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Bill Mayne writes:

> Why is the rule there that you don't use your own tanks to do deco,
> since you aren't doing a gas switch anyway? If forced to use only
> air for a dive I always breathe down my doubles on deco instead of
> switching to a deco bottle, although I generally have an extra

I have never actually seen this written down. I was told this in no
uncertain terms recently by a regional instructor who sits on our
National Diving Council. I had been discussing helping out some dry
cavers in the North of Scotland because they are short of divers and
they have some exiting prospects if they can link some of their systems.

> By the way, on a single stage (cave) dive, where I am using a stage
> bottle to extend penetration, I still prefer to breathe down doubles.
> I breathe 1/3 of the stage bottle on the way in before switching to
> my doubles. Then if all goes well and I turn the dive after consuming
> 1/3 of my doubles I should be able to make it out with plenty to
> spare without using any more from the stage bottle. If the dive is
> so extreme that I can't then decompress of what is left in the doubles
> I would most likely be using another bottle with nitrox or oxygen
> for deco. I feel that having a big reserve in a stage bottle gives
> me more options in case of problems. If a buddy has an air loss it
> is easier to give him my stage bottle than to share air from my
> doubles.

To answer this I think I need to give some background info on cave
diving in the UK. My apologies for the length of this message.

Cave diving is significantly different in the UK than in the US. My
understanding is that in the US most of your diving will be in caves
large enough to use back mounted doubles and probably dived in pairs.
Visibility tends to be good and the water temp is comfortable, certainly
in Florida. There is therefore a significant chance that, in the event
of a problem, a buddy rescue is feasible.

Furthermore entry is normally via a spring and the entire cave is
submerged, in terms of not having to leave the water and port your gear
to the next sump.

This is not the case in the great majority of caves in the UK, certainly
in Scotland. The viz is crap and the water is cold. You have to port all
of your equipment through dry caves to get to the sumps. This may be a
half a mile or so and involve SRT and abseiling. Dives in these caves are
primarily aimed at extending dry exploration. Once you have dived a sump
you may use a molefone to locate the dry region and attempt to find a dry
way in, or you may have to explore further and dive more sumps.

Now before I go into this I think it is important that I point out that
I am not an experienced cave diver. I expect to carry out my first
original exploration this Summer. As Bill will know, I am planning to
come over to Florida to get some extra cave diving experience within the
next couple of months. Although I think this will be valuable experience
I  don't think it will all be directly translatable to crawling through
the muck in UK sumps.

The sumps are often shallow and long. There are many points which are so
narrow that you have to de-kit and push your stuff ahead of you, or perhaps
dig your way through, thus side-mounted cylinders are always used with
completely independent systems (no manifold). Divers go through the sumps
one at a time. If you get into trouble you have to attempt self-rescue.
Because the tunnels are narrow we have specific techniques for locating
lost lines in poor viz. Losing the line is a real possibility. For the
first diver it may not be so bad, but subsequent divers have to go through
in almost nil viz. When you come back you will certainly be swimming
through your own stoor.

The problem is compounded if you have to pass several sumps. The more sumps
you dive the further you are from base and the more isolated you are. These
caves are also remote from civilisation so that it may take several hours to
get a cave rescue team to the site. Even then these would be dry cavers with
no diving experience and it may be too dangerous to send them through. I think
the team that I am joining has only three divers and they are primarily dry
cavers who dive only to pass the sumps. The nearest cave diving experts may
be in England. Remember also that each dry section may involve more pitches
using SRT and hauling all the diving equipment with you.

So to answer your question. You may incur decompression penalties far from
base. You may have to surface and wait out SI's alone before diving the
next sump on your way home. It is not feasible to take the staging tanks
with you. It is therefore important that you have as much air in your mounted
tanks as possible. Stage tanks are generally collected on subsequent dives.
You will be diving solo so forget buddy breathing, and getting signals back
to base is unlikely to be possible as the line will have been belayed.

This is getting very long so I will leave it there and if you are not happy
that I have answered your question then I will take it further in a
subsequent message.

Alan

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