CO2 test method development

Method 1 is something I've seen you and VaughnH (Hoppy for anyone from TPT) talk about. I'd love to give one of these a try eventually. If they could be manufactured to fit with a few common brands of pH meter, I'm sure they'd change the hobby for the better. Even +/- 2.5ppm would be significantly more meaningful to the average hobbyist than playing with a drop checker, especially given the increased responsiveness.

Your second design seems like it'd be a bit more expensive, but if it handles low range better then I can see the appeal. What sort of response times would it give compared to the membrane? Could it be moved around within the tank as easily?

-Philosophos

 

Method 2 is a great idea. the IR analyizer might be DIYable with an IR source and a detector. Not sure how to control the IR source wavelength though without a prism.

 

Hi,

Are we talking something like this Extech Desktop Indoor Air Quality Carbon Dioxide (CO2) Monitor - Mfg# CO200 ?

Biollante

 

First, great project

I also find method 1 technically easier and very interesting. I see some major issues with it: it will need more time to react than method 2 which will lead also to fluctuations due to CO2 going on/off. We know it can get very low in 30mn in well aerated aquariums while it could take 2h to show in the KH4 solution. It will cause 1h30 gap with low CO2 before probe reacts. Once the probe reacts, CO2 will be injected. It will take 30mn to 2h to diffuse well in the aquarium, thus leading to +2h low CO2 fluctuation periods. Also, the most sensitive PH regulators have a +/-0.05 PH trigger before switching the control on or off. With a KH4 solution this leads to an adjustment of +/-6ppm CO2 at the best. Finally, the biggest problem will be deviation and the need for a regulat calibration of those PH electrodes

For method 2, we will need a calibration to adjust readings between the CO2 concentration in the bell and the CO2 concentration in the aquarium. Again, using a mist effect can affect drastically this correlation, also the flow near the bell and also the concentration of other gases, mainly oxygen. I'm really not sure that a correlation can be found with method 2 to the 1ppm precision

 

Co2 & N2

Hi,

If I understand method 2, great precision should be possible by calibrating against a known mix of nitrogen and carbon dioxide.

Not being very smart, I know I tend to irritate, that is not my intention.

Biollante

 

It might be possible to just pass the IR through a column of water itself, provided that the detector is sensitive enough and you can tune the source down to a very specific wavelength. I'm not sure if 30ppm is a high enough concentration to detect though, maybe a long path length can increase the sensitivity. I think a detecting CO2 concentratinos in a gas bubble above water will be pretty tricky since CO2 solubility and the vap pressure of water both vary with temperature.

 

Yeah that's why i don't like thinking in ppm, I prefer molarity, more useful for calculations too. 30ppm of co2 in water works out to be about 0.00068M? Sounds pretty detectable. I couldn't find any databases for absorption coeff. of CO2 but i found this Determination of the molar absorption coefficient for the infrared absorption band of CO2 in rhyolitic glasses -- Behrens et al. 89 (23): 301 -- American Mineralogist
CO2 at 2349 cm-1 is 1214, so at 30ppm with a path length of 1cm, you'd get absorbace of 0.82. nice number within the detectable range. I think 2349 is pretty unique to CO2.

 

Ah, then that eliminates the need for the air bubble
wish i had the money to try these things out. Good luck with the project, subscribing and keenly awaiting the results.

 

Hi Tom,

I may not have fully grabbed onto the concept of method 1, but I thought that is basically what most of the cheap CO2 electrodes do. I understood them to basically be pH electrodes within a buffer sollution separated from the sample by a selective membrane. Electrodes direct has some designed to be built in line
pH Electrodes Products. I am not sure about the accuracy though.

Bill

 

DIY Sub US$ 350 ? Maybe

Hi Bill,

I think the object is to do what the electrodes and so forth that cost US$ 1,000 plus for US$ 200-400 instead.

I do not have the brand you reference, but they seem accurate, as a result of questions (objections?) Tom Barr raised, I had a fellow with appropriate letters behind his name verify my equipment, storage, processes and calibration procedures and tells me (certifies) accuracy for my purposes as a minimum of 0.08 ppm and technically I could reach 0.02 ppm which is significantly better than I had anticipated frankly +- 0.1 ppm makes me happy, for hobbyist purposes I figure +- 1 ppm is good.

If I understand it is Method 2 that really seems to have the greatest hope for the best accuracy at a low cost, I think around US$ 300-350.

It is possible we could pick up the Original Equipment Manufactures (OEM) NDIR (non-dispersive infrared) CO2 sensor. We might be able to knock the price down under US$ 200.

I am seriously unsure of the calibration or reference samples, but if I understand the process it is not terribly difficult to get a good N2 CO2 reference sample.

I am giving it a shot with a Extech Desktop Indoor Air Quality Carbon Dioxide (CO2) Monitor - Mfg# CO200. It is going to take some serious messing with, but I figure I have equipment to tell me how accurate I can be with this thing.

Biollante