Infrared CO2 sensors for capnography

Capnography means measuring CO2 concnetrations in gas streams.

Here is a particularly crude IR spectrum of CO2

Here is a detailed picture of the P and the R branches

Tony Williams reported a particularly sad tale of an LED which emitted bang on one of these narrow absorbtion lines all through lab testing, but which shifted a bit when used on the colder shop floor so that it missed the line ... he went over to a broader-band source that always sampled a fair number of lines (and would have given a non-Beers Law calibration curve).

How hard would it have been to keep it at constant temperature, e.g. with a Peltier thingy?

Paul Burke

You'd have to ask Tony. These days it would be pretty easy, though not all that cheap. My 1996 Measurement Science and Technology paper talks about stabilising a block of glass to +/-0.001C and my colleagues subsequently simplified the circuit to stabilise a laser diode in the same machine to +/-0.01C, which kept the beam pointed in the same direcion to better than a second of arc, which was all they needed.

Sorry, I've only just seen this thread............

I ended up with a small aluminium block carrying the LED, temperature sensor, and heater, running at about 35C. It was good for about 0.05C, but had a very small low frequency hunt... and those damned IR absorption lines in water can be very steep. The combination effectively thickened the line on the chart recorder and it was a question of tuning each LED temperature to move it away from the worst absorption frequencies.