Hi Andy,

I have meditated over your problem and have come to an interesting
solution (patch) to your current system. Maybe. It relies on using a
volumetric method on the water that fills the tank.

Briefly, if your tank fills and empties through a piece of transparent
tubing and you have a small leak bubbling air into this tube all the time
(syringe needle + needle valve fed from your emergency recovery air or CO2
tank - the air consumption will be very low), then you can measure the
speed of the bubbles (regardless of their size and frequency) using two
photocell + LED assemblies that look 'through' the tube. You probably want
a reflective arrangement for bubbles (LED and detector at 90 or less
degrees). On top of it, this is a PIC project ;-). Of course I assume that
water is incompressible, that the tube diameter is known and that the
amount of air bubbled is small wrt. the water volume in the volume between
the photocells. You can probably run some tests using an aquarium air pump
and some transparent tubing for same.

You can further refine this by using a central electrovalve to shut off
bubble air to all tank bubblers when not required. This way you will not
run out of blow air by mistake, and there will be no slow draining by the
air bubbling into the tanks.

The system can be implemented in a few hours using 1/2" aquarium tubing
etc, and will take some work to make it practicable on the software side
(but the PIClist is here) ;-).

I hope that this helps,

Peter

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