Bob wrote a few things I would like to comment on: >Peter Neubert <neubert@BOW.INTNET.MU> wrote: >>CO2 has this strange property which makes it absorb infrared light at a >>ceratin wavelength (I think 490 nm - check it out, I am really Really not > > The wave length in question is at least ten times longer than this, > more like 5000 nm. As I said - cant remember - however look a bit further down in this text > >>1. finding a LED which emits in that spectrum > Normally an incandescent lamp is used. For 5000 nm yes maybe :-) > >>2. Finding a Ligth sensitive diode which only responds in this spectrum - I > > There are photo detectors and filters made for this use but they are > expensive and temperature sensitive. Either the sensor must be in a > temperature controlled environment (Peltier coolers are one common > technique) or temperature compensation must be incorporated in the > softare. Yes you are rigth - however they are availble from specialist companies and the prices are not as high as you think - actually the price of an optical filter is higher than a specialised photo detector > Typically, the light beam is chopped by a rotating disk, which tends > to raise the complexity out of the range of a hobby type application. > > > Quantifying CO2 is a non-trivial problem. > >>3. LED driving circuit - normally the LED's of this kind must be pulsed in > > Forget LEDs. They are not available in the wavelengths that are > absorbed by CO2. Well this is where you are wrong - as I said i did this once as a project at Loughborough University - and they are available >>5. CO2 is - what many people dont know - very dangerous - and a >>concentration of just 10% will cause unconsciousness - remember this when > > Are you confusing this with carbon MONoxide? Your expired breath > typically contains 5% CO2. Nope ! as you write yourself Your expired breath typically contains 5% CO2 - then you would agree that it would be a bit difficult for the body to get rid of its own surplus CO2 if the surrounding concentrations are high :-) causing hyperventilation, dizziness and eventually you would be lying on the floor -------------------------- Please read the following cut/paste from http://sablesys.com/ca1.html Please note i dont work for the company but that text confirms part of my original text Regards Peter Neubert ----------------------------------------- To understand the CA-1's unique features, you need to know a bit about how carbon dioxide analyzers work. Start geekspeak. Carbon dioxide is a big gas molecule reminiscent of Mickey Mouse's head, with a central carbon to which is attached two oxygens at the ends of springy double bonds. Shake it at the right frequency and it resonates. The "right frequency" is in the infrared at a wavelength of 4.26 microns. Shine light at this wavelength through a sample cell and then blow carbon dioxide through it. The intensity of 4.265 infrared light transmitted through the cell will drop because it is absorbed and re-emitted at longer wavelengths (this is why carbon dioxide contributes to the greenhouse effect). Now, detecting infrared light is non-trivial because at room temperature everything glows in the infrared including the sample cell, the detector's housing and the detector itself. The infrared source must therefore be modulated to distinguish it from the background. The detector can be made to pay attention to the modulated signal alone and, with the help of a lot of electronic processing, give information on carbon dioxide concentration. In most carbon dioxide analyzers this infrared modulation is achieved by a chopper - a rotating disk with cut-outs. It mechanically interrupts the infrared beam at a known rate. The disk's motors consume power, are unreliable and are the primary cause of failures in carbon dioxide analyzers. End of geekspeak. The Sable Systems CA-1 is an infrared-based carbon dioxide analyzer that has no moving parts. We have completely eliminated the mechanical chopper by directly modulating the infrared light source itself. This greatly improves the CA-1's ruggedness and reliability, and simultaneously reduces its power consumption still further because no motor is required and the infrared source is only "on" for half of the time!