Sorry guys, got a bit of a newbie question here: please don't slap me about too hard
The ScotSAC lectures suggest planning for an additional third of your dive air requirements be added on as a reserve.
For example, if I work out a descent/bottom/ascent/safety-stop will take 1000 litres of air, I should add +333 litres of air as a reserve - meaning I shouldn't go on this particular plan with less than 1333L of air.
My question arises when we add a pony cylinder/bailout to the mix...
Am I right in thinking that the reserved +33% of the required air should still be added into the main cylinder (and that the pony bottle's sole purpose in life is to get you safely to the surface with the required stops, and is sized accordingly to the depth you plan on having your accident)?
My reasoning is that if you run into trouble - say, from entangelment, and use a lot of your reserved air freeing yourself - you then still have the pony to get you to the surface - whereas if your pony was the reserve, you've only got whatever is left in there. Is that reasoning correct, and as such, am I right in saying that in no way, shape, or form does a pony bottle increase your available bottom time?
Many thanks - and sorry again... I feel like this is something I should just know, but it has only been touched upon in the lectures as "a pony is an independent air source" and assume you'll gain the rest of your knolwedge of gas management through your club and technical courses if you go down that route! Most of the other divers in my club use twinsets as well - so I've never even thought to ask until now!