Mailing List Archive Mailing List Archive
In a message dated 8/4/00 1:50:05 PM Eastern Daylight Time, ScottBonis writes:
> I am not arguing against anything. I have learned a great deal from
> studying about, analyzing and adopting various DIR techniques. I would
like
> to understand why the apparent objection to using heli-air.
>
Scott,
First, I want to point out that I do not represent myself as a DIR
spokesman. My personal views and objections concerning heliair may differ
somewhat from what DIR doctrine would be. So with that clarification, I'll
attempt to answer your questions to the best of my ability.
> I see one important (to me extremely significant) advantage for heli-air
and
> one other nice feature. The significant advantage is that at any time, I
can
> use an oxygen sensor and determine exactly what is in my tanks. This is
not
> true with other trimix blends. And the nice feature is that because I can
> analyze my blend, I can have confidence in topping off my half filled tanks
> after each dive, with Helium and then air to optimize the mix for my next
> dive.
>
As we attempted to convey to you last night, the percentage of helium in
the mix can vary by several percent without creating a problem. Helium is a
realitively friendly gas, it's not going to kill you. It's the oxygen
content you need to be concerned about. If you're using some of the tables
available in the market you can be assured that they tend to be heavily
"padded." Discrepencies of 5%, possibly as much as 10% in the helium and
nitrogen content will not result in a substantial DCS risk.
If you're really that concerned about the helium content in your tanks,
assuming you blend your own, always do an intermediate analysis before toping
the mix with air. This will establish an absolute He content that is not
going to change with the addition of air. If you have a blender prepare your
mix for you, request they supply you with the results of the intermediate
analysis. This should provide you with more than an accurate enough analysis
of the contents. Since you now have an accurate analysis of the orignal
contents, you have an accurate figure for doing a top-up of the tanks.
> In both of these cases I am not dependent on the accuracy of any
compressor
> pressure gauges or the conscientiousness of fill station operators or any
> other personnel, in determining exactly what I will be breathing. And when
I
> need a bunch of tanks for a dive team, this becomes IMHO, particularly
> important. I teach my students that it is real risky to dive using any
tank (
> nitrox or trimix) that they haven't personally analyzed with an instrument
> that they have personally calibrated.
>
While I agree that it's important for the end user to conduct a final
analysis of the mix, the fact remains it is the oxygen content of a mix that
has the greatest potential of killing you. If the oxygen content is too low
you risk hypoxia. If the oxygen content is too high you risk CNS OxTox. In
contrast, the helium is present primarily to reduce the level of inert gas
narcosis experienced by the diver. Assuming you're planning dives using a
conservative ENDs, a variation of a few percent of He shouldn't make a lot of
difference. Because helium both on gases and off gases quickly a variation
of a few percent should have minimal impact on your DCS risk exposure.
> As far as the use of heli-air as a bottom mix is concerned, I don't
> understand the objections. It's just like other trimix blends but with a
> somewhat lower oxygen content. I pick an acceptable END and that then
> defines the mix. Typically the PP(O2) comes out to a little below 1 ATA at
> depth and remembering that we are talking here about bottom mixes and not
> deco mixes, a slightly lower PP(O2) would not necessarily seem to be such a
> bad thing. And of course for any mix for a dive of less than say, 400
feet,
> once I am down to 20 or 30 feet, the mix is no longer hypoxic, so I don't "
> need" a travel mix.
>
It is the travel zone from the surface to depth, both during entry and
exit, where hypoxic mixes can become problematic. It is possible to
experience a problem upon entry that delays descent. If the diver is
breathing the hypoxic mix the risk of black-out and drowning is high. (A
diver drowned on the surface at the beginning of a Doria dive last season for
this very reason.) Should the diver choose to use a decompression gas to make
the entry and exit, the chance of being on too hot a mix, or using so much
decompression gas that there is not a sufficient quantity for the deco
obligation is real.
Another consideration is the fact that it is next to impossible to get
the ideal mix for a particular dive by using heliair. The mix is almost
certain to result in an END greater than 100 FSW or with too low or with too
high of a PO2 to result in a desirable decompression obligation or with a
combination of these factors.
> So could you please explain, in slow and simple terms, what problems are
> being created? Have I missed something? Since I obviously would have deco
> gasses available for any required surface swimming, why is 18/50/32 so much
> better of a mix than 10/50/40 for a say, 250 foot dive?
>
In the first place, I wouldn't use 18/50 at 250 FSW, the PO2 would be too
high. In contrast I might use a mix of 16/50 for such a dive. Your 10/50
heliair would result in an END in excess of 110 FSW. Using the calcualator
feature in the GUE Decoplanner, my trimix would result in an END of about 88
FSW. Applying DIR principles, as I understand them, the maximum planned END
should not exceed 100 FSW.
Using the same software, and planning on a 25 minute bottom time, using
EAN50 and O2 for decompression, your heliair results in a 167 minute runtime.
In contrast my trimix results in a 134 minute runtime. I've reduced my time
in the water by thirty-three minutes. If we're talking open ocean diving
here, a lot of things can happen in 33 minutes. A squall could move in,
placing the boat and all onboard in jeopardy, a strong current could move in
making decompression dangerous, a diver that had entered earlier could be on
board and in need of transport to a medical facility with that transport
being delayed by your long decompression obligation.
Taking this one step further, using a RMV rate of .5 cubic feet per
minute and applying the rule of thirds you would require 280 cubic feet of
heliair, 75 cubic feet of EAN50 and 75 cubic feet of O2. In contrast I would
require 247 cubic feet of trimix, 59 cubic feet of EAN50 and 60 cubic feet of
O2. While the difference in these figures may not seem huge, I could charge
a pair of 95s to 3500 psi and sling two aluminum 60s vs you needing to charge
a pair of 104s to 3600 psi and slinging two 80s to do this dive.
> I'm not trying to start any kind of argument here, but I would like to
> understand the rationale for precluding the use of these heli-air mixes. I
> have attempted to explain "What do you believe in your system is better
than
> what we believe in the DIR system and why???" Although I would not use the
> word "better" but rather "different."
It's really all about simplification..... aka KISS; Keep It Simple Safe.
There are situations where a hypoxic gas is required, but for dives that can
be done without one, it's less complicated to eliminate the need for using
the deco gas or a travel mix. By increasing the depth of the END any
problems encountered during the dive become more complicated due to narcosis.
By choosing an ideal gas for the plan, rather than settling for the closest
compromise, in water time is reduced, reducing risk in the process.
Beyond this attempt at an explaination I can only offer a quote from a
good friend; "To some, no explanation is needed; to others, no explanation
is possible."
Regards,
Bob Decker,
www.SportDiverHQ.com
--
Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'.
Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
Navigate by Author:
[Previous]
[Next]
[Author Search Index]
Navigate by Subject:
[Previous]
[Next]
[Subject Search Index]
[Send Reply] [Send Message with New Topic]
[Search Selection] [Mailing List Home] [Home]