Condition of my well, some thoughts about caps, and other rambling.

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Hatsuwr

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Alright, hold on to something, this is going to be a long one.

Since I finally got my pump freed, I now have the opportunity to overthink things and fix them until they break.

This will be the first of three posts, since apparently everything at once is a bit over the character limit...

The Well

My well is in the coastal plain of Maryland, so I expect mostly sediments on the way down. I won't go into the full water composition, but notable are extremely high iron and manganese levels, and a pH of 5.6. The state apparently lost records from the period when the well was drilled, but they believe it was in 1962. The casing is 4" Schedule 40 PVC in 20' sections. Strangely, measuring the distance between joints has most of them at almost exactly 20', but the second and third sections down measure over 4" too short. Maybe they were damaged on the ends and cut before installation? The total depth is about 83.4 m (273.5 ft), although there may be a layer of sediment that stopped my measurement short.

Regarding the condition of the casing, I found that several sections are very... bumpy? There is also a damaged section at around 50 m (165 ft), right next to where the pump previously sat. That probably isn't coincidence, and I'm guessing the pump must have overheated at some point and weakened the PVC. I couldn't get a great view, but it almost appears cracked. The pump was stuck on this section, so I'll just reinstall right above it. I'm considering trying to get a length of 3.5" PVC pipe into the damaged section to reinforce it, but it's difficult to assess the situation under water with such high turbidity.

Here's a video trying to show the damage (on the left): https://streamable.com/4u5s9e

And here's a bumpy part of the casing compared to a normal one:

bumpy_casing.JPG


The static water level is around 39 m (128 ft). Drawdown levels will have to wait until I have the pump back in. I measured the water level with a camera at the end of a distance-marked fiberglass cable, but also tested echo distance measuring using my phone to emit a signal. A phone could also be used to record the signal and echo, but I used a microphone connected to my laptop so that I could skip transferring the recording, and probably get a higher quality recording. I used Audacity to measure the time between signal and echo.

Here's what three pulses and their echoes look like:

pulses.png


The pulse itself is very clearly defined, but the echo is a bit dispersed. I used the the first point where the echo became distinguishable from the noise background as reference. If we assume the timing can be measured to within a millisecond, that corresponds to a max distance measurement error of about 17 cm (6.7 in). Things like pressure and humidity can influence the speed of sound by a fraction of a percent, but more significant is temperature. An estimate is fine here though, since the change in air temperature from 10-25 °C (50-77 °F) is less than a 3% change in the speed of sound. The well's water temperature seems like a decent estimate for the air temperature. There are plenty of online charts and calculators that show the speed of sound for a given temperature.

Anyway, for my well, I estimated 342.3 m/s as the speed of sound. I measured the delay for five echoes to a precision of 0.5 ms and got 0.2295 s for all five, so repeatability is very good. One thing to note is that despite me setting the pulse frequency to 10 kHz, the highest amplitudes of the signals and echoes were well below 1 kHz. The timing was the same when isolating to these frequencies, so I don't think it matters much, but it might be an indication that 1 kHz would be a better choice than 10 kHz.

That works out to 342.3 m/s * 0.2295 s / 2 = 39.28 m (128.87 ft) measured acoustically, compared to the 39.09 m (128.25 ft) measured directly. Assuming my direct measurement is correct, and assuming the water level didn't change in the hours between these two measurements, that's less than a 0.5% error. Not too bad. If we assume my timing and my direct measurements were correct, that would mean an actual speed of sound of 340.7 m/s, which would relate to a temperature difference of just 2.8 °C (5 °F) from what I estimated for my measurement.

Making an app that does all of this at the press of a button wouldn't be too difficult, I'll probably give it a try when I have a little more free time. Just don't go dropping your phone down a well. I also plan to create an acoustic distance sensor to install ~120' down to give me continuous monitoring of levels over time, but I don't have time for that just yet either.

The Cap

My cap is from MAASS Midwest, model WTCC-4. I wanted something that was aluminum (or at least, not plastic/steel/cast iron), vented, and had a connection for electrical conduit. This fits all of those requirements, but execution could be a bit better. I'm happy overall with it though - it does its job and was fairly cheap.

cap.jpg


Some thoughts for improvement:

1. The casting isn't as flat as I'd like to see for the mating surfaces, but honestly it's as good as it needs to be considering this is a vented cap. The opening for the casing is also much larger than it needs to be. It has a screw to tighten against the casing, but a tighter initial fit would have been nice (or having three screws for tightening to a more centered position.) I cleaned up the mating surface of the upper section to have a continuous flat path along the seals, and will do the same for the lower section next time I take it off. I use Fluid Film to slow down corrosion on the aluminum parts.

2. The cap has a 1" FNPT port for connecting conduit. Makes installation a bit more difficult than it needs to be, and makes removal of the lower half of the cap impossible without cutting the conduit. A better connection, in my opinion, would be a hole just large enough to admit 1" MNPT threads. Sealing would be accomplished by a smooth surface on the bottom with a gasket between it and the male adapter. Securing would be done with a thin 1" NPT threaded locknut inside the cap. Pretty much how liquid-tight non-metallic conduit connects to an electrical box.


3. The gasket around the casing and the conduit openings is nitrile rubber from what I can tell. EPDM probably would have been a better choice, and a design that uses standard size O-rings would be an improvement. For venting, it has a #30 stainless steel mesh screen surrounded by what seems to be a neoprene gasket. The three gasket pieces seem to be adhered instead of bonded, and mine broke fairly easily at one of these joints. Cyanoacrylate adhesive can do a good repair job, but the manufacturer also quickly sent me a replacement without any charge or issues. I restored the NBR gaskets with methyl salicylate (this was after the break, so not the cause of it), and applied a thin layer of silicone grease to them and the neoprene gasket.

4. Doesn't matter for my current setup since I don't have a ground wire anyway, but a grounding screw seems like it would be a sensible inclusion, especially when used with plastic conduit and casing. Easy enough to add one though.

5. A bit nitpicky, but rather than the included stainless steel screws/nuts, just use aluminum screws and tap holes in the bottom part of the cap. Regardless, I use metal-free antiseize on these screws and the one clamping the casing.

6. Instead of a mesh-screened hole for venting, it wouldn't be too hard to expensive to have a pair of spring loaded vents - one for intake, one for exhaust. A very low cracking pressure would be best for most situations, but this could be adjusted by changing out the spring. You could even have a mechanism that would allow the cap to be fully sealed.

Here's a very rough model I threw together for an improved cap. Blue areas are sealing surfaces. Casing and conduit are obvious. Small holes are screw holes, the two remaining holes are for intake/exhaust. Could be machined very easily from a $10 piece of aluminum, and the remainder used to cast the top half. Only thing I'm not happy with is that the red section of the O-ring around the casing wouldn't be compressed by the top half. A small piece could be added to that area that would be pushed down by the top when it's screwed into place. Not sure if I'll have the time to make this anytime soon unfortunately.

cap_model.png
 

Valveman

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Those caps don't fit tight enough to be used in a submerged location anyway. Don't spend too much time trying to seal that cap. Any old screen will do. Wouldn't use the spring loaded vents either.

The echo thing is cool. But when you put the pump back in, run a little 1/4" poly line down with the wire. A pressure gauge and a Schrader valve on a tee up top will let you blow air in the tube. What ever the pressure gauge says multiply by 2.31 and you will know the feet of water standing above the pump or the end of the tube. That is really the only reliable way to get an accurate water level while the pump is running.
 

Hatsuwr

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Yea, the current cap won't ever be waterproof. It can be sprayed down with a hose and not leak, so I'm happy with it. But when thinking about designing a new cap, it's really a small jump from that standard to fully waterproof. What's the concern with using valves for venting? Cost or reliability?

For absolute depth measurement, a problem I was having when using 1/4" poly rope is that it stretches quite a bit even with small loads, and the amount of stretch increases over time. No problem for making a good estimate, but I was aiming for a bit more precision. By using the fiberglass camera wire as a reference, I was able to mostly eliminate any stretching issues, but you do need a decent weight on the end to make sure it's straight

I really like the pressure method for seeing the change in water level though. Wouldn't have any issues with noise from the running pump potentially interfering. If you can measure to 1 kPA (0.15 PSI), that's accuracy to around 10 cm (4 in) of water. I do think an echo based sensor closer to the water level will be much more accurate since the majority of its error is from unknown air temperature, but that's at the cost of greater complexity and the increased accuracy wouldn't be needed in the vast majority of situations anyway.
 

Valveman

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If you install the 1/4" poly when the pump is installed, there is no need for any weight. Just tape the open end of the tubing exactly at the top of the pump and it will always be exactly in the same spot. Never found a more accurate way to measure a water level. It is as accurate as the gauge you use to measure it. Could even use a digital gauge if you wanted. Works in aerated or churned up water as happens when running the pump, especially when testing wells. Learned how to do it with a well that had a cascading water problem. Deep well, perforated midway and at the bottom. There was enough water coming in the top perforations to really aerate the water down at the pump. Echo, shot, and electrical meters told me the later level was where it was cascading in from, the air hose didn't lie. Even if you weight a 1/4 poly line and drop it down an existing well, the stretch is of little importance, but could also be calculated.

Just don't see any need for a check valve on a screened vent. Cracking pressure could be an issue and it is just another opportunity for a failure.
 
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