2airishuman
Contributor
What's the difference between isolated doubles and a pony? I mean, in terms of gear. I understand that from a planning point of view, isolated doubles are closer to a stage bottle than a pony, but from a redundancy point of view, it's the same thing, right?
::shrug::
The idea with isolated doubles is that they are, well, doubles, that is, identical in terms of their size, contents, and how they are carried. In most cases one is connected to the BC and the other is connected to the drysuit (if used and if a separate inflation bottle is not used). Each cylinder has an SPG that is visible during the dive so that the diver can assess the condition of each cylinder.
During the dive, you alternate regs so as to keep the amount of gas roughly equal in the two cylinders.
With a pony, well, by definition, it's smaller than the primary cylinder, and it's not connected to anything except a second stage, and maybe an SPG, which may or may not be visible during the dive. For most people it's a reserve only and it's neither part of the gas planning process (except as a contingency) nor is it used on an alternating basis during the dive. If it's a stage, ordinarily, you use however much of it you plan to use at a particular point in the dive, rather than alternating with the primary; in technical diving, it may have a different mix.