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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