Wave guide for Ultrasonic Sensor (HC-SR04)

Ultrasonic level sensors work by measuring the time of flight of a sound wave. The wave must travel in a clear path to and from the top of the liquid being measured in order to provide an accurate liquid level measurement.

As with any wave, the sound pulse generated by the ultrasonic sensor does not travel in a straight line, but rather expands as a function of the distance traveled. As the wave widens, it will bounce off of anything in its path. In many cases this is not an issue, but, occasionally, conditions within the tank or vessel being measured cause sound waves to be reflected from objects or surfaces other than the intended target.

Detecting the wrong target causes inaccuracy or false readings from the sensor. Things like large weld seams, pipes, mixers, or access hatches inside tanks can present false targets to the sensor. The use of a standpipe as a waveguide for the sensor can help eliminate these problems. 

This rule of thumb gives you a pretty good indication of whether the ultrasonic transducer will work in a standpipe (or nozzle). The necessary ratio of standpipe diameter to height is 3:1. So if you have a 6″ diameter standpipe or nozzle, and it’s 18″ high, your transducer should work without problem.
And ideally, the standpipe nozzle should be cut at a 45° angle (as shown), and the standpipe side wall should be seamless and smooth.

A standpipe serves as a waveguide to focus the sound wave away from possible false targets and directly onto the desired liquid target. Any smooth-walled tube or pipe can serve as a waveguide as long as care is taken to de-burr the end of the tube and cut the end at a 45° angle. This avoids the creation of standing waves inside the tube.

Use of a standpipe may affect the maximum sensing range of the level sensor because the waveguide also serves to limit the sound energy coming back to the face of the sensor. This should be taken into consideration when applying a waveguide.

Let us know if you have any questions about using a standpipe as a waveguide for an ultrasonic sensor. Level measurement can be drastically improved when the circumstances are just right, and the standpipe is applied properly.

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