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Thread: Ir detection limit

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    New Member joshhan2000 is on a distinguished road joshhan2000 is on a distinguished road joshhan2000's Avatar
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    Ir detection limit

    We have all been hearing about IR detection of CO2. I can personal say I use the technique for many years in the lab. For the past 6 month I have be experimenting with the free radical separation of CO2. But there are limit to IR detection of CO2. One limit that has to be over come is at higher then one ATM gas is no longer following the ideal gas law. I know we have all read about PV=NRT but how many really under stand that this is called ideal gas law. It is name this because it is the way gas behaves in ideal condition. That means 1 ATM or lower and close to 20 deg C. After that we need to Van Der Waals equation which is:



    This it the way you must look at gas in the non-ideal state. Now that you understand a little how gases work. I can come back to my point as the gases get more pressurized there are more electro static forces. Hydrogen bonding, Van Der Waals forces, London dispersion forces. These force make the CO2 behave differently then when under ideal condition. So not only do you have to deal with the Moisture content of the systems you also have to deal with Quantum Mechanics of the system. I am sure there has been a lot of very ingenious ways of dealing with the moisture content I would just like to know how they are planning to deal with a gas that is at 10 Atm. The follow are helpful web site.


    http://www.licor.com/env/PDF_Files/co2_abs.pdf

    http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Thermal/vdWaalEquatOfState.html

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    Re: Ir detection limit

    Quote Originally Posted by joshhan2000
    But there are limit to IR detection of CO2. One limit that has to be over come is at higher then one ATM gas is no longer following the ideal gas law. I know we have all read about PV=NRT but how many really under stand that this is called ideal gas law. It is name this because it is the way gas behaves in ideal condition. That means 1 ATM or lower and close to 20 deg C. After that we need to Van Der Waals equation
    Are you sure? The VdW values for CO2 are, admittedly, quite different from O2/N2/He but up to about 10bar the deviation from ideal is trivial (<5%) in biological terms. I quite like IR systems as they tend to give a bulk measure and with a bit of curve fitting their idiosyncrasies can be ironed out as they are predictable.
    I think the big snag is, as you say, 100% condensing humidity.

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    Re: Ir detection limit

    I was not talking about the deviation of the Pressure. There really is not much difference in that at all. I was talking about how IR work in detection of CO2. IR can only detect molecules that have a dipole moment. You can figure out the module by using symmetry operations theory. But needless to say CO2 has a dipole moment. Were O2 N2 He and H2 do not. This is what makes IR good at detecting CO2. But when you increase the pressure by 10 bars you have also decreased the space the gas occupies by 10 times. This decrease in space leads to formation of CO2 crystal structures. Although these crystal are quick forming and quick to fall apart they are still there. So when you use the 4500 nm wave length of light to activate the CO2 you will get a falsely high reading form the crystals and will not achieve the 97% accurate that we all want. I not saying that IR can not be adjusted using things like Fourier Transform or by doing a calibration scale but in the lab we just simply cut the gas pressure down. I know that last year when using IR to measure CO emission of cars it took 25 know samples to calibrate the IR to read acutely. IR is not a linear scale that we are all use to on the O2 sensors. It is polymeric and will take a lot of work to calibrate the scale properly.
    Last edited by joshhan2000; 21st December 2005 at 12:12.

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    Re: Ir detection limit

    Quote Originally Posted by joshhan2000
    I was not talking about the deviation of the Pressure. There really is not much difference in that at all.
    Sorry. That's how I took VdW.
    But when you increase the pressure by 10 bars you have also decreased the space the gas occupies by 10 times. This decrease in space leads to formation of CO2 crystal structures. Although these crystal a quick forming and quick to fall apart the still are there.
    This is new to me. I clearly need to read up.
    So when you use the 4500 nm wave length of light to activate the CO2 you will get a falsely high reading form the crystals and will not achieve the 97% accurate that we all want. I not saying that IR can not be adjusted using things like Fourier Transform or by doing a calibration scale but in the lab we just simply cut the gas pressure down. I know that last year when using IR to measure CO emission of cars it took 25 know samples to calibrate the IR to read acutely. IR is not a linear scale that we are all use to on the O2 sensors. It is polymeric and will take a lot of work to calibrate the scale properly.
    I make a lot of my living doing measurement systems but mostly these days lumps of metal trailing nasty shock waves so I find this area interesting. Do I assume that the problem here is the CO2 is a sublimating solid? Does it even have a gaseous state at 10 bar? Sheesh. I think I'll go and sit in a darkened room and hold my head.


    Thanks for the reply

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    Re: Ir detection limit

    I was not saying that CO2 is sublimating that will not happen till about 1000 bar. I was saying that the CO2 will emit the IR radiation back out. This is why IR works. If the CO2 is 10 times closer then normal the gas will have a higher thermal conductivity.

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    Re: Ir detection limit

    Quote Originally Posted by joshhan2000
    I was not saying that CO2 is sublimating that will not happen till about 1000 bar. I was saying that the CO2 will emit the IR radiation back out. This is why IR works. If the CO2 is 10 times closer then normal the gas will have a higher thermal conductivity.
    ?? are we talking about the same PPCO2 or the same FCO2 at 1 and 10 bar

    ?? is the effect so big when we have the same PPCO2 at 1 and at 10 bar, because this is what we want to mesure

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    Re: Ir detection limit

    I think the point he's trying to make is that the IR method of determining CO2 levels is disrupted to some degree (e.g. linearity is impacted) by the total pressure of the gas in the system (that is, P(absolute))

    Determining absolute pressure though is easy. Indeed, you already have ambient pressure in most units, in that the processor has a depth sensor fitted to it in one form or another. This becomes a matter of computation and thus is solveable.

    The larger issue has always seemed to be humidity. There are ways around that (drying the sample is not terribly difficult to accomplish - but it takes a god-awful amount of power, which is at a premium in a RB.)

    Taken as a whole they present a problem that so far has resisted a compact, energy-efficient solution.

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    Re: Ir detection limit

    No the point I was trying to make that even if you solve the moisture problem you will still have to solve the high pressure problem. CO2 reacts under high pressure. If you do not believe me ask some one that has had the Argon bottle toped of with CO2. Under pressure the CO2 reaction to form Carbonic acid. So even if you get all the moisture out of the loop you will not be able to get an accurate reading. There is a difference between precision and accuracy.

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    Re: Ir detection limit

    Quote Originally Posted by joshhan2000
    CO2 reacts under high pressure. If you do not believe me ask some one that has had the Argon bottle toped of with CO2. Under pressure the CO2 reaction to form Carbonic acid.
    Argon? Carbonic Acid? Small sharks?

    I thought it was a reaction between CO2 and water that produced carbonic acid, hence acid rain. And that was all ambient pressure.

    Ho hum, something new every day.......

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    Re: Ir detection limit

    at higher perssure it will react faster.

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