Do you have your own basic kit already, or would you be buying it all
together?  I'd recommend getting yourself sorted with all that first, before
looking into extra kit.

What you need depends on the diving you're doing.  Personally i'd ignore
Spare Air (almost as much as a pony setup, with a volume which means any
ascent will always have to be dangerously rapid), leaving the choice between
pony and twins.  At your level, i'd say get a pony.  Theres no particular
reason not to get twins if you decided to go that way, but a pony is
initially lighter, easier to carry / kit up etc, and perfectly fine for
diving to around 30-35m, which is all you'll be doing I expect for at least
a year, maybe more.  If you look at the cost of the pony, if/when you
upgrade to twins you'll only have lost the ~£60 cost of the 3l cylinder, and
of course you can keep that for other uses in the future.  Helps spread the
cost a bit more if you're thinking twins later.

Just make sure you can justify to yourself what you're getting and why -
don't try to future-proof yourself too far into the future, because kit
changes, devalues, tanks need servicing, etc.

David

I think you'll get a lot of opinions against.

My quick calculation is that I dive with 12l and usually last around 45
minutes with 50 bar when I surface. The equivalent of using a 9l tank to
empty.

9l = 45 minutes
0.42l = 2.1 minutes

So I'd get 2 minutes worth of air at my average depth.

Someone else has done more calculations (based on the smaller 0.2l
version) at:

http://diveweb.oneandoneis2.com/spareair.htm

however they are flawed in one respect.

The bailout calculations give the maximum bailout until taking your last
breath. In reality what the spare air would mean is that you could bail
out from 20m and it would be empty, for example, when you got to 5m. You
wouldn't suddenly drown the moment you sucked your last breath from the
canister. I'd don't know about you, but I would much rather run out of
air at 5m when the spare air emptied than at 20m without one.

I'd rather have one than not have one. But I haven't got one. And I'm
not likely to buy one either. So unless someone's giving them away....

You're good. It took me a few buddy dives to figure that out. You
figured it out quick!  8^)

Great advice already given!

My preference is for a pony, but it suits the solo, non-deco shore
diving that I like to do, which are almost always long dives, draining
my main tank almost completely. Since I drove 200km to get to that dive
site, I want to have a lot of time in the water!

I would just add that if you decide on a pony, I'd get one with
significantly _more_ capacity than the _minimum_ needed for bailout at
your anticipated max depth. The higher capacity pony won't weigh or
cost much more, but will give you more options under a variety of
circumstances.

Frankly, relying on a buddy for redundant air supply never seemed
reliable enough to suit me.

Spare Air, from what I've read, doesn't have enough capacity for the
ascent _and_ a safety stop from some of my deeper dives. Or at least
not enough capacity to deal with issues that might arise. I don't want
my only alternative to be an ascent.

I don't have any personal experience with twinsets, but I assume their
redundancy would be best for overall capacity and for deco.

My compromise is to have a large capacity single high pressure steel
tank (120 cubic foot, or ~14.3L, 238 bar) _plus_ a substantial aluminum
pony (30 cubic foot, or ~4.3L, 200 bar). I like this arrangement
because I've got enough air just in the main tank for long, fairly deep
dives (25 to 35 meters) and then plenty of time for a swimming safety
stop, and can use that main tank down to near zero in the shallows
because I have the pony. Makes for a nice, relaxing cruise at the end
of the dive.

Changng to a fresh main tank between dives is a simple matter of
unscrewing one bolt in the Highland stainless steel band that holds the
pony rigidly to the main, and sliding the band off and then onto the
fresh tank.

Although the pony mounts to one side of the main, I haven't felt any
significant change in trim. Maybe it's counterbalanced by the camera
and catch bag on the other side!  8^)

I use the pony on all my dives, even if shallow, even if I'm with a
buddy. It's almost as simple as a single, IMHO.

Regards,

Dave C

Craig
The only thing I can add to the good advice already given is:
When I weighed my 3l pony plus reg and clamp when empty, and in water,
it weighed 1 kg. I removed a Kg of lead to avoid being overweighted.
Also, whilst it didn't seem to affect my trim in "normal" diving, when
demonstrating a hover, it was apparant. I now wear more lead on the
non-pony side.
Oh, and practice reg swapping.
Have fun.
Cheers
Graham