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The diluter demand and pressure demand masks are not "required" by regulation to my knowledge, but are recommended for above 25,000 feet.
Cannulae are approved only for up to 18,000 feet. Above that you are required to use a mask.
The Nelson systems all include the adjustable flow meters inline, where each user can see and adjust as necessary without messing with any flow control at the regulator.
It seems that one can dump the pressurization on a P model and safely take it higher than 20,000 with an oxygen system and masks. The trick would be the legal argument about certification. If anything happend I bet lawyers would have a heyday.
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