I think even with the fingertip sensor there is at least one demo you can do to newbies. That is simply, put it on, and tell them "try to hold your breath untill it goes below 90".
Most of them will not be able to...But the point is, to show people who are afraid of brain damage etc, that you really are not dying of low oxygen even if you hold your breath for a minute or two.
Of course, it highly depends on the person how low it will actually drop, so test it first before demoing
From my experience, fingertip sensors pretty much "loose it", below 70-80% range. In water, even sooner than that...But on dry apnea and especially on newbies, you may get better readings because the vasoconstriction might not be that strong
