Need help mixing gasses, but not for diving.

[CodonAUG]

Thanks for the help everyone. I think I will see about getting some gas custom mixed for the lab since we don't have the money for a sensor.

 

[adelman]

How much blended gas do you need and how accurate do you need the mix to be?

If you used a scuba-style mixing stick and tested the gas mixture with a scuba-style O2 sensor located after the stick,
an O2 content of 19.9% would be a CO2 content of around 5%. If you could test the O2 to within .2%, then the Air content
should be good to within 1%, hence the CO2 content to within 1% (meaning between 4 and 6% CO2).

Not the most precise way to do this, but it could be cobbled up on the cheap.

 

[Sleepdr]

my recommendation would be to get a couple of flow meters with control valves McMaster-Carr set the pressure the same on both sources then you can tweak flow rates to get the right mix

That's essentially how we dial in specific gas mixes for anesthesia. A few flowmeters are ganged together and exit through a common gas outlet. Oxygen at 1 LPM, Nitrous at 2 LPM, and you have a 33/66 mixture coming out at 3 LPM. (ignoring, of course, some types of anesthesia volatile agent vaporizers that add additional stuff to the mix). The expense for flow meters isn't bad - $50 or so for a CO2 version on the internet. Analysis is the expensive part. If you need it to be documentably precise & repeatable, the previous suggestions to just buy a premix bottle are good if you don't need massive quantities of gas.

I'm not certified to mix gases for diving, but do have more than a passing familiarity with real-time gas mixing & analysis. Any comment I make should not be used for gases intended for human breathing, etc.

 

[Graeme Tolton]

If you say that this is on a budget, I would say that there are two options.
1. As mentioned above, get a custom mix from a gas supplier.

2. Partial Pressure blending.

Partial pressure blending with CO2, you must keep in mind that at room temperature, CO2 condenses around 700 - 800 psi, meaning that if your mixture went above that, you would require a dip tube in the cylinder, so that at it's full pressure, you would still be getting co2 out of it, rather than the co2 just sitting in the bottom of the cylinder in liquid form. On the same note, you could attempt to stay under the condensing point at a given temperature ( a chart can be found by googling R-744 pressure chart, R-744 being the refrigerant name of Co2).

With partial pressure blending, you can use an o2 sensor to calculate the co2 content of the mixture. for example, blending known gasses (air and 100%co2), you analyze your mix to have an o2 content of 17%. Knowing that you started with 20.9% o2, you can use the formula
1-(analyzed o2 content / o2 content of air) = co2 content of final mixture.
Hope this has helped!

 

[tbone1004]

partial pressure implies you have enough gas at high pressure to really do it. and a place to store it...

 

[adelman]

Although CO2 condenses easily at relatively low temperatures, this happens because of the relative affinity of the CO2 molecule for itself and the relative frequency of CO2-CO2 collisions in the gas. It shouldn't take much air to poison the CO2 and prevent it from condensing. I'm not sure if anyone has developed empirical tables for this, and the first-principle quantum calculations are quite difficult, but since you'd only be adding 150psi of CO2 total to get a 5% mix in a 3000 PSI cylinder, I would intuitively think by the time you could get to a pressure where the CO2 would condense, it would be sufficiently mixed with the incoming air to poison any chance of it condensing out. I haven't seen tables for the compressibility of CO2, but I suspect that it is FAR from an idea gas, and I'm sure some big correction to the ideal gas laws is going to be needed to accomplish a partial-pressure blend.

You'd also want to be sure that the air was very dry, or else the moisture would combine with the CO2 and make Carbonic Acid, which would be corrosive to the storage cylinder as well as any pumping equipment. You definitely don't want to continuous-blend this mix with moist atmospheric air fed into a standard scuba compressor, but post-compressor scuba air would be more than dry enough to mix with the CO2.

So how pure do you need the gas to be?

Don't forget you're not going to want to breath air with elevated CO2 levels like this. Are you planning on a mechanism to vent this gas to the outside?

 

[Thalassamania]

I'm not sure, but I suspect that two of these "panel mounted flowmeters" would do it (we use them to control oxygen flow in submersibles), they're about $50 a piece: McMaster-Carr

 

[NAM001]

ho exact does the 5% have to be

if close is close enough then put an o2 meter on it and adjust co2 for 19.9%

if you can get the whip and a small tank then partial presure mix a tank of it. if you dont need a lot of volumn then a tire fill tank whould work.

I am a researcher at the UC Davis MIND institute and the lab needs to mix some gasses in-line prior to dispensing and none of us in the lab are confident we have the right idea. Our lab doesn't have a lot of funding, so we can't afford a normal gas mixer but in my research it seems that these are not actually necessary and I think that mixing in-line shouldn't be too hard.

The lab wants to have a gas mixture of 5% CO2 and 95% Air. The gas is going to constantly blow at a low rate (a few L / min) into a small plastic container (4"x6"x2") that has a 2" diamter hole in the bottom.

I was thinking that all that we needed was to maintain a proper ratio. My idea was to have an air tank set to 95 PSI and a CO2 tank set at 5 PSI and then have the lines merge before reaching the end.

Would this work? Would it be roughly 5% CO2 and 95% air?

 

[NAM001]

i would think that the precision would be much closer than 1 percent. i would guess 1/20 of a percent.

How much blended gas do you need and how accurate do you need the mix to be?

If you used a scuba-style mixing stick and tested the gas mixture with a scuba-style O2 sensor located after the stick,
an O2 content of 19.9% would be a CO2 content of around 5%. If you could test the O2 to within .2%, then the Air content
should be good to within 1%, hence the CO2 content to within 1% (meaning between 4 and 6% CO2).

Not the most precise way to do this, but it could be cobbled up on the cheap.