And may we ask WHY you want this? I'm trying to see the application.

---
James.
 
 

> -----Original Message-----
> From: piclist-bounces@MIT.EDU 
> [mailto:piclist-bounces@MIT.EDU] On Behalf Of Russell McMahon
> Sent: 2006 Jul 03, Mon 05:59
> To: PIC List
> Subject: [EE] Caphograph design
> 
> I wish to build a Capnograph.
> Don't know what that is?
> I didn't either until yesterday - even though I wanted one.
> It's an instrument to measure exhaled CO2 content.
> Exhaled CO2 content is typically in the range 2% - 10.% Y.CO2.M.V.
> 
> Real ones cost real money.
> Most straight forwards method is measuring absorption at about 4.2 -
> 4.3 nm where CO2 has an absorption peak.
> This usually involves producing light which includes this 
> wavelength and then filtering it with a suitable filter. 
> These are commonly enough avail;able due to their use in 
> Automotive gas analysers, and, not surprisingly, in 
> Capnographs. Such filters are nastily  expensive - typically 
> hundreds of dollars. This is a 'discretionary'
> project and while such a cost is bearable it would be much 
> better if a cheaper solution was available.
> 
> One possible solution is the use of a diffraction grating or 
> possibly a prism.
> 
> Other people may have alternative innovative suggestions for 
> addressing the 4.2 nm line approach OR may have alternative 
> methods. I want a potentially "breath by breath" response and 
> ideally electronic logging so chemical means are unlikely to 
> be useful.
> 
> I can imagine a "colour wheel" approach where CO2 is absorbed 
> on a substrate causing eg colour change, read by a sensor and 
> then regenerated by eg heating, all on a rotating wheel, 
> might provide a viable real time solution. How one achieves 
> such a colour or other change I don't yet know.
> 
> Pushing that a little further, exhaled air could be bubbled 
> through a solution causing transparency change and light 
> occlusion and the solution subsequently regenerated. Much 
> messier than I would like, but such "idea starters" may lead 
> to other things.
> 
> FWIW the End Tidal value (the CO2 level at the end of 
> exhalation)  is an amazingly sensitive and valuable indicator 
> for many medical situations. ET value gives far faster 
> response than Pulse Oximiter blood oxygen readings to 
> oxygenation changes (one breath versus a minute plus), is a 
> sadly accurate indicator of resuscitation prospects and a 
> good indicator of whether resuscitation is working (reading 
> at 20 minutes resus gives near certain indication of 
> death/life prospects). It's invaluable as an intubation 
> indicator and monitor for patients during critical transfers.
> 
> 
> 
> 
>         Russell McMahon
> 
> 
> 
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