Public Lab Research note

Shallow note on depth

by donblair | January 14, 2015 17:52 14 Jan 17:52 | #11522 | #11522

This is just to hold some pics for now ... more deets later!








And a great study of capacitive depth measurements is here:

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Hey Don -- How well does the depth feature work? @stevie and I were just talking about it, and are needing a reminder. :)

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Hi @gretchengehrke :)

I'd never really testing it very thoroughly, and I haven't done sufficient background research to know if the 555 approach I was using would be particularly robust in the field. Could be a fun project for folks to look into with us -- if it works well enough for a someone's particular use case, it'd be a pretty inexpensive way of getting water depth ...

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Don have you described somewhere how in the world this thing measures water depth? I need this and I discovered today that my spring house up in the woods is within range of the home wi-fi signal.

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This sounds like it might prompt a PVOS research trip up to Bernie Country ...

I need to write up a research note, but here's the basic idea (courtesy of a hacking session with Dan Beavers and Scott Eustis a while back): we're exploiting the fact that a capacitor placed in a dielectric will have an enhanced capacitance. We can make a capacitor out of a long extension cord, or another two-wire cable; the measured capacitance in the extension cord increases as more of it is placed in water (a dielectric). So, if we can measure the capacitance, we might be able to correlate it with water depth. Here's what I'd tried, in more detail:

  • We used a 555 oscillator circuit, in which the output frequency f goes as 1 / RC, where R and C are a resistor and capacitor connected to the 555.
  • We chose a fixed resistor R and place it in the circuit; for C, I used a two-wire extension cord. Both ends of the extension cord were cut: the two wires on one end were hot glued, so that no wires were exposed to water; the two wires on the other end were connected a screw terminal on the breadboard, where the capacitor C is indicated in the schematic.
  • The schematic on this note is the basic layout for the circuit. In that note, we later choose a fixed through-hole capacitor for C, and connect a photodiode as a variable R; in this case here, we want to choose a fixed resistor R, and use the extension cord as our variable capacitor C.
  • The 555 when powered and connected in this fashion will output a frequency that is (if I recall) f = 0.7 / (R*C). We then hook up pin D2 or D3 of an Arduino to measure the number of pulses per second (D2 and D3 have a special 'interrupt' feature which allows for counting fast pulses).
  • The hope is then that the frequency can be nicely correlated with the length of submerged extension cord. The 555 frequency will go down as the capacitance in the extension cord increases; the capacitance increases when more of it is covered in water. So, if we arrange the extension cord in the configuration of a 'meter stick' in the water, as the water level rises, the 555 frequency should go down.
  • I found that the measured capacitance C of tap water using an extension cord dipped in water only a little was on the order of 10 nanoFarads, it helps to know C when choosing R in order to get a frequency output in a reasonable range. A typical Arduino is 16 MHz; I don't know what maximum frequency it can reasonably measure, but likely significantly less than 16 MHz.

Do you have power in that spring house?

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Why does this work when the two wires (the extension cord) are insulated? It must be that the electric field extends outside the wires into the water. Electricity is weird.

There is no power at the spring house. There is a constant flow of water like a little waterfall, so we could fix up a turbine generator.

It might not be an important project because I already get data on the water level at the house -- when it gets low enough nothing comes out of the faucet.

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Exactly. Was confusing for me, too, as I'd been nursing a rather fixed mental picture of a parallel plate capacitor with a dielectric slab sandwiched in-between the plates. But for long, parallel wires, I guess there is fringing of the field lines into the water.

The trick here is whether the measurements are repeatable and stable; the link I referenced in my initial comment on the note is a study of capacitive depth sensors, and tries to assess how conductivity biofilms affect the measurements.

That said, if you want to measure "is the water level above X feet or not", I would think you could fairly reliably measure that with this system by placing the end of the wire at the threshold of interest -- the difference between water being present, or not, should be fairly dramatic.

That said, in such a "binary" case you could just use two wires directly, perhaps -- no special circuitry needed.

Regarding power, I suspect that this won't do the trick (insufficient pressure?), but it looks pretty neat anyway -- a little hydro-generator, with internal battery.

(Aside: if you could: please refrain from voicing the "... but if I needed to check that environmental parameter, I could just walk over and look at the ..." critique in a public forum? You'll blow my entire gig.)

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That hydro generator says it needs 3 liters per minute, which we have.

Sorry about revealing your irrelevance. Also, we wouldn't need the internet if you would visit more often. Only 20 more days of winter for SNOWFEST to happen (assuming the snow part is non-essential).

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Oh, exciting ... it'd be fun to demonstrate anything powered by moving water. Maybe this thingy should be ordered immediately.

A visit ASAP sounds very much in order.

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For $25 that hydro generator has got to be a good thing. It is an impartial solution for my spring house because it can't freeze (operating range 4° to 80° C).

The thingy has a 700 mAh battery, and it charges that battery, so it does not have to be running all the time to supply power. So it can be inserted into plumbing where the water flows intermittently and power a microcontroller continuously. The only plumbing I have where I also have heat in the winter is in the house where I also have power and therefore don't need to generate my own.

It might be fun to experiment with a long hose in a mountain stream to see how far upstream (how far uphill) you have to put the inlet for the pressure to be sufficient to run the generator. Or we could ask an engineer who understands the Bernoulli and Darcy–Weisbach equations. I'm guessing you would need a couple hundred feet of garden hose and 20 feet of head to run the generator (that's a steep stream). Solar power will probably be more widely applicable, but if someone wants to power Riffles in a shady mountain stream, this might be a solution.

Or you could just walk over to the stream and see if it looks okay.

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