Quote:
Originally Posted by orcafood
I'm taking the optical grade mirrors from the cameras, they should work well as the entry and exit mirrors they are probably close to a wavelength flat.
The concave collimating mirrors would be pricey your right. Perhaps less so out of old equipment though. Next question why even use the collminating mirrors? Why not just a tube with a silicone collimating lens (passes 1-10 um) at the end, no mirrors, just one lens which runs around $200. Of course the bandpass filter is still needed. Gas standards would be tricky to make. Could they not be made using a working CO2 meter? For example remove the CO2 from a gas space with soda lime and see how low it can go. That is the first point on the calibration. Add CO2 with a tank until it goes up 50 ppm on the working meter then set the point. 5-10 points would probably do?
Good information on the IR-lasers I was trying to find more about them, I was worried they would be cost prohibitive. (Off topic but holy cow I wish I had one of those Xe/Ar lasers, so much fun to use in the labs doing Raman. I never set it up, but I watched them do it for a little bit, takes foooreeevverrr)
A=-log(I/I0)=Elc brings me back, extinction coefficients are so useful. Absorbance and concentration seem linear in that equation or were you referring to how the linear range breaks down at the maximum linear response?
CO2 only has 4 vibration modes. This is its symmetric stretch, asymmetric stretch, and then the two scissors. Only 3 of these are IR active, the asymmetric stretch at 4.3 um and the degenerate scissors at 15 um which have a much weaker IR intensity. So it seems like the best choice is to focus on the 4.3 um area.
All that said I agree that a sensor would be much easier to setup and repeatable for all of us reefers. Please let me know if you can find a sensor with ppb resolution for a decent price, that would be a game changer! The $100 NDIRs seem to have a resolution around 20 ppm which is not good enough.
I couldn't find one so far that fit the bill, thus looking for alternatives.
Another thing I was reading about was that many of the HVAC NDIRs have a built in baseline correcting algorithm that assumes that the lowest value it can get to is around atmospheric CO2 levels. The detector drifts to lower CO2 concentrations so the baseline constantly gets changed under the assumption that indoor co2 will only be above atmospheric CO2 and never below. Not good when a headspace in contact with seawater may have less than atmospheric co2.
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To the mirrors there is not necessarily a need for them. There is no inherent reason the components can't be laid out linearly removing the need to reflect the light. This is good because standard mirrors used in NDIR instruments are gold plated.
You don't inherently need to collimate the beam. It helps greatly with optical through put and eliminates/minimizes sensitivities to surface characteristics of the optical bench though.
For standards, sure you could mix gas as a transfer standard from a working meter. But that means you have a meter already that has a greater accuracy than what you desire in your DIY version. You can also use precision syringes and a precision mixing volume to generate standards. This might actually be doable pretty cheaply as I think about.
As to absorption as a function of gas density. It may appear linear over a narrow range, but it is inherently not. Probably best to think about it on a narrow wavelength basis and build up from there. This is important for thinking about where a gas absorbs as well. Because while you are correct on major regions of absorption, CO2 has individual lines and small groups of lines peppered through out the IR region. Some more useful than others depending on your IR source, filters and detection limit.
In any case, absorption on a per wavelength bases (Av) can be expressed as:
Av = 1-e^(-kv*l*d)
Where l is the optical path length and d is absorber density. kv is the absorption coefficient at v, which is a function of line strength and halfwidth. Line strength saturates at some point.
I have lofty dreams of trying out some of the cheap NDIR sensors but haven't gotten to yet; too many more important projects to get through first. But when I do I will report what I find.
I had seen most had limited resolution and had hoped that there may be some way of working around that. Hopefully we shall see at some point.
I wasn't aware of the drift correction. The direction of the correction seems curious to me though. Do you know if they are using a reference gas cell? Diffusion into the reference cell over time would make sense for the need to correct upward. Or maybe its a source aging issue.