The bag is important. It's the loose fit around the neck that allows co2 to get flushed downwards and out from the continuous incoming pressure + flow of your chosen nit/heli/arg gas.
A mask wouldn't allow that because if it was loose enough to let gas out it would also be loose enough to allow oxygen back in every time you inhaled. The edge of the mask would be so close to your mouth that would happen.
It may be possible to rig up a divers breathing full face mask somehow. The exhaled air gets expelled via a valve that opens under pressure that shuts when you inhale. I'm not up on the specs but it'll work if you had the correct gear.
That big thing on the right is the filter. If the airholes prior to the filter were sealed and you drilled a hole right through you could insert your tubing from a nitrogen tank straight in. Turn on and breathe as normal, your exhaled air gas gets pushed out that valve assembly lump in front the mouth area.
Nitrogen and scuba mask is one of the methods I would like to use. Nitrogen tanks have a very different valve than scuba tanks so they can't get confused accidentally and I am having trouble finding adaptors. Just another hurdle to work through I guess.
Thank you all for your comments in this forum. I discovered it recently and it is being really helpful. I am totally determined, and I am just purchasing the last details and trying to decide between the bag or the mask. Regarding the bag, I have a question... I hope this does not sound stupid, but I cannot find an answer:
If the bag needs to be loose around the neck for the CO2 to be expelled... wouldn't a little amount of O2 go inside the bag as well?
Also, how loose does it needs to be? I understand it is important to have some "room" for the CO2 to go out and for the bag not to explode, right? But I am afraid of making it too loose.