I need to flux 250 ppm of CO2 at 1000 ml/min.

I have a gas system with a tank containing CO2 (CO2 at 5000 ppmvol, Oxygen 21%vol and Nitrogen the rest), connected with a mass flow controller (MFC1, flux max at 150 ml/min) and I have a tank of dry synthetic air (O and N), connected with a second mass flow controller (MFC2) that can be fluxed at a max of 1000 ml/min. Since I don’t have the possibility to measure the ppm of the CO2 after the dilution, I need to know a right way to dilute the CO2 in my tank (that already contains a mix of CO2 and air).

I know that I can digit the values of 50 ml/min for MFC1 and 950 ml/min for MFC2, obtaining a final flux of 1000 ml/min. But, in this way, I don’t understand the final concentration of CO2. I don’t know if applying this formula 5000ppm*(MFC1/tot flux) [that means 5000ppm*(50ml/min : 1000 ml/min)] I am double diluting my gas (because I am considering two times the air, once mixed with the CO2 tank and the other ones in the other air tank). Or, I don’t know if this formula [5000ppm*(50ml/min : 950ml/min] is the correct one, since apparently I consider air in the CO2 tank and air in the other tank.

I don’t even know if my approach to this problem is wrong and both formulas are not correct (e.g. I should consider other parameters; the carbon dioxide is a pollutant and so it follows particular rules/formula/conversion).


  • 1
    $\begingroup$ Recalculate both flows to flows of pure CO2. Both values must match. $\endgroup$
    – Poutnik
    Jan 4, 2021 at 18:53
  • $\begingroup$ If I consider to flux 1000 ml/min of dry air, I don’t have carbon dioxide; if I flux 1000ml/min only from the carbon dioxide tank, I have 5000ppm of CO2 + air. If I consider to have a tank of pure carbon dioxide at 5000ppm, I can hypothesis to flux 50ml/min from the carbon dioxide tank and mix it with 950ml/min of dry air, obtaining a tot flux of 1000m/min. This way, I should obtain 250ppm of CO2 (I applied 5000*(50/1000)). But I have carbon dioxide+air and I add other air, so, since I cannot experimentally verifying the final ppm I am fluxing, I would like to be sure on what I am doing. $\endgroup$ Jan 5, 2021 at 10:53
  • $\begingroup$ @Poutnik looking at my previous comment, to me the calculations do not match so, I would like to know which calculation is correct or where and why it is wrong. $\endgroup$ Jan 5, 2021 at 10:54
  • $\begingroup$ 1000 mL/min of 5000 ppmv CO2 is equivalent to 5 mL/min of pure CO2. To have 250 ppmv CO2, you need the total flow 5 *1000000/250=5*4000=20000 mL/min, So 20 times more. In fact, it is very trivial high school calculation. Do you need 20 times diluted solution? You need 20 times bigger volume. That simple.20 times higher volume is achieved by 20 times higher flow. $\endgroup$
    – Poutnik
    Jan 5, 2021 at 11:04


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.