Capacitive sounds like the ticket to me

http://www.analog.com/library/analogdialogue/archives/46-05/cdcs_in_healthc=
are.html

Eoin


On 2016-08-26 6:34 AM, James Cameron wrote:
> Living in the outback ...
>
> Looking for a way to estimate liquid levels on the other side of 4mm
> plastic; a potentially flammable methane-air atmosphere, without
> necessarily cutting holes in the plastic.
>
> Composting toilet collection bin has a coarse filter 50mm from the
> void at the base, and a liquid drain.  A DC fan pulls air through the
> bin contents, through the void, into a chimney.
>
> Composting works best when the void has less than 20mm of liquid.
>
> Sometimes the drain becomes blocked.
>
> Liquid levels increase slowly, blocking the airflow, until the gas
> generation of the bin contents exceeds the airflow, then the residents
> really notice.  Pong.
>
> How to measure the liquid level?
>
> Float switches would become fouled.
>
> IR reflectance sensors would be great, but expensive, and if covered
> by muck wouldn't reset until the bin was serviced.
>
> Stainless steel bolts through the plastic might work, but there's a
> conductive path through condensate on the inside surface.
>
> "Can't you just point an instrument at the tank from the outside and
> have it tell you the liquid level?"  ;-)  Magic is wanted.
>

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