Assuming a recreational dive to no more than 100 -110 feet, with a mandatory deco stop, and an OOA emergency, would a 6 cu.ft. pony be enough for the diver to make a proper ascent without issues? Someone earlier mentioned the rule of 3rds for a pony, so diving with an AL80 would "require" a 30 cu.ft. bottle, a little bit unwieldy for a rec. diver not doing a cave or wreck penetration dive, don't you think?
Thoughts?
Thanks - Samer
The rule of thirds is for all your gases and allows for usage there, usage back and emergency supply (for you or your buddy). Given the pony isn't part of your "normal" gas usage I see it as emergency use only. Your back gas should be rule of thirds. The pony bottle (for me anyway if I was diving recreational), is enough gas to ascent from the deepest point in my dive, do deco and surface with some air remaining (only on the emergency pony supply). This means you then have to plan your dive ahead of time (which most rec divers don't usually do).
So, on that assumption, what's the absolute deepest depth you will be exposed to (assuming some error due to down currents), maybe 130-150 foot maximum. What's the run time from that depth to deco stop, calculated deco stops, what’s your normal air consumption, and adding some for Murphy. Your pony should allow for all of this. Its all in the maths.
The minimum size pony I have is 3 litre (25cu ft) which I consider a baby bottle. I wouldn't say it’s unwieldy at all. Next size is 5.7 litre (40 cu ft). Good size for deco stops etc and good backup. If I was going to use a pony for emergency in a rec dive I would use 3 litre (24 cu ft) as a minimum and that's just off the top of my head no planning assuming I am diving to say 30 metres. It’s a small cylinder, easily slung and used and not heavy or bulky by any means but should get me safely to the surface with air to spare.
For me, all these “spare air” cylinders I have seen are toys made to a price, sold to divers who want to feel good about their safety; they are not serious gear to be used (but to me) feel good items only. Now that's just my opinion and I am sure many divers do like them and what they offer (I know some air is better than non at all). A 6 cu ft cylinder will give you about 2-3 minutes at depth max, a 25 cu ft will give you about 8 min at depth max. If you are pushed to say 55 metres by a down current your air usage will go way up due to stress so assume SAC 30l/m by 6.5 ATA=195 l/m and your toy cylinder holds 169 litres so you have about 40 sec air, the 3 litre cylinder will give you about 3.5 minutes of air.
Plan your dive and do the maths. Feeling good and not feeling bulky might be a great feeling but it just doesn’t work out when you do the maths. If you don’t like a sling tank then an alternative is twin independents or manifolded twins, BUT yes they are bulkier and heavier and you do need to understand how to handle them as well but they do give good backup. All food for thought I am sure.
Before I went tech I used to do the “normal” rec diving, suck the cylinder way down with not a lot of reserve. Maybe take a pony sometimes. After the course it opened my eyes to how I should be diving and how to properly calculate air supplies and reserves. All of a sudden one realises that an 11 litre (90 cu ft) cylinder doesn’t hold much bottom time gas if you plan properly, so its either less bottom time or more gas.
