Thanks for the replies on liquid level sensing.

Another aspect of the problem is detecting fan failure, which can be
done by measuring current or a rotation sensor, but doesn't actually
tell if the fan is moving air.  Blocked intake makes fan spin faster;
it is not moving as much mass, but the same could happen if the
chimney is working well from the heat of the day.

I'm interested in comments on anemometers, air flow sensors, mass flow
sensors, or pressure sensors relative to ambient.

The space to work within is a few yards of 3" diameter plastic pipe.

I'm fine with drilling it, because if I change my mind I can plug it
with kapton tape or silicone caulk.

The environment can be excessively dirty or wet.  Occasionally an
insect gets into the system, lays eggs, and a few days later a swarm
will try to leave.  Occasionally the dew point is just right for
condensation in the pipe.

The fan (at the moment) is a 90mm 12V DC rated at 59 CFM, 3.8 watts,
0.32 amps.  The corners get cut off and foam padding added to fit it
into 100mm nominal (4") plumbing pipe.  Wish for circular fans.  ;-)

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