I almost didn't post this, but I feel obligated to do so. For the record, I advocate the 30 or 40 cubic foot pony, and I think spare air is crap. I know at this point in the thread that we are past spare air. My main concern is the fervent, almost blind belief in the alternate second stage.
There is only one o-ring (-011) on a 3/8" port plug. That is a fact. Stay with me, this is going somewhere.
There is also an o-ring on the first stage side of a low pressure hose (-011 again), and another where the second stage hose meets the orifice (-010). There is typically at least one where the orifice mates to the second stage housing (-014 typically). There is a soft seat where the poppet is on a downstream second stage. Then, there is the diaphragm and mouthpiece, and a zip tie. Those are 7 potential failure points. I am being generous as most second stages these days are balanced, so they have many more (especially if the have an end user adjustable cracking pressure, venturi, etc). So you are adding 6 more (minimum) soft failure points every single time you add an alternate second stage to your gear. Normally, this is preferable because most people have no idea how to buddy breathe, or because it is PADI standard in some of my dives, or because it is how you have been taught. Normally, I dive with an alternate as well. Again, stay with me.
Everyone rails on about how diving with only one second stage is heresy. Why? If I have a 30 or 40 cubic foot pony bottle with a first and second stage, then a first and second stage on my back gas, why do I need one? Even PADI safe diving practices say that you need an alternate, safe second, OR redundant SCUBA. A pony with its own first and second stage meets this requirement. In my opinion there is zero need for an alternate second stage on your primary first stage if you have a pony bottle of appropriate size for the depth and planned profile.
It just blows my mind how people just leap onto their keyboards when someone merely mentions diving with only one second stage. When I dive vintage gear I only use one second stage (because you can typically only use one), and we plan our gas and practice buddy breathing. I understand that most people do not dive vintage, but the skills are still valuable.
There are times when diving with only one first and second stage is fine. If you are trained and willing to practice the skill of buddy breathing, it is not the end of the world. Last weekend I dove the jetties with my girlfriend. We both dove vintage regulators and we planned out our gas. Neither one of us had alternate second stages (no place to mount them). If you stay close to your buddy, and dive your plan, then what is the huge issue? I am not saying that everyone should dive with just one second stage, but there are situations where it is perfectly fine. Would I dive 100 foot profiles with no pony bottle and one second stage? Nope. Still, on shallow dives, river dives, or numerous other types of dives, having only one second stage is preferable in some cases.
I just do not want people to think that the freaking world will end if they take off their octo. I know 90% of the time it is a good idea to carry it, but there are other times.
at the safety stop with 500 lbs and an empty BC (make sure it REALLY is empty), you are very close to target weighting. If you have to fin or add air at the safety stop, you are too heavy. If you are having trouble staying down, too light. Most of us call it "close enough" at about the weighting you describe, give or take a couple pounds, because you can easily compensate for a couple pounds just with lung volume.
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