How an infrared gas sensor works

Originally Posted By: phinsperger
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http://www.new-technologies.org/ECT/Other/InfraredGasSensor.htm


Originally Posted By: Jay Moge
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i feel smarter, thanx icon_cool.gif


Originally Posted By: Caoimh?n P. Connell
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Hello Paul:


Refinements in IR optics notwithstanding, IRs are still considered to be qualitative and not quantitative or even semiquantitative. The reason is that IR spectra are fraught with overlapping signatures and the resulting ID and measurement by the instrument consists of ?good guesses? and in many cases, even ?damn good guesses,? but they remain guesses nonetheless.

Field IR specs are great tools when used within the limits for which they were designed, but could be dangerous if used with overconfidence and if used with disregard for their limitations.

Personally, I like field IRs for many applications, and I've use them extensively? for example, monitoring the air while exhuming buried gasoline tanks. In an application such as this, one already knows the general profile of the source hydrocarbons and uncertainties about actual identifications are not particularly important. But one would not be particularly wise to use an IR spec to attempt to definitively characterize a completely unknown gaseous component. For that, one would still need to perform proper air monitoring.

Just a thought?

Cheers,

Caoimh?n P. Connell
Forensic Industrial Hygienist

www.forensic-applications.com

(The opinions expressed here are exclusively my personal opinions and do not necessarily reflect my professional opinion, opinion of my employer, agency, peers, or professional affiliates. The above post is for information only and does not reflect professional advice and is not intended to supercede the professional advice of others.)

AMDG



Originally Posted By: phinsperger
This post was automatically imported from our archived forum.



Caoimh?n P. Connell wrote:
Hello Paul:

Refinements in IR optics notwithstanding, IRs are still considered to be qualitative and not quantitative or even semiquantitative. The reason is that IR spectra are fraught with overlapping signatures and the resulting ID and measurement by the instrument consists of ?good guesses? and in many cases, even ?damn good guesses,? but they remain guesses nonetheless.

Field IR specs are great tools when used within the limits for which they were designed, but could be dangerous if used with overconfidence and if used with disregard for their limitations.

Personally, I like field IRs for many applications, and I've use them extensively? for example, monitoring the air while exhuming buried gasoline tanks. In an application such as this, one already knows the general profile of the source hydrocarbons and uncertainties about actual identifications are not particularly important. But one would not be particularly wise to use an IR spec to attempt to definitively characterize a completely unknown gaseous component. For that, one would still need to perform proper air monitoring.

Just a thought?

Cheers,

Caoimh?n P. Connell
Forensic Industrial Hygienist

www.forensic-applications.com

(The opinions expressed here are exclusively my personal opinions and do not necessarily reflect my professional opinion, opinion of my employer, agency, peers, or professional affiliates. The above post is for information only and does not reflect professional advice and is not intended to supercede the professional advice of others.)

AMDG


Caoimh?n,

I am barely on the limit of being able to understand what you wrote. Guess that keeps the mind sharp ![icon_biggrin.gif](upload://iKNGSw3qcRIEmXySa8gItY6Gczg.gif)

So are you saying that an acoustic wave sensor is better than infrared sensor for our line of work or is there something else we should be looking at?


Originally Posted By: Caoimh?n P. Connell
This post was automatically imported from our archived forum.



Hello Paul ?


The SAW sensors or IR sensors or MOS sensors or PID sensors or FID sensors may be very well suited for your work. Your data quality needs will drive the appropriate type of sensor, and I don?t know how you intend to use your meter or what your data quality objectives are.

My point was simply that just because the IR instrument tells you that you have hexane or toluene or whatever in the air at a particular concentration, it ain?t necessarily so ? it might be so, or it might be a very different gas at a very different concentration. All of the direct reading instruments have their limitations, not just the IR.

For example, I have a pretty neat little MOS toluene sensor that I carry as a cop on meth-lab raids that reads ppm toluene. The reality is that if the contaminant in the air is benzene or xylenes or monocyclic terpenes or even propane or an hundred (or thousand) other organic compounds, the instrument will still read ?XXX ppm toluene? because in truth, it doesn?t know the difference. (Of course, in meth-labs, my main concern is airborne lead, if you get my drift?)

The instrument you select should be capable of meeting your needs. If you, as an HI, absolutely need to know the gas is xylene instead of toluene, then you probably don?t want an IR detector; if you are actually interested in combustion/explosive levels of gases, and don?t really care about the chemical structure, then maybe an IR or better yet a simple pellistor meter (CGA) is what you want (unless of course, you are in the presence of regular gasoline, etc). For carbon monoxide, I would recommend either a pellistor or chemical cell detector, for combustible gases, I would recommend an O2/pellistor meter (remember, you don't know what the explosive level is if you don't know the oxygen level, regardless of the meter reading).

The key is to be aware of the limitations of your instruments.

Cheers,
Caoimh?n P. Connell
Forensic Industrial Hygienist

www.forensic-applications.com

(The opinions expressed here are exclusively my personal opinions and do not necessarily reflect my professional opinion, opinion of my employer, agency, peers, or professional affiliates. The above post is for information only and does not reflect professional advice and is not intended to supercede the professional advice of others.)

AMDG



Originally Posted By: kwilliams
This post was automatically imported from our archived forum.



Caoimh?n P. Connell wrote:


I carry as a cop on meth-lab raids (Of course, in meth-labs, my main concern is airborne lead, if you get my drift?)



Mr Connell, You go with the police on the raids ? ![icon_eek.gif](upload://yuxgmvDDEGIQPAyP9sRnK0D0CCY.gif)


Originally Posted By: lkage
This post was automatically imported from our archived forum.



Jay Moge wrote:
i feel smarter, thanx ![icon_cool.gif](upload://oPnLkqdJc33Dyf2uA3TQwRkfhwd.gif)


Still feel smarter, Jay?

Caoimh?n, your posts are a pleasure to read...I still feel dumber than a box of rocks sometimes but I enjoy attempting to understand. And sometimes I do.

Thanks for sharing your expertise...like Paul says it keeps the mind sharp.


Originally Posted By: Jay Moge
This post was automatically imported from our archived forum.



Larry.


to be totaly honest, if someones post is longer than my screen, has 12 letter words and i get lost in the first sentence, i usualy skip it and read the next one. that way i can still feel smart, even if i’m fooling myself, i damn good at it. icon_cool.gif


Originally Posted By: lkage
This post was automatically imported from our archived forum.



Jay Moge wrote:
Larry.
to be totaly honest, if someones post is longer than my screen, has 12 letter words and i get lost in the first sentence, i usualy skip it and read the next one. that way i can still feel smart, even if i'm fooling myself, i damn good at it. ![icon_cool.gif](upload://oPnLkqdJc33Dyf2uA3TQwRkfhwd.gif)




...been there, too.


Originally Posted By: rcooke
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Jay Moge wrote:
Larry.
to be totaly honest, if someones post is longer than my screen, has 12 letter words and i get lost in the first sentence, i usualy skip it and read the next one. that way i can still feel smart, even if i'm fooling myself, i damn good at it. ![icon_cool.gif](upload://oPnLkqdJc33Dyf2uA3TQwRkfhwd.gif)

Skipping these posts could make you miss information that might keep you out of court.
Roy sr


Originally Posted By: Caoimh?n P. Connell
This post was automatically imported from our archived forum.



kwilliams wrote:
Caoimh?n P. Connell wrote:


I carry as a cop on meth-lab raids (Of course, in meth-labs, my main concern is airborne lead, if you get my drift?)



Mr Connell, You go with the police on the raids ? ![icon_eek.gif](upload://yuxgmvDDEGIQPAyP9sRnK0D0CCY.gif)


Good Morning, Mr. Williams -

I AM the police! Although I am a practising forensic industrial hygienist, I am also a law enforcement officer. Sometimes these two professions and activities coincide (for example during meth-lab investigations). But when I'm not a cop, I also perform private industrial hygiene consultation. Thus the disclaimer at the bottom of my posts.

Cheers!
Caoimh?n

www.forensic-applications.com

(The opinions expressed here are exclusively my personal opinions and do not necessarily reflect my professional opinion, opinion of my employer, agency, peers, or professional affiliates. The above post is for information only and does not reflect professional advice and is not intended to supercede the professional advice of others.)

AMDG