You're in the water at 30m and a hose ruptures or the cylinder neck O ring
blows, or your reg goes into a massive uncontrollable freeflow (HP seat
problem in 1st stage possibly), or any one of many other possible scenarios
that can happen to you OR YOUR BUDDY.

It would be nice to have a completely separate gas source to breathe from
whilst either fixing the problem, ascending (under control) or to get over
to your dive buddy.

I won't even mention the matter of enjoying the dive so much that you run
out of gas!

Since you seem to have a liking for having redundant air supply, please
specify the difference in redundancy between a bigger volume bottle with
 independent valves, and an add to clumsyness additional pony.
Except for a clogged valve filter, which is all so common...;-)

 I use it down to around

Why not? Can't count to two?

 and there's no point in rushing into getting

Single + pony _is_ doubles. Handicapped doubles, that is.

Matthias

No worries, thanks for the input.

Sorry if you read it that way.  All I was saying was that compared to the
comprehensive level of knowledge that SCUBA divers get from the club based
system in the UK and most of Europe, I've often found that when my wife & I
dive abroad we see first hand how little can be learned from just quick PADI
resort based certification courses.

This is no reflection on the divers, as they often don't even know that
there is a better alternative to the 'Put Another Dollar In' way of learning
SCUBA, its just a reflection of the way the industry and (as it used to be)
SPORT has been taken over by commercialism in the last 10 years or so.

I was just trying to point out that although you may gain much useful
information from sources such as this NG, trying to learn the detailed
points of SCUBA diving, equipment and moving on to becoming a better diver
via a keyboard and screen is like trying to train a neurosurgeon with a
playstation.

Only by meeting, talking (and listening) to other divers, getting hands-on
with the gear and diving with them will anyone improve (always assuming that
you are meeting with more experienced and better divers).

Tony

LOL, Fair enough.

There's really no need for much of this. A new chap on the group has asked a
perfectly civil and reasonable question. Just because he is used to cylinder
volumes being quoted in different units than we are used to - that doesn't
make him an idiot, nor badly taught. Perhaps I should start using Mega
Pascals rather than archaic units like Bar ? I think some of the posters
were mistaking this (in my experience) friendly and helpfull newsgroup with
its US equivalent.

That aside, although speaking as someone with substantially fewer dives than
the poster, I seemingly have more UK experience, and have been through this
earlier this year. I bought a pony for a Scapa Flow trip (more or less my
first serious dives) and consider it would have been foolish not to have it
for the extra safety margin it gave. Pretty soon after I bought a twin-set
as it's more suited to the 30 to 40m diving I'm doing. I have therefore
tried both, to a limited extent, and had the various discussions on pros and
cons.

1.    "Spare Air" - (virtually) unanimously derided as nonsense so no more
to be said.

2.    Proper pony bottle - normally 3 litre (in European money).
      This is (debatably) just about enough air to get you up from 40m or
so.
       Or perhaps more likely, gives you enough to finish an ascent, or
avoid cutting short a safety stop if you outstayed your margin eg due to
faff at the end of the dive, or free-flow, bungling or even equipment
failure.  Some folks deride pony bottles as not being enough, but it is
undeniable that it is 3l more air than not having it. Though the equivalent
of 50 bar in a 12l is not really a lot. I never had to use mine in anger.
   It is said to make the rig lopsided, but I can't say I noticed a
problem.
   It's not a sensible margin for deco diving (in most people's opinion at
least)

3.    Twins - there is an opinion that someone might not "be ready for
twins". I've never entirely understood this logic myself, but it is often
said, and by much more experienced people than me, so there may be something
in this (quite what, I don't know). A proper twin set with an isolation
manifold (which I've now got) is the majority config, though the alternative
of independant twins is favoured by a sizeable minority. The pros and cons
can be argued at length, and in my (relatively inexperienced) opinion, there
is something in both views. Manifold twins are presumably not really
practical for holiday diving as you can't take them with you. I don't know
if it's practical to hire two singles on holiday - I can't see why not- but
perhaps you'd have a lot of hassle with the dive-guides etc. I've only dived
in the UK, so couldn't say. |Once I'd tried twins, and the wing you'll also
have to buy, I wouldn't go back as the whole set-up is more comfortable, and
knowing I've loads of air, make me more relaxed, hence I use less air still,
safer, and all the rest.

However, twins are heavy (on land eg beach diving), you'll have to buy a
wing, are alledged to be a nuisance on a RIB (haven't tried myself) and
somewhat more costly (in that you'll need a wing &manifold, though won't
need extra 2nd stage and guage)

Assuming you want to do UK boat diving. I'd see if you can borrow someone's
twin-set and wing, to see if it suits you, as it did, me. If it's for
holiday diving, I guess that might push you down the pony route - but can
you take a pony (empty obviously) on a plane ?

(By the way, if you do buy twins and wings, I heartily recommend the
one-piece-harness / stainless backplate rig)

And a final dig at some of the responders - I've actually learnet a great
deal about diving (and much else), from the internet, from advice
face-to-face and in newsgroups, and from reading books - probably far more
than from my original training (which was perfectly good in itself, and got
me started safely).

Hywel