Oxygen 760nm spectroscopy - quick 'n' dirty

  • Context: Graduate 
  • Thread starter Thread starter JGarry
  • Start date Start date
  • Tags Tags
    Oxygen Spectroscopy
Click For Summary

Discussion Overview

The discussion revolves around the use of a 760nm laser for measuring molecular oxygen concentration in a gas, focusing on experimental techniques and challenges in spectroscopy. Participants explore various methods, potential absorption characteristics, and the feasibility of different approaches in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant inquires whether modulating the diode current and amplifying the photodiode output is sufficient for measuring oxygen concentration, suggesting a 1m path length might yield around 0.1% absorption.
  • Another participant questions the possibility of finding a wavelength with stronger absorption for oxygen, expressing skepticism about detecting absorptions below 1.0% and suggesting that higher absorption percentages would be easier to work with.
  • A participant notes that the 760nm wavelength is considered the best for oxygen detection, referencing correlation methods that require tuning the laser current for optimal absorption, but acknowledges the limitations of these methods.
  • One suggestion involves detecting weakly scattered light at a 90° angle to the main beam, proposing that this might simplify the measurement of small absorption differences.
  • Another participant points out that scattering effects would be similar for O2 and N2, complicating the detection of oxygen in a nitrogen buffer, and suggests using a calibrated photodiode or a reference path for measurement.
  • One alternative proposed is mixing a small sample of oxygen with hydrogen to measure water production, which could be detected using IR spectroscopy, assuming no water is present in the original sample.

Areas of Agreement / Disagreement

Participants express various viewpoints on the feasibility and methods for measuring oxygen concentration, with no consensus on the best approach or the effectiveness of the proposed techniques. Multiple competing ideas and uncertainties remain throughout the discussion.

Contextual Notes

Participants acknowledge the challenges of detecting low absorption levels and the limitations of the chosen wavelength, as well as the potential influence of scattering on measurements. There are unresolved questions regarding the practicality of the proposed methods and the assumptions underlying them.

JGarry
'Morning all,

I wish to use a 760nm laser to measure the concentration of molecular oxygen in a gas. I've a laser (multi-mode, not TDLAS:QLD-760-10S), a photodiode (Si), and I've built an oven to hold the laser diode at a constant temperature to avoid mode-hopping.

Is it really as 'simple' as modulating the diode current at some frequency, amplifying the PD output, and then deconvolving the sampled output with the modulating frequency?

At RTP, with 20% O2, I've a gut feeling that 1m path length ought to give something like 0.1% absorption of the total laser power.

Any comments about technique?
 

Attachments

  • Optical lash up.png
    Optical lash up.png
    5.5 KB · Views: 685
Science news on Phys.org
Just a quick comment or two=I have done a fair amount of experimental spectroscopy and radiometry, but haven't ever attempted something quite like this. Would it be possible to find a wavelength where oxygen has a resonant absorption, or at least a stronger absorption? I think it's going to be difficult to detect and quantify absorptions that are less than 1.0%. It would be much easier to work with numbers like 25.0 % absorption or 30.0 % absorption.
 
Regrettably, the thicket of (narrow and faint) lines around 760nm appears to be the canonically 'best' choice for molecular O2 detection. Correlation methods (Luo et al., Oxygen measurement by multimode diode lasers employing gas correlation spectroscopy, 2009) appear to work in a non-realtime fashion for path lengths as short as 3cm - but they use a ramped laser current to always tune for optimal absorption.
I think.

Thanks for the interest though, and 'yeah' - oh that O2 had deeper bands!
 
  • Like
Likes   Reactions: Charles Link
JGarry said:
Regrettably, the thicket of (narrow and faint) lines around 760nm appears to be the canonically 'best' choice for molecular O2 detection. Correlation methods (Luo et al., Oxygen measurement by multimode diode lasers employing gas correlation spectroscopy, 2009) appear to work in a non-realtime fashion for path lengths as short as 3cm - but they use a ramped laser current to always tune for optimal absorption.
I think.

Thanks for the interest though, and 'yeah' - oh that O2 had deeper bands!
Just an idea: I don't know if it's practical or not: Is there a wavelength that you could pick up through scattering? Detecting a weakly scattered component, e.g. ## 90^o ## to the main beam, especially with a lock-in amplifier and/or boxcar integrator, might be a whole lot easier than resolving differences from 100% that are quite minute. You might be able to accurately quantify even a very small scattered component. ## \\ ## Editing: An additional idea would be to bring the main beam to a focus inside the gas sample and collect the scattered light from that focused point with a lens.
 
Last edited:
:/ Any scattering (say, Rayleigh) will be pretty much identical for O2 as it is for N2 (we're looking at O2 in a N2 buffer) - given the similar sizes of the molecules.

So basically, (I think) it comes down to either having a temperature-held laser diode (and a calibrated photodiode sensing a fraction of the beam) to establish beam power, or building a reference path (beamsplitter, mirrors, etc.) that passes through either pure O2 or pure N2.

<tosses coin>

Again, thanks for the commentary - glad that this tickles some folks' neurons too!
 
  • Like
Likes   Reactions: Charles Link
One other alternative that might prove easier would be to take a small sample and mix with 100% hydrogen (It might require a slight heating, and it will give a slight "pop"), and see how much water you get, which could be readily measured even with IR spectroscopic techniques=assuming there is no water in the original sample.
 
:D
Lasers *and* flames! What can go wrong?