Converting CO2 ppm to mmHg

newbie1234

I am trying to make a carbon dioxide (CO2) intubation device. I understand how to convert the voltage that my CO2 sensor reads into a corresponding CO2 ppm. However, all of the documents that I have read on CO2 intubation refer to CO2 measurements in terms of mmHg. How are ppm and mmHg related? How do I convert from one to the other? That is, how do I get my sensor to output mmHg instead of ppm?

Bean

I think mmHg is a "pressure" measurement and ppm is a "concentration" measurement. I don't see the two can even be related.
It would be like trying to convert percentage to PSI. I don't think the two are related at all unless I'm missing something important...

Bean

Dave Hein

CO2 contributes a "partial pressure" to the overall atmospheric pressure. The ratio of the partial pressure to the overall pressure is identical to the ratio of the number of molecules. So the answer is (PPM / 1,000,000) * 1 ATM. Since this is in relation to a contest, I'll leave it up to the original poster Duane to figure out how to convert 1 ATM to mmHg.

Duane Degn

Wouldn't it be:

mmHg(CO2) = mmHg(system) * (reading from sensor in ppm) / 1,000,000

I'm trying to remember my gas laws (which I used to teach). I think each molecule of gas contributes equally to the overall pressure (within reason) so there's a 1:1 relationship between PPM and pressure.

So if you had standard pressure then the mmHg of "system" would be 760mmHg.

So in this case:

mmHg(CO2) = (reading from sensor in ppm) * 0.000760

Edit: The above will only work if the parts per million is a mole fraction (same as volume fraction in many cases). If the measurement is ppm mass fraction, then you'd need and additional conversion factor.

Rforbes

I'm thinking Dalton could come to the rescue on this one. He's good buddies with Charles and Boyle.

If a volume of gas were to consist of 98 % helium and 2% CO2, the amount of pressure the C02 contributes to the "system" pressure of the entire volume within its container miiiiiiiiiight be a little different than, say, a mixture of equal parts helium and CO2 of the the same volume and same "system" pressure.

Duane Degn

I don't see a CO2 sensor in Parallax's store. I was trying to find out if the ppm was mass fraction or mole fraction.

The equation I gave above would apply to a mole fraction ppm.

Invent-O-Doc

They don't have one - there is a carbon monoxide sensor. I've seen compatable sensors for carbon dioxide on ebay and futurlec.com

NWCCTV

There are a couple that I am sure could be hacked on EBay: http://www.ebay.com/itm/MG811-MG-811-CO2-Carbon-Dioxide-Sensor-Module-for-Arduino-and-other-MCUs-/120804868793?pt=LH_DefaultDomain_0&hash=item1c208802b9 http://www.ebay.com/itm/1-MG811-CO2-dioxide-Sensor-Module-New-/110888030682?pt=US_Amplifier_Parts_Components&hash=item19d17111da I am sure there are more but these were the first couple that I found.