Living in the outback ...=20

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.

--=20
James Cameron
http://quozl.netrek.org/
--=20
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