Sonic depth sensor for a deep water well

Other Hardware Science and Measurement
1

Hello,

I have found a few random references to this here at this forum and others but nothing that has been completed of has any follow up. It’s possible that’s a bad omen, but I’m still feeling bullheaded about it.

I have a deep water well - drill depth is about 800’, water level at rest is 130’ and well pump is at 400’. I don’t know the drawdown characteristics of the well, but I do know that the previous owner instructed me to provide a well rest period of one hour between irrigation cycles to protect the pump from running dry. I’ve lived in the house for over a decade and haven’t had any issues but I’m interested in monitoring the depth of the well.

The reason I’m interested in measuring the depth of the well is because I’ve volunteered to contribute to a groundwater monitoring project in my area. Ideally, the solution I develop will be reasonably inexpensive (<$500), reasonably precise if not terribly accurate, and reproducible so that I can offer the plans to other contributors in the area.

There are a number of ways to measure the depth, and the following are the ones that come up most frequently:

  1. Pressure transducer: A long cable would drop the pressure transducer to either the bottom of the well or to a known depth, and one can use the pressure in order to determine the height of the column of water above the transducer. This one is somewhat cost prohibitive, as the cabling plus transducer alone reach about $1000.
  2. Bubbler/pressure differential measurement: In this case, one tapes a 1/4” clear poly tube along the length of the well wire to a known depth. Using an air compressor, determine the pressure needed to just start to send bubbles out of the bottom of the tube (or where pressure eventually stops increasing). Use this pressure to calculate the depth of the water. This one requires pulling the well pump, which is also cost prohibitive and obtrusive.
  3. Sonic depth measurement: This is my preferred method as it’s relatively portable and doesn’t require obstructing with the well operations. It’s also the method that they are currently providing for contributors to the project. They are loaning out a well depth sensor, the WL650, which merely needs to be pointed into the well access port and measured a few times. It emits a series of clicks, measures the echo time, uses a preprogrammed temperature setting (well temperatures are relatively stable), and returns a depth value.

It feels reasonably possible to reproduce some version of the third option. I would need something to emit the sound, which should have a very short time period and relatively low frequency, and something to detect the echo’s return.

The speed of sound is 343.2m/s at 20°C. The water depth at its highest is approximately 125’ or 38.1m. The sound would have to travel there and back, so it would be going 76.2m and the sound should return in 0.222s. Therefore, the duration of the sound should be relatively short, and the time sensitivity of the device should be able to pick up the reflected sound accurately in that short of a timeframe.

I’ve never done a single arduino project. I have limited experience with electronics and soldering, mostly from my youth. I’m very handy, though, I have an engineering degree, and I’ve fixed every appliance in my home numerous times, so I’m hopeful I can tackle this project for this community and my community’s desire to monitor groundwater concerns.

Any thoughts, suggestions, or help are very much appreciated. What board/clicker or piezo/microphone/etc would be most precise? Budget is reasonable at sub-$500 but obviously the cheaper it is the more likely others will repeat and we’ll get a better view of our groundwater table.

Thanks in advance!

2

Do you have two years or more to experiment and develop that device? Do you have time to do that? At this point you do not even know if they used an off-the-shelf device to send/receive the sound or did the custom manufacturer it?

3

I've got time, in answer to your first two questions.

It's not a complicated problem from the scientific side; it's simple physics. I just have to see if I can make something sensitive enough to pick up the echo. I can hear it with my own ear when I use a rock to tap on the well cap. There are multiple devices that do this same thing. Low frequency, timer, detect echo, stop timer and do the math.

I understand you're less optimistic and that's fine. Appreciate you taking the time to respond, even if it is to discourage me from "wasting" time on the project. I don't believe there's such a thing as wasted time, so I'm interested in learning what I can along the way.

4

It is no problem to have an Arduino monitor a microphone and determine the time between a start pulse and a echo return.

As an illustration of the principle you can buy an HC-SR04 ultrasonic ranger for less than $4, but its range is limited to about 4 meters, because sound intensity diminishes so rapidly as it propagates in 3D.

For sound propagation in a 1D case like a well column, that intensity reduction is vastly reduced. My approach would be to put a board across the well head, hit it with a hammer, and use a mic/Arduino setup to reproduce the basic HC-SR04 approach.

2 Likes
5

My guess is the the main expense is in the physical construction of the sensor head. The emitter needs to generate a high amplitude pulse that is highly damped so that the co-located receiver doesn't get confused by the still ringing transmitter when the weak echo arrives.

For a well you probably want the sound wavelength to be the same as the diameter so that the wave propogates as a single front passing minor obstructions without being reflected. This puts the frequency in the low KHz range.

Most regular piezo alarm/speaker devices designed for use in that range are designed for efficeincy with little regard to damping.

Why does a bubbler require transparent tubing and why do you need to pull the well pump?
Fundamentally it's just a tube with a weight at the end to overcome the buoyancy provided by the entraped air.

1 Like
6

This is a good point I hadn’t considered. I suppose it will depend on the amplitude of the echo compared to the residual sound. Experimentation should help me sort that out.

It doesn’t. I don’t know why I said transparent, except that I was imagining it that way. You’re right that I could weight it. I don’t know how far it would have to go below water level but likely not all the way down to the pump. The downside is still that I’d need to keep an air compressor there which would cycle on whenever a reading was due to be taken. I am also hoping to power this device by solar and battery, the idea being that it would go to sleep between readings which would be perhaps hourly at the most, but more likely 4-6x a day. That wouldn’t be as feasible with an air compressor.

Appreciate the input.

7

You could drop a loosely fitting floating ball into your well and measure the distance with inexpensive laser range finder module.

9

Interesting idea, but not all wells are dead straight and at just the right temperature the mositure laden air will form fog. Plus you have the piping which is often unrestrained and could end up anywhere in the optical path.

10

Please be aware that the speed of sound also depends on the humidity and inside a well this is probably higher than at the top. It might even vary with depth.

My SRF05 library is not suited to solve your problem but has information about the effect of humidity and might help you to tackle this effect.

How about "a long rope with knots", throwing it in the well and count the knots?

These days throwing can be replaced by a rope between two gears to measure the length of the rope. When it touches water tension changes, or maybe better use an electric cable. As long as it doesn't touch water infinity Ohm, touching water means a drop in resistance.

In my home town we have a water meter for ground water and it is a transparant tube about 10" (25 cm) in diameter in which a styrofoam cylinder is placed. This styrofoam floats on the water (defined depth) and if the water rises the foam rises. Its height above the ground is almost 7 foot (2 meters) its accuracy is better than 2 inch (5 cm). See picture

watermeter
watermeter682×1663 244 KB

11

Sure not, and I have personally experimented only with two 50m+ irrigation wells, but both of them had straight line of sight to the water. Fog might be big problem though.

12

Hang one of these on a cable?

13

It had better be restrained at the pump or eventually the starting and stopping of the pump will unscrew the pipe somewhere.

14

For well related videos the Youtube h20 mechanic channel has lots of good practical info. Customers want everything done cheap and contractors are happy to oblige.

Strangest thing was watching how little effort was put into waterproofing submerged mains voltage electrical connections.

15

Another thought on the shoulders of this one...

Mount the LASER (upper case) on one wall. Point the LASER beam to the junction of wall and water and measure the angle. The diameter of the well is the tangent of the angle (if I remember into last century).

height = diameter / tan (angle)
16

It’s possible I could use a LASER range finder. I could purchase an off the shelf version and see if I can get a line of sight or not. I do not believe bouncing off of the wall works, and I cannot mount to the inside of the casing of the well.

There is a 1/2” access port on the top of my well for taking measurements. This is the point of access that I plan to use. It’s how I measured using the commercial sonar device. With a well as deep as this, I’m unsure if I can use light without interference but I’m open to it.

I’m already getting close to a prototype to test the sound theory. I hope to start experimenting at some point next week.

17

Carpenter's tool.

https://www.lowes.com/pd/Bosch-BLAZE-400-ft-Indoor-Outdoor-Laser-Distance-Measurer-with-Backlit-Display-Bluetooth-Compatibility/1000492089

18

Have a look at commercially available equipment .
Level measuring stuff in tanks etc .

A wire with float on the end is a basic simple system that works well ( well ha !)

19

The space NOT filled with water is easier to detect than the space filled with water.

20

A solution with a pressure transducer does not need to be that expensive. I built a depth meter with a MPX4250A as sensor. It lies on the well bottom, safely in a small plastic bottle filled with silicon oil. This sensor has an analog output which can easily be read in a device above ground. There is another MPX4250A which measures the air pressure. With a differential amplifier (or micro processor) you can obtain the water pressure and thus the depth. The sensor was about 15 €, but it looks like it is not available any more. You might look for another absolute-pressure sensor.