Ultrasonic sensors use the sonar principle in air: they send out an ultrasonic chirp, then convert to the receive mode and wait to detect a return echo from the surface of the target. With the speed of sound in air (or other gas) as a given, the user can calculate distance. By placing an ultrasonic sensor at the top of the tank and using the liquid surface as a target, the user can easily determine the level in the tank. In addition, variables such as temperature may be sensed and compensated for, resulting in highly accurate readings. Noncontact ultrasonic sensors have a number of advantages over in-contact mechanical systems, as they have no mechanical moving parts to wear out, stick, or corrode because of constant contact with a liquid. Today's electronics and microprocessors permit many features to be programmed into the sensor. These include high-low alarms, filters to remove waves and other transient disturbances, rate-of-use information, and diagnostics to report a malfunctioning sensor.
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