You might also want to read this tread
Do I want a Spare Air
The first thing you need to ask yourself is if you are going to hand over the bottle ISHTF. This will impact how you can carry it and, importantly, what YOUR ascent looks like. I'm going to assume you choose to have the option to hand of the bottle for now and will come back to it later.
- Consider the envelop of dives you might do, now and in the future. Consider the worst case scenario you want to use it for
- Based on that, determine how you would want to ascent from that scenario. Based on your ascent profile and your (or your buddies) gas consumption you can determine how much gas you need. Search for "rock bottom" or "minimum gas"on the forum if you like to know more. Keep in mind that if somebody needs a pony, the fan is already dirty so gas consumption is likely quite high
- Add some extra gas to avoid sucking the bottle dry / add more conservatism
- you now know the size of cylinder you would need as a minimum
As far as the regulator is concerned, I would advise going for the same or similar reg as your normal one. If you are going to need it, it suddenly becomes your primary regulator so it better be a good one. This also means the hose should be long enough to be comfortable, and that depends on the way you rig it. More on that later.
SPG doesn't matter that much (like other said, you want to know empty / full but other than that you are on your way up anyway) for your current diving, but if you are considering using the reg in the future for tec diving or sidemount diving you might as well go for a proper SPG straight away. Put it on a short hose something like 6-9" according to taste (I am metric minded and would say 15-20 cm, so if I picked examples that are not available, use that as a reference and pick something that fits).
Now the rigging. There are a few basic options:
- "permanently" attach it to something like your main cilinder
- attach it to something with clips / bungees
Since you mentioned you want to bring the extra gas for others, I would always make sure you have the opportunity to hand it off if needed. You want to avoid the out of gas buddy forcing you in an ascent you don't want. This means you want to clip / bungee it in place.
Again you have a few options:
- sling it like a stage
- sidemount it
I am sure people can come up with other options as well, but let's discuss these 2 for now.
If you sling it like a stage, you have 2 bolt snap attachment points (easy to google or find here how to make a rigging kit); 1 for your hip, 1 for your shoulder d-ring. Make it easy for yourself and put it on your left side. Yes, you can put in on the right as well, but this complicates longhose deployment and is more of a PITA to deal with than clipping your stage on your left hip d-ring and clipping your SPG on top of that.
Added benefit is that rigging like this is really common, easy to rent or travel with. It's the simplest solution that just works.
If you decide to sidemount, you still have the hip attachment point, but now instead of clipping of to the shoulder d-ring, you use a bungee to secure it. The big benefit is that you can streamline it more, so it is completely parallel to your body. This is it's own downside as well, since it is a bit trickier to finetune. This also complicates renting and traveling with it a bit more. Handing it off is a bit harder (for the receiving diver now looking for the top boltsnap) as well.
In either case you need to make sure the regulator hose is long enough. Ideally, place it like this: coming from the left side, it runs behind your head to your right shoulder. There it bends and feeds the reg (from the right side of the reg) with enough slack in the hose to turn your head all the way to the left without pulling on the reg. This also means it is long enough to donate, and sort out the situation while you consider handing the bottle over or not.
have a look at how tec stage bottles are configured, that should give you a pretty good idea of the end result.
