Using Sound Waves to Measure Water Levels
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.
Other sonic meters have been developed for measuring through pipes using a much lower frequency sound close to 200hz. At this frequency, the sound waves spread out and pretty much fill the space available to them. This makes it unsuitable for measuring a stream level from a bridge or level in a tank for example. But when confined to a pipe, the wave fills the pipe and travels as a single wave, eliminating the multipath issue with the ultrasonic waves.
But even at this frequency, small imperfections cause a significant portion of the sound pulse to be reflected and can appear like it has reflected off the water. Here is where Eno Scientific has taken this sonic technology to the next level. The Eno Scientific Well Sounders were developed to use an even lower frequency, down to 50hz. At these frequencies, the sound pulse acts more like you would expect from an air pressure wave. At these slow speeds, the air fills all the space along the route flowing around obstacles. The reflections from most pipe couplings and minor obstructions become insignificant.
Not only has Eno Scientific raised the bar in its ability to use very low frequency pulses to measure the depth to the water, but also in the standard features included to make the users job easier. Features such as the built-in data logger to automatically record those measurements over time as well as data from external devices such as an associated flow meter. Also the ability to transmit the measurements over serial data links and analog levels to remote data systems. The powerful internal software is intuitive and easy to use with many advanced functions, such as the ability to compensate for methane gas or use in small tubes, and the ability to measure flow rates and calculate recovery rates while the well is pumping down.
Why doesn’t everyone do it this way? The advantages are obvious. The answer is that it is much harder to do. Generating a clean low frequency pulse is more difficult. It is also much more difficult to determine the exact location of a slowly changing pressure wave than a quick one. Eno Scientific was able to solve these problems using powerful digital signal processing techniques and electronics.
Many of these features are just not available on other devices at any price.
This is what makes the Eno Scientific Well Sounder the most advanced sonic well measurement system in the world!