Electronic – Low power device in sealed unit — design help

low-powersensor

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?

Best Answer

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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