O.K., I just now spoke to Nigel on the phone, and now I understand the confusion. We're talking apples and oranges here. Whereas I was thinking in terms of *spotting* bad oxygen (my post), Nigel was thinking in terms of *intentionally diving* with bad oxygen (his posts), and allowing the system to calibrate pre-dive in spite of having non-oxygen as your oxygen.

I have to be a bit frank, in that now I'm the one who is a bit staggered, stunned, speachless, etc., in that like Nigel, I was unaware that people *intentionally* and *knowingly* did CCR dives with bad oxygen. This one is news to me. I guess if you've got something like 94%, it's probably OK -- but in that case calibrating as if it was 100% is probably also within reasonable margins of error anyway.

So, now that I understand the context of the conversation better, my questions to the list members are:

1) What percentage of impure oxygen are you willing to *intentionally* and *knowingly* dive on a CCR?

2) When you do know that the oxygen is not 100%, how do *you* deal with the oxygen calibration point?

As Nigel said on the phone just now, it's not a matter of figuring out a solution -- it's a matter of deciding which, among several, solutions is optimal in the context of the MK-VI diving paradigm.

So...the obvious answer in the MK-VI is to allow the diver to specify both gas mixtures (not just the diluent). The fear, of course, is the diver screwing this up. I'm just curious how other rebreathers deal with this issue of calibrating with gas that is known to be some percentage of oxygen other than 100%?

And...I'm a little curious as to how far of a deviation from oxygen CCR divers are willing to dive (and how do you know what the non-oxygen stuff is)?

Aloha,
Rich