How to find the water depth in a well ... with a cell phone!

Have a water well? Want to figure out how deep the water level is? So did I, so I made an overly complicated contraption to do so. But before I get into that, there's a much, much easier way to figure it out. You simply need to time the echo of a loud noise traveling down the well and back, and use the speed of sound in air to estimate the length the sound traveled. To accomplish that, the hardware you need is a cell phone... and that's it.

Begin recording a video (or just sound) on the phone. Place the cell phone on top of the well, then knock the top of the well with your knuckles or with an object. You should be able to hear the echo of the knock reverberating in the well. Stop recording the video, then load it to a computer. Convert the video or sound recording file to a .WAV sound file (there's several places online that will do this, I used this one). Open the resulting .WAV file with a program that allows you to visualize the sound waveform. I used Audacity (here).

I made a short video showing how to use the Audacity program here:

Zoom in on the knock sound, and if it all worked out, you should be able to see high amplitude echoes well above the background noise level. Use the software to measure the time between the leading edge of the initial sound and the leading edge of any echoes you observe.

I like the Audacity program because if you choose the Selection tool from the top toolbar and click and drag within the waveform area, the Selection start and end time is given in the box at the bottom, with a precision of one thousandth of a second.

In my case, the echoes were about 0.060 seconds apart. Next, we need to translate that to a distance. The gas in the well above the water should have a composition close to air, so we can assume the speed of sound is that of air. The speed of sound varies with temperature. The temperature of the air in the well is probably closer to the ground temperature than the ambient temperature, so assuming 72 F (yep, it's hot here), that gives me a speed of sound of about 1130 ft/s or 344 m/s from the table here.

Then, it's a simple matter of multiplying the echo interval by the speed of sound to find the total distance that the sound traveled each echo.

Sound travel distance = speed x time

Sound travel distance = 1130 feet/second x 0.060 seconds

Sound travel distance = 68 feet

Don't forget that the sound has to travel down the well, reflect off of the water, and travel back up the well before you hear the echo. The total travel distance for the sound is therefore twice the water depth. Divide the total travel distance by two and that will give you the water depth!

Depth = Total travel distance / 2

Depth = 68 feet / 2 = 34 feet

One little caveat here - we're assuming that the echo we're hearing is from the surface of the water, and not the sound bouncing off of a change in the diameter of the well or some trash stuck higher up in the well. To confirm that the echo response is indeed the water, try running the pump for a few minutes and measuring the water depth again. The water level will fall while the pump is running in most wells, so you should see a slight increase in the water depth in the second measurement. If you don't see a decrease, either the water level is not drawn down appreciably while the pump is running, or there is an obstruction higher up in the well that the sound is echoing off of, masking the water level.

And there you have it! I bet the salesman didn't tout the high precision water well depth estimating capabilities of your phone when you bought yours! The next time you upgrade, be sure to insist on this feature, along with the ability to make phone calls!

For my next trick, I want to be able to log the well depth over a period of time, and watch as it rises and falls as the pump is turned on and off. Since I don't want to sit there whacking the well all day, I'm putting together a microcontroller datalogger to do the whacking, listening, and data interpretation. More on that next time.