On Tue, 21 Sep 1999 17:38:35 +0100, John Thornton wrote:

>Oh Absolutely!! I think that over here we have a problem with all the
>gear on one side, as I said in my post we all agree that simple is best
>but having all the deco on one side I have never understood. I did
>assume that it was a development from the days that you had no deco gas
>and the torch cannister was the problem, it is big for what you do so
>the only place to put anything else ( at first only one tank?) was on
>the other side, as things developed then all ended up on the other side.
>Frankly I find that regardless of how well it is signposted having
>gasses on opposite sides is easier and less inclined to cock up.
>Remember, no scoots, no big batteries for torches, streamlined for our
>specific interests.

In addition to clearance for the light canister and for discharge
thrust from the scooter, there are other reasons for keeping all deco
gas on the left side.  This position produces less drag as compared to
bilateral mounting, because the bottles are all in each others
slipstream.  Try this if you don't believe me - much easier to swim
with unilateral mounting.  Attitude offset is not an issue if you use
the correct (aluminum) cylinders for stages.  Another reason is
consistency, since with all bottles on the left the regs all feed the
same way, every time, in the same fashion as both the back gas
regulators.  In an emergency, conditioned responses are possible due to
this consistency.  With the stages mounted orifice up, the valve knob
is to the left, and in the event of a reg failure, can be easily
operated manually to deliver gas, with the left hand riding on the
bottle.  To do this on the right side, you have to mount orifice down
(inconsistent rigging) or have some sort of convoluted hand position,
and have the regulator rigged either from the wrong side or with a hose
that is too long - neither one an attractive option.

>Rodger Rodger, I have never had a prob with this, its the gasses off one
>side that I struggle with, it has to be easier to have your marking
>system with the high and low gasses on different sides, the confusion
>has to be less.

The problem with mounting high and low PPO2 gases on either side (other
than the mere disadvantages of bilateral mounting mentioned above) is
that this allows the diver to associate one side with a particular mix,
which can lead to breathing off of a bottle without proper positive
identification of the gas being breathed.  This is a CORE point of DIR:
that a gas be identified solely by the prominent MOD markings and not
by position, content decals, colour, mouthpiece, poodle jackets,
tactile aids or any other stupidity.  If you cannot positively identify
a gas, you do not breathe it, period.

>G you mark them in feet, we mark them in metres, not our fault, it is
>the measuring system we use. With 50% our depth is 22m, normally21m is
>marked to avoid a 1.6 po2. Confusing when an air tank marked 21% could
>be close. Our bottles are turned off as well, not sure what you mean by
>parked, is this left on the line? I assume so, we dont do this, probably
>because we dont dive in caves and have options to surface.

It doesn't matter if you use feet or meters as long as you are
consistent.  When I dive with Americans, we decide which units will be
used, and then ensure that ALL bottles are marked identically, and that
everyone dives with tables and gauges in the appropriate units. 
Confusing an MOD with contents should never be a problem, if ALL
bottles are marked with MOD only, and every member of the dive team
should know and agree what a 3" high number on both sides, horizontally
in the orientation of the tank in its normal riding position, means. 
Anything else is an invitation to disaster.  Parked means that the
second stage is clipped off at the first stage, near the neck of the
tank and not anywhere else where it could potentially be mistaken for a
different gas.

>Simple hose, simple guage and thats optional, thats all. The point is
>you have to run with it on, I accept that. But remember if you use
>redundancy to its full then this has to apply to all stages of the dive,
>do you have redundant deco gas? If not then how do you cope with the
>same failure scenario that dictates the whole twin set back gas
>scenario?. The schedules used should always have a back up,the option
>here is do you take loads of 32%, 40% or whatever( I am in your camp
>about the 50% here and staying on the he) to cover the failure? or do
>you have redundant backup? the back mounted o2 is there as a bonus and
>should be regarded as such, ie shorten the deco, if it fails the your
>set up is still safe and the desired result of getting home is still
>okay, it just takes longer

John, your buddies are your redundant deco gas.  Dive better than
thirds on everything, and for long dives, reserve the backgas for
emergencies, oxygen breaks, and wing inflation only.  Carrying the
oxygen is not the issue.  Carrying in on your back, in a high
entrapment potential location, where you cannot reach it to free it
should you become entangled, where it adds unnecessary drag that could
be avoided by mounting it another way, where the bottle identification
can not be seen, where it can not be quickly and easily handed off to
another diver, where a first stage failure can not be immediately dealt
with and gas delivery done manually, and where the second stage is not
clipped to the bottle neck and stowed in such a fashion as to make it
absolutely impossible to accidentally breathe off of this regulator
without first making a positive identification of the gas, is the
issue.  I can see you have given this a great deal of thought.

>Simple I agree with, your system is very specific and we all can learn
>from it, it is not the answer to all types of diving.

True, but for self contained applications other than 1 atmosphere
suits, it works everywhere.  Let me know if George takes you up on your
offer to come out there and set things straight - I might be interested
in a trip to Scapa Flow.

-Sean


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