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Main question: can I build an arduino-based sensor that'll use such low power that something like inductive charging will suffice to power it?

Details: I've got a "holding tank" on my boat ... a 3-foot tall polyethylene tank that's typically part-full of human waste mixed with salt water, which is a pretty horrible environment for electronics. I want to be able to know how full it is. External sensing hasn't worked well, so I'm considering putting some assembly into a piece of PVC plumbing pipe, with sealed ends, and sticking this in the tank. My plan is to use capacitive sensing (as in this design) with temperature compensation using a low-budget temp/humidity sensor I've got lying around. I plan to get information from this sensor to a display via bluetooth or some similar coupling. I typically need to know the tank-fullness about once or twice a day ... this doesn't require continuous monitoring.

My plan is therefore to use something like sleep mode most of the time, waking up now and then (or perhaps waking up in reponse to some sort of trigger -- I'm not clear on how to do this), and powering the thing from something like a lithium battery. But I need to recharge the battery, even if it's a very low power system overall. I was thinking that I could use inductive coupling (from a coil of wire outside the poly tank to a coil of wire inside the PVC pipe), and then take the resulting current and rectify and filter it a bit and use that to recharge the battery.

Any thoughts on

  1. whether this is even slightly plausible? The PVC is probably 1/4" thick, and the poly tank is about 3/16"...so the inductive coupling would have to be over a distance that's on the scale of 1 inch, more or less, and

  2. Whether there's an easy way to waken the system from low-power mode in response to some external signal, and

  3. Whether using bluetooth, when the processor is going to be in sleep mode 99% of the time, is plausible, or am I going to be spending all my power in reestablishing the bluetooth connection?

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  • \$\begingroup\$ Spontaneously it sounds feasible, though I don't think bluetooth is the best solution (even though I'm Swedish!). Bluetooth LE seems to be "the shit" at the moment, which also suits your specific project.. \$\endgroup\$
    – pipe
    Apr 21, 2016 at 17:36
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    \$\begingroup\$ Just wondering, but, why not put a 10x10x10mm, or other popular and large size, Neodymium inside some foam (floater) inside the tube and have one or more magnet switches outside the tank? A "die-sized" Neodymium magnet will surely trigger a small reed contact through 25mm of plastic, let alone a hall-effect sensor. \$\endgroup\$
    – Asmyldof
    Apr 21, 2016 at 17:43
  • \$\begingroup\$ Most magnets have enough iron in them that when immersed in a solution of salt water and urine and other stuff, they rust pretty rapidly. Also: slime grows on stuff, and then it doesn't move up and down as well as you thought it would. Inside a "holding tank" is a pretty darned ugly environment. Experience shows that almost nothing survives it. (If you've ever used a men's room with stainless dividers between urinals and see how rusted they can be, you'll have the general idea. And those often have janitorial staff cleaning them!) \$\endgroup\$
    – John
    Apr 21, 2016 at 19:39

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A few ideas come to mind:

  • Your tank is polyethylene. You should be able to sense liquid level through the wall of the tank using your capacitive sensing. If that doesn't work it won't work in a PVC pipe. Advantages: no wireless, no battery, no retrieving the unit out of the tank ...
  • I'd consider an ultrasonic level sensor. These are used in all sorts of industrial applications, quite reliable and can go in the top of the tank where it shouldn't get dirty. If it does get dirty you may have problems. Advantages: simple, analog output for level display, easily retrieved for adjustment, etc.

I've never considered the matter, but is there any risk of methane buildup making the airspace an explosive atmosphere? If so you need to take appropriate measures with any electrical devices inside the tank. From memory ATEX equipment is limited to 10 V or so at 8 mA. (Someone will jump in with the correct numbers.) This is low enough to prevent a spark in any circumstances.

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  • \$\begingroup\$ I'd be inclined to think of some solution involving a sealed bladder in the base of the tank, connected to the outside world by a tube through a sealed hole in the tank wall, with a pressure sensor on the end. The physics of that situation is very simple and it would be easy to calibrate. And there's no electronics anywhere near the poo. \$\endgroup\$
    – Ian Bland
    Apr 21, 2016 at 18:16
  • \$\begingroup\$ The external capacitive sensor seems like a good idea...but the external metal strips are in contact with the wooden cabinetry...and end up doing sensing on the moisture content of that wood, etc. As for ultrasonic sensors: the environment is just too nasty. They don't work THROUGH the tank (I tried) and anything that involves being inside is hopeless. As for methane: not really an issue. There's a fairly constant flow of fresh air pushed through the tank to aid decomposition. \$\endgroup\$
    – John
    Apr 21, 2016 at 19:43
  • \$\begingroup\$ @IanBland's idea is a good one, although it involves modifying the tank. If I can spinweld a new NPTF fitting onto the top of the tank, pointed inward, I could solvent weld a NPTM fitting on the end of my PVC and get wires out the top of the tank pretty cleanly. I'm not certain about spinwelding in place however, with ordinary tools...I'll have to look into that. Thanks for the simplifying idea! \$\endgroup\$
    – John
    Apr 22, 2016 at 10:54
  • \$\begingroup\$ My basic idea here was (as a former maintenance engineer with some experience dealing with wet systems) to get the data out of the tank by mechanical rather than electrical means. Once you've done that you can collect the data electronically if you wish. Hence, measure the height of the fluid column in the tank by the pressure at the base of the tank (Pascal and all that). I'd want to avoid wires in the tank, full stop. Steam engines measured boiler pressure without microcontrollers and sensors, remember :) \$\endgroup\$
    – Ian Bland
    Apr 23, 2016 at 1:45
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I wanted to put this in a comment, but apparently I don't have the cred to do so yet. Seems backwards I can answer but not just comment...

I know this takes all the engineering fun out of it, but have you considered using a clear polyethylene tank like this one so you can visually see the level? I'm not sure if that would work in your setup or if your tank has more to it than described.

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  • \$\begingroup\$ I considered the "sight glass" solution. But the tank's in an awkward location, and a clear tank isn't really an option without buying a new custom tank, and that's beyond my plans. I like your general out-of-the-box thinking though! \$\endgroup\$
    – John
    Apr 21, 2016 at 19:44
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You do not need to put the sensor or control unit into the tank, you can get away with just inserting an empty, sealed pipe of some kind. I just wrote in another answer:

If you use a differential pressure sensor you can build a solution that will be very robust because it has no moving parts, is not impacted by temperature/humidity, water cleanness is irrelevant (both with regards to salts and transparency), and no electrical sensors are in contact with the fluid (so if you want to meassure level in an acid tank, no problem).

and I think that solution is just as applicable in your case.

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