Hey Jack,
Welcome aboard. You've got the makings for a great oxygen therapy kit with a welder's O2 cylinder and regulator already at home.
All you really need now is a reservoir bag and mouthpiece. You can make one easily for less than a couple dollars. I've made several of these using the following instructions and they work great!
A half inch PVC pipe union/coupler makes a perfectly functional mouthpiece. You should be able to pick one up at Lowes or Home Depot for less than 50 cents. After that all you need is a clean 40 gallon plastic trash bag, some electrician's tape and some duck tape.
Cut a small corner off the closed end of the trash bag a little less than a half inch in length and force the PVC coupler half way through the hole and seal with a wrap of electrician's tape.
Then fold over the open end of the trash bag a couple times and seal with duck tape for an air tight seal.
If you want to get fancy, get 6 feet of oxygen tubing and barb fitting shown below. The barb fitting will attach to the oxygen hose fitting on the regulator. Push the oxygen tubing onto the barb fitting and tape the other end of the tubing through a hole in the reservoir bag with electrician's tape to hold in place.
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If you don't want to get that fancy, all you you need to do is cut a half inch tip off the closed end of the trash bag opposite the mouthpiece, slip it over the oxygen hose fitting on the regulator and rubber band it in place.
Once you've everything attached, open the regulator's cylinder supply valve then open the regulator valve to inflate the trash bag reservoir. When the trash bag reservoir is fully inflated, close the supply valve and start sucking oxygen through the mouthpiece.
You may need to refill the reservoir bag another time or two but you'll find there's no restriction on inhalation. There's no exhalation valve so you'll need to exhale away from the mouthpiece. Exhaling back into the bag is bad... That will cause you to re-inhale exhaled breath high in CO2 and that can make an abort with oxygen therapy impossible.
The best technique is to exhale as fully as possible away from the mouthpiece, then inhale as deeply as possible through the mouthpiece and repeat as rapidly as you can.
If you start at a respiration rate of 20 and a tidal volume of 3 liters inhaled oxygen with each breath, you'll be sucking it down at an effective flow rate of 60 liters/minute. Keep this up for one minute if possible.
After a minute at a respiration rate of 20, slow the respiration rate to 15 breaths a minute with the same 3 liter tidal volume and continue at that rate until the abort. That should work out close to a flow rate of 45 liters/minute.
Take care and keep us posted.
V/R, Batch