2airishuman
Contributor
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I saw Caruso's comment, but do not agree that, for a solo diver (or any diver), a pony should be part of gas planning by letting you deplete your reserve. Use your primary cylinder for gas planning--the pony is only for an emergency, so do a gas plan as if it will never be used. If you want more bottom time, use a bigger cylinder, not the pony.
Plan your dive, dive your plan.
Sometimes I bring two cylinders on a dive. Usually they're the same size, so I maybe have a pair of 72 cf cylinders, or a pair of 100 cf or 120 cf cylinders. Ordinarily when I do this I have a gas plan that involves using gas from each cylinder and some sort of minimum turn pressure. Sometimes the cylinders are part of a twinset, but not always.
Sometimes the cylinders are different sizes. So I might have, for example, a 120 and a 40. Usually when I do that my gas plan is to start on the 40, stay on it until I reach some minimum reserve pressure -- say 1000 PSI depending on the depth -- and then switch to the 120 for the remainder of the dive.
I suppose I could start on the 120 and then switch to the 40. It doesn't make much difference from a standpoint of safety or the amount of useful gas. However, it would be necessary to maintain a suitable reserve in the 120, because the gas in the 40 is not sharable since it only has one regulator.
I don't think there's anything wrong with planning dives in those ways.
In practice the reserves most divers/DMs plan to with an AL80 are not sufficient to get themselves and an OOA diver to the surface (e.g. 500-700 PSI). Therefore it would be unwise to further reduce these reserves when diving with a pony. (In most cases the low reserves don't matter because most recreational dives involve a group with widely varying RMVs; many include a DM; so there ends up being enough gas to go around). That said, if you actually perform a rock-bottom calculation and plan your dive around the result, I believe it is within the bounds of prudence to plan one diver on the pony for the ascent, which reduces the reserve requirement somewhat.