> What about mass / specific gravity / density? Sure, breath is a 
> messy soup with various gasses and moisture and such, but most of 
> those things seem considerably easier to measure.

Gotta admit I thought of that and then left it out of the post to 
think about as it seemed too good ;-).

Mass is low but doable.
Water vapour needs removing.
Air mass is 1.3 grams per litre and this increases by about 0.5% per 
1% of CO2 or about 50 micrograms per litre per percent of CO2. A 
balance system would be easy enough to trial.Direct strain gauge 
weighing or (name escapes me) electronic balancing against a voice 
coil or ...

Also moment of inertia type approach or ...



        RM


-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist