a complete solid-state sensor with no moving parts. The reliability of this approach can be high but ultrasonic sensing devices can cost more than several other level switching methods. Units can range from $150 to as much as $500 depending on the packaging and features.

Ultrasonic switches are constructed by molding or gluing piezoelectric transducers in the switch body. The body can be stainless steel for high temperature/pressure uses or a plastic such as polypropylene. Teflon or other special materials can be used to handle particularly corrosive liquids.

Ultrasonic switches can work in most liquids that are not aerated or which have a lot of foam. Air bubbles or foam can look much like ordinary air to an ultrasonic wave and thus may cause reliability problems. Similarly, liquid hydrogen has transmission qualities close to that of air and is not a good candidate for these sensors. And there may be a delay in signaling for cases where sufficiently viscous media must drain away from the sensing elements.

There are two types of configurations for ultrasonic switches. The most compact package locates the sending and receiving transducers across from each other in a gap at the end of the switch body. The advantage of this style is that the sensing elements can sit within 0.25 in. of the tank bottom. The other style puts the sensors across a gap in the middle of the switch body. The advantage is that this style can use slightly larger sensors that are in turn less susceptible to noise as from foam or air bubbles in the media.

Ultrasonic switches can carry either solid state or electromechanical relays in capacities ranging from 1 to 10 A. The input power supply is typically 110 Vac though small units can be powered from 9 to 36-Vdc supplies. Switching repeatability generally ranges from ±0.5 to 2 mm.

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

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