Need help mixing gasses, but not for diving.

[CodonAUG]

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?

[saxplayer1004]

short answer is no. you need equal pressures at different flow rates. if you have the pressures that different the ratios will be all over the place and may get back filling from the air into the co2. also next to impossible to set something at 5psi accurately. there's no real easy way around this to get it accurate. best way to do it would be to do something similar to partial pressure blending, or talk to your gas supplier and see if they can blend it for you

[CodonAUG]

That's a bummer. Thanks for the advice, I appreciate it.

[TSandM]

It's quite possible to do this, if you can analyze the gas coming out of the system. This is the way continuous blended Nitrox systems work -- but depending on the length and diameter of the tubing that's channeling the gas, you may not be able to predict accurately what the flow rates are going to need to be. But it is possible to bleed a small amount of a second gas to make a continuous mix.

[SteveAD]

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

[Peter_C]

Guessing you are trying to do this at a low pressure? If so it wouldn't be too hard to accomplish you just need a home made mixing stick, some way to get and regulate compressed air, a CO2 tank with regulator, and a way to measure the CO2 content output.

Without knowing what you have available for air I can't comment there.

CO2 can be purchased from most any gas supplier, IE Praxair, although they are not my favorite, I use Complete Welding Supply for their better prices. A regulator could be purchased used and welding type regs are fairly common on ebay. I think it is a CGA-320 fitting. Also used for home beer brewing.

The mixing stick is just made up from PVC pipe of some kind and has wiffle balls or some type of media to cause the air/CO2 to mix. Nothing extravagant, and can be built for less than $40. Of course you can dream up anything you want. You also will not need an air filter but instead two inlets for the gases, and one outlet into your project.
Mixing Stick

The CO2 meter is not going to be cheap. Maybe another dept has one you could use? They seem to all read in PPM so you are going to need to run a conversion.
Nuvair - CO2 Analyzers
CO2 Meter | Carbon Dioxide Meters, Sensors, Monitors, Data Loggers - Welcome


The reality is it might just be easier to have a gas supplier blend you a tank full of the mix you desire, then use a regulator to control the flow. Depends how many CF you need??

[saxplayer1004]

see why i said no real way to do it accurately? If it's plus minus a few percent then sure it's not that difficult or expensive, but to get it to a true 95/5 mix is going to be rather difficult. Hell the nitrox blends we get are usually +-2% and those are continuous blend. just something to think about

[Peter_C]

Hell the nitrox blends we get are usually +-2% and those are continuous blend. just something to think about

Much of that depends on the meter/sensor you are reading it with. I have four O2 meters and a few O2 sensors. They all read just a little bit different. I can keep my 32% mix pretty close to +-.5% though. If accuracy of 5% CO2 is very important it is going to cost the OP $,$$$.

[saxplayer1004]

I was talking about just going to a dive shop and getting a "32%" fill. that's very difficult to hit 32 on the nose. I can get pretty darn close doing continuous on my own compressor, but when they fill their banks its hit or miss

[Doppler]

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?

I would suggest two options if you wish an accurate mix.

1) call a gas supplier and ask them to supply cylinder(s) of the gas ready-mixed. I believe most suppliers can provide this service... the gas will be laboratory grade and therefore will be expensive.

2) mix by mass. One mole of air comes in at around 29 grams (I am pulling these numbers out of my arse so check them) and a mole of CO2 is about 44 grams. Since you require some degree of accuracy, don't bother with oxygen sensors, nitrox sticks and partial pressure bull****.

All that stuff is fine for diving, but you work in a lab, correct?