Reprhasing the question is a more open ended manner:
Method 1: Use a pipe and measure it's resonance.
Given a closed pipe from 10 to 300 feet in length, and roughly 5/8" in diameter, made of 1/16" polyethylene, can the length be easily determined from it's resonate frequency?
(This is really an acoustic engineering question, but there isn't such a stack exchange group yet)
Edge damping: The very high length to diameter ratio (up to 600:1) may cause the signal to fade into mush before the reflection off the end and propagation back.
Q factor. Resonance may not not be sharp enough to provide useful information.
Detection. The nominal resonant frequency is around 1 Hz. This is difficult to measure. In theory it would be possible to use a harmonic instead, but it may be difficult to determine which harmonic is being used.
Perch a suitable tank on the well casing and make a helmholtz resonator. This uses an unrealistic amount of power, I suspect to move the required amount of air. Not clear how to build it to resonant at the fundamental, and not a harmonic.
Method 3. Run a length of wire down the well, and measure the capacitance between the wire and the steel well casing. Water has a high dielectric constant so the capacitance should vary substantially as the water level runs up and down. Make the capacitance part of an LC circuit, and count oscillations.
3a. You already have two wires going into the well to supply power to the pump. Should be possible to use one of those, if you can tune the LC to a frequency that is high enough that the motor ignores it. (Might put a 1 KHz signal on every device for a long way around. Isolation is a big issue here. I think I prefer a separate wire.
Where this comes from:
I want to measure the drawdown of a well continuously. (Once a minute) Ideally this device logs to a memory stick, and has a LCD display of the current level to make it easy for the well owner to track changes in his well behaviour over time.
Pressure sensors are expensive for this application, are at best only good to about 2% of range, are subject to hysteresis, require an A to D converter...
When testing wells, the common way is with an insulated tape with bare end and a continuity tester. You have to keep making and breaking the contact.
My first kick at the cat was to use an ultrasonic pulse and bounce it off the water surface. However there is a lot of competing junk in the top end of a well. Once you are below that, you still are working in the annular space between the riser pipe and the well casing. You have possibilities of spurious reflections off of casing joints, the torque arrestors, the well pump, and the bottom of the well. And ultrasound attenuates quickly.
My second thought was to use a wavelength that was a small multiple of the casing diameter. The transducer is placed below the wellhead junk, and emits a short chirp -- about 20 cycles. That won't work. The reflection off the well head will overlap the chirp. The chirp has to be under Vsound/20 or about 1/50 of a second. This would mean a 1 foot wavelength. Given well casing diameters of typically 4 or 6 inches, this is close enough that I can see all sorts of weird modes, and an unbelieveable amount of junk in the return signal.
This is my third thought. It's easy to lower a weighted 1/2" (which is really 5/8) plastic water line down to the bottom of the normal drawdown zone. The water level in the pipe is the same (within a second or two) of the level in the well. And there is no junk in this smaller pipe.
Ok, then, you say, do the 'bounce the sound wave' trick in the small pipe. My intuition is at a stretch here: I think to get efficient transmission, the conduit has to be large, or at least not small compared to the wavelength.
Resonance has the advantage that you are getting a very specific amplification. But it takes a long time (at least several cycles) to pump it up.
For this application the ability to get one measurement a minute would be sufficient.
"But what about the temperature? Vsound is temperature dependent."
The temperature once you are more than a few feet underground is constant. If the well is in frequent use, there is a 1.25" diameter column of fresh water in the middle of the casing. That water is at average ground temperature. In mid winter, the steel casing, which stick out of the ground a couple feet, will create an annulus of cold air, but I don't think that this can chill any signficant length of the pipe.