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SCUBA-L  September 1999, Week 1

SCUBA-L September 1999, Week 1

Subject:

Re: Mixed drinks (Nitrox and Air)

From:

Art Greenberg <[log in to unmask]>

Reply-To:

Scuba diving discussion list <[log in to unmask]>

Date:

Thu, 2 Sep 1999 08:46:40 -0400

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (96 lines)

On Thu, 2 Sep 1999, Lee Bell wrote:
 
> > There is no reason to limit a clean cylinder to nitrox use - EAN21 is
> > fine, long as the source meets the appropriate standards for hydrocarbon
> > content.
>
> The logic here is clear, but begs a question by the uninformed.  What are
> the standards and how does one know they are being observed?
 
The standards that are followed by *most* SCUBA training agencies, and
therefore their affiliated shops, are established and published by the
Compressed Gas Association (CGA). The breathing air grading system that
uses letters like "E" and "J" is the CGA system. You can purchase CGA
documents, including a complete description of that system, from the CGA
online at http://cganet.com - not cheap, but very enlightening.
 
According to the CGA, the minimum acceptable purity level for SCUBA
breathing air is grade E. The next better grade is J, which looks like it
would be quite expensive to produce. It seems to be common practice in our
industry to accept a "modified" grade E as "oxygen compatible". This is
essentially grade E with the hydrocarbon limit between that of grades E
and J. This is usually acheived by adding an extra set of filters to a
compressor that delivers grade E air.
 
You can tell if standards are being observed by looking for one or more
certificates that show the results of the most recent air system tests at
your fill station. There are independent companies (Lawrence Factor for
one) which provide testing services. If a fill station can't produce
documentation showing their air has been tested to grade E (or better for
oxygen compatible air) within the last three months, I would go elsewhere
for fills.
 
BTW, you can get documents that describe oxygen cleaning and oxygen
service requirements from the CGA, and other neat stuff like cylinder fill
procedures and inspection standards as well. Again, their documents are
not inexpensive, but if you're very curious or serious about knowing this
stuff, they are very worthwhile.
 
The CGA is a *very* influential organization. Much of the DOT regulation
regarding compressed gas cylinders is based upon CGA work, as is the SCUBA
industry VIP.
 
> > Even a few fills of Grade E (the minimum CGA grade for SCUBA
> > applications) in a season would be ok, as that's "pretty clean" - it would
> > take a number of borderline Grade E fills to add enough hydrocarbon to
> > cause concern.
>
> This does not seem like something I would be comfortable with.  Again, as
> the uninformed, I would chose to avoid a risk which I don't fully understand
> when the possible result of a mistake could be catastrophic.
 
Its simple arithmetic. I don't have the documents here, so I can't cite
actual numbers. But some number of fills of gas just meeting grade J will
equal one fill of gas just meeting grade E with respect to hydrocarbons. I
think that number is in the range of 10-20. So, if you get 100 fills per
season and have your cylinder and valve cleaned at VIP time, you could
have gotten 5-10 grade E fills and had the same amount of accumulated
hydrocarbon. Clearly, then, 1 or 2 such fills won't upset things -
provided the fills are grade E or better gas.
 
There is little chance that a handfull of grade E fills would result in
anything catastrophic. If you take your oxygen service cylinders on a trip
and have a couple of days of diving booked, and your only choice is to
accept grade E fills or not dive, I would take the fills. If you wish to
play it safe, you can always have your cylinders serviced when you return
home.
 
> The basis for my decision to have the regulators oxygen cleaned is
> primarily based on a preference to avoid the cumulative damage to
> materials not oxygen compatable.  As far as I know, the one time cost
> of replacing incompatable parts is relatively small relative to the
> benefit of reducing the probability of regulator failure when I least
> want a failure, on an expensive dive vacation for example.
 
Not unreasonable. OTOH, I would service my regulators before an expensive
dive vacation anyway, especially if the location was remote enough to have
concerns about the availability of repair parts or services. And I would
carry a spare.
 
Oh, and the cost is not one time. An oxygen service regulator must be
serviced at least once per year, just line any other regulator. To
maintain its oxygen service rating, it will have to be cleaned and rebuilt
to oxygen service standards each and every time. The premium for this
service can increase the cost of an annual greatly, depending upon where
you get your work done and whether or not any warranty terms apply.
 
> At my present risk tolerance level,
 
This is key. All each of us can do is to be as well informed as
circumstances permit, and make risk judgements based on what we know. We
all do this every time we jump in the water.
 
--
Art Greenberg
[log in to unmask]

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