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Good or Gimmick - CO2 Digital Analyzer

ukco2guy

Member
Joined
9 May 2010
Messages
221
Location
Swindon UK
Hi,
Non-dispersive InfraRed (NDIR) sensing technology is a valid technology for gas detection. Basically, gases have a certain absorption characteristic over the Infrared range of wavelengths. Visible and photosynthetic wavelengths are from about 400 nanometers to about 700 nanometers. Infrared starts at 700 nanometers and extends to about 10,000 nanometers. So each gas absorbs these various wavelengths in different amounts which, if measured accurately generates a collection of absorption values which, in effect is sort of "fingerprint" of that gas. This set of measurements is referred to as an "absorption spectrum" and the science of fingerprinting is called "gas spectroscopy" or "gas chromatography". The magnitude of the absorption is proportional to the concentration of the gas. The energy of the IR light that gets absorbed actually results in various types of vibration of the molecular bonds. The study of Infrared Spectroscopy is all about what kind of weird molecular vibration patterns occur (stretching, bending, symmetric and asymmetric atomic movements) when each frequency of IR radiation contacts the molecule in question.

Here is a typical fingerprint of CO2 absorption as a gas dissolved in air.
co2_ir.gif


So the unit in your link would be calibrated to look for absorption of the individual frequencies shown along the horizontal axis in the ratios as seen by the height of each peak. That will identify the CO2 in the sample. The actual numerical values will determine how much CO2 is in the sample. Knowing this profile, one only needs to pass those wavelengths where you see a peak. So you have a choice: take your IR light source and pre-filter it to select only those few wavelengths and then pass those wavelengths through the gas sample, or pass the full spectrum of IR light through the sample and filter afterward. To pre-filter the light, you have to break it up with something like a prism. This is called "Dispersive" method.

Of course, this requires a gas sample, but your tank is not a gas sample, therefore this would not be useful. It is more appropriate for use in a greenhouse, not a tank. You would need a unit that measure CO2 in water. When CO2 is dissolved in water the absorption spectrum changes drastically because the water also absorbs the IR. Not only that, but the CO2 concentration in our water is an order of magnitude lower than in air. That means you need a meter that is 10 times more accurate. That means you'd be looking at units costing nearer to £2,000, not £200, and which probably use a completely different technology, something like OxyGuard CO2 I think the probe detect CO2 diffusion rates through a permeable membrane by partial pressure measurement.

Hope this helps. :geek:

Cheers,
 
You know Clive i actually followed that from start to finish and i got most of it :) What i don`t get is how on earth i would convince the wife to let me part with £2k to measure some gas in my Aquarium, for now the DC will have to do ;) .

As Chris said, legend!

Cheers,
 
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