How about IR gas absorption sensors? The thermopile devices (from 
Perkin-Elmer) are a little of the costly side, but I've been able to 
design some very accurate sensors using them.

Douglas Wood


----- Original Message -----
From: Russell McMahon <apptech@paradise.net.nz>
Date: Monday, July 3, 2006 9:32 pm
Subject: Re: [EE] Caphograph design
To: "Microcontroller discussion list - Public." <piclist@mit.edu>

> > 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
> 
> 
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