A Nitrox mixing stick does not to be high quality, it just needs to mix air and oxygen without creating too much resistance to airflow.
I used about $10 of PVC plumbing pipe, and a $10 air filter. Commercial Nitrox sticks cost a few thousand dollars. The cost savings really make it worthwhile looking into a homemade project.
People here on this forum can direct you to some plans for making your own Nitrox stick, and also give you information on how to assess and adjust the performance of your homemade Nitrox stick.
Basically a Nitrox stick is an extension of the compressor intake. You feed in oxygen at the top of the stick, and the baffles inside of the Nitrox stick mix the air and oxygen so that it is a homogenous gas as it enters the compressor intake.
An oxygen analyzer at the bottom of the stick tells you whether or not your Nitrox stick is mixing properly. If the O2 reading is stable, then your stick is mixing properly. If your O2 reading bounces around a lot, then your stick is not mixing adequately and you need to add or modify the baffles.
Also, if your O2 analyzer reading drops a lot when you turn on the compressor, then your stick has too much resistance to airflow and you need to remove or modify the baffles.
The oxygen analyzer and blue T-tube can be purchased from Oxycheq.
My baffles are PVC end caps with holes drilled through them. The holes of each baffle are offset from the holes on the adjacent baffles so the air has to zig-zag through the Nitrox stick. Initially I had too much resistance, so I just drilled the holes bigger and removed one baffle.
I use a lawn mower air filter on top of my Nitrox stick
I use a standard welding regulator for my oxygen supply. The clear pastic tubing feeds into the top if my Nitrox stick. With the compressor running, you adjust the oxygen flow to produce the desired Nitrox mix by watching the oxygen analyzer. I leave the compressor bleed valves open until I have the Nitrox mix dialed in, which takes less than a minute. Once I am producing the proper Nitrox mix, I close the compressor bleed valves to start filling the tanks. The welding regulator valve is adequately sensitive to achieve any desired Nitrox mix. The second valve shown in the photo below (just downstream from the welding regulator) proved to be not necessary for fine control of the oxygen flow .
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Harry