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George,

These are really good points, especially seeing as I usually
end up doing my gas-mixing at 11PM after a couple of beers.  :) 
A formal gas-mixing checklist is a great idea - its 
way too easy to get cavalier with a fill whip.

The problem with guaging low-o2 mixes is especially problematic
with a CCR, since you are typically filling a dinky bottle.  After
a bit of experimentation, I found mixing in small bottles to be
virtually impossible.   So what I started doing is mixing up
the Heliox or mix in a set of double 104's, then trans-filling to
the smaller cylinders (6,12,or 30 cu ft).  At least that gives
a greater margin of error.  And 240 cubes of mix will last 
damn-near forever on a CCR.  I mixed up a doubles-full of 10/90, and 
another of 13/44, and I expect I'll make it half way
through next season without refilling (besides, I had to have
*something* to do with those now-useless doubles).  That, and
a couple of AL80's of the same mixes for OC bailout (which I will 
probably never need to use), and I'm set - travel diving aside,
of course.

-Will

On Fri, 14 Nov 1997, G. Irvine wrote:

> Will, this 2 percent problem is not too bad when you are partial
> pressure mixing for middle readings, like 35%, for example.You have the
> double check of the partial pressure and the reading, and the use of the
> gas is not a problem from either a toxicity or a decompression
> standpoint. Where this gets tougher is maxing the real deep mixes, but
> then the partial pressure still applies and the totals are still valid.
> You can start to see where the rebreather becomes a problem, however.
> 
> The big trick is to be sure you actually added the gases in mixing ,and
> did not have a valve off while you THOUGHT you added 35 psi of oxygen,
> or some such number, when in fact you did not, and then the miniox
> reading seems "acceptable". 
> 
> A real good anal way of doing this proceedure is what is necessary to
> get it right. Bill Mee and I do it together, and we have a whole
> checklist to go through before disconnecting the tanks, and in the whole
> process.
> 
> Adding oxygen to a high helium mix can feel like yoiu are adding it when
> youi are not. Just presurizing a big fill line fr9omt hat pressure to a
> small increment higher, even with a very accurate digital guage is a
> fooler since you can hear the gas moving even when the tank is not in
> fact open to accept it - a real dangerous situation. You have to
> depressurise the line afterwards and note the tank change on the same
> guage that the system was on to begin with , and you must do it quickly.
> There is no way the pressure has risen without the addition, even if the
> number is thrown off by the cooling - it can not decrease.
> 
> Little double checks like that , and then immediately throwing the
> analyzer on the result will give you some comfort. The go and throw it
> ont he pure helium to be sure it is not offset. A lot of work, but you
> are always betting your life with this stuff.
> William M. Smithers wrote:
> > 
> > On Thu, 13 Nov 1997, Jim Cobb wrote:
> > 
> > > Julian-
> > > If you are worried about it, throw in a 5% safety factor instead of a 2%
> > > saftey factor, which is what we do when mixing in adverse conditions.
> > 
> > You know, how many of you guys have actually read your MiniOx I manual?
> > +-2% is what the sensor is good for.  That's 4% total, which
> > is either pretty scary, or pretty indicative of the default conservatism
> > that's built into modern tables and deco algorithms.
> > 
> > The fact that it also quotes such potential "skew" factors
> > as 0.000X% for a Helium component is *such* a joke, in light
> > of the overall precision.
> > 
> > -Will
> > 
> > --
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