> I have just tried out something: A dyi oxygen sensor. It's a hearing aid
> battery. It uses air for operation. The air is admitted through small
> holes on the back of the button cell battery. The internal resistance of
> the cell changes with oxygen content in the air. Thus by loading the cell
> with a constant current load, the i.r. can be measured indirectly.

 I tried this very briefly quite some years ago and results were not
marvellous.
Maybe I missed some major point (such as maybe critical load levels or
whatever).
Reading the rebreather discussion below it seems to depend on many factors
including brand//model, load, temperature, more ...
Could be worth revisiting.
Was interesting enough for a quick Google:

Varta Zinc-Air O2 sensor that looks suspiciously like a ZA button cell


http://www.varta-microbattery.com/en/mb_data/documents/data_sheets/DS4650.PDF

Excellent and extended multi-person discussion - from the rebreather diving
people.
Well worth reading as they discuss many factors.
"William M. Smithers" did some good looking tests which suggested that they
have too many problems.
The discussion ends in name calling.

        http://www.metacut.com/rebreathers/Nuggets/ZincAir.htm

Here's a simple but interesting experiment.
They use a very heavy load and accept a short sensor life.
Note initial stabilisation of cell under load.

        http://www.customsensorsolutions.com/zn-air.htm

Diving rebreather links page - just to see what other classes of madmen
there are in the world :-)

        http://www.portup.com/~dfount/drflinks.htm



            RM

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