Using Sound Waves To Measure Water Level
Anyone who has tried to measure the static water level in a well knows that there is no perfect method. Lowering tapes down the well can be challenging. The tapes get caught and hungup on wires and pipes. They get false indications from cascading water and condensation. And it is labor intensive reeling the tapes down and back up. Then there are the decontamination issues. Pushing a tape down a crooked or sloped well can be nearly impossible. The submersible pressure transducers have similar issues with the added uncertainty about their location and reliability.

Then there are the acoustic or sonic devices which send sound pulses into the well. While they are very convenient and fast, they are susceptable to imperfections in the well such as open perforations, open fissures or pockets, or even pipe fittings on the drop pipe, all of which can cause incorrect readings.

The bottom line is that there is not a perfect solution for every well. But where the tape may not be able to fall into a sloped well, the sound pulse has no problems. And where the sound pulse is lost in an open fissure, the tape may go right past.

While the submersible technologies have been well established, the acoustic or sonic meters have left room for improvement. This is where our story begins.

One of the biggest problems with the sonic meters is that the sound pulses tend to travel in straight lines and reflect off any imperfection in its path. The degree to which it does so is a function of the sound frequency. The higher the frequency, the more directional the beam, and the smaller the obstruction that will reflect the sound. Ultrasonic detectors are commonly used for measuring streams from bridges and levels in tanks through open air. This is possible because the very high frequency sound travels as a beam in the direction it is launched. It is also able to detect small objects crossing the beam. It cannot detect the water level through a pipe, however, because the ultrasonic beam takes many paths through the pipe. A very small part of the beam travels straight through the pipe, while part takes one bounce off the wall, and some may bounce 20 times making the path much longer. This is combined with the reflections from every pipe seam and wire bend in the pipe. The net result is noise.

Eno Scientific - Model WS131 - Well Sounder Compatible Flow Sensor User Manual