----- Original Message -----
From: "Bjorn Vang Jensen" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, January 07, 2002 3:29 AM
Subject: Re: [SCUBA-SE] Call Me Old Fashioned


> Lee,
>
> > My computers (I
> > usually carry two) require me to reset my gas before every dive once
I've
> > used something other than air.  If I don't, they set themselves to 50%
> > oxygen and 79% nitrogen (very conservative on both gases just in case).
>
> One computer sets itself to a mix that totals more than 100 ??? Why ?
> - And why is 50% oxygen more conservative ?

Keeping in mind that I'm not all that happy with the feature, the correct
response probably should be "ask Oceanic."  On the other hand, I'm pretty
sure I understand their reasoning even if I don't agree with it.  79% N2 is,
of course, what is found in air and unless I'm forgetting something, the
highest N2 level any diver will experience.  At 79%, the computer allows the
least no deco time of any gas that might be in use.  The 50% does pretty
much the same thing for O2.  The computer is only designed for use up to
50%.  In this case, it's O2 clock, both CNS and Pulmonary plus MOD that's
caculated at the most conservative rate.

> > Had I left the computer set for air when diving nitrox, the importance
of
> > understanding what's going on becomes even more important.

> You mean the other way around, right ?

I don't think so, but I certainly could have been more clear about it.  I
was running close to my O2 limit with the computer set properly.  If it had
been set for air while I was diving nitrox, the chances that I would have
been over the limit while my computer was still telling me I was fine are
pretty high . . . I think.  To tell the truth, I worked a 12 hour day to day
after driving 5 hours to get here.  I'm tired enough that my mind is no
longer working real well.  Perhaps you also recall that I'm just a bit
paranoid about PPO2/MOD.

Lee