Redundant Air questions

[tkaelin]

It depends on a large degree on what type of diving you are doing, what you want the back up for, who you are diving with, what your comfort level is and what your philosophy is towards diving and personal safety/responsibility. I have read all the posts here and in the past and understand all the arguments. I can't really suggest what might be right for you based upon what little you provided but I can tell you what I do and what my thoughts are. For my shallow solo shore diving (30' or less) no overhead, I stopped carrying my small 6 cft back up. On warm clear boat dives with a buddy with no overhead environment I don't bother as well. On my warm clear solo shore dives to depths greater than 30' with no overhead environment, I carry on my side attached to my BCD a 6 cft bottle. I am comfortable in this environment going down to 100' or so with this rig. I know people like bigger bottles.

My thinking is that I will likely never use it. I would be an extremely rare situation that I will ever need it. If I ever do it will be simply to assist getting to the surface. I don't dive anywhere near deco on my deeper solos and would not plan on a safety stop on an emergency assent. This would be for a true emergency situation where on the next breath from my main there was no air type situation. I am confident with my training and experience that I could safely make it to the surface from depth with little extra air needed. Remember the standard assent rate was modified with the safety stop recommended added. I started diving way back when when the assent rate was faster with no recommended safety stops.

As I side note, over my 40 years of diving I note the trend for divers to "overgear up" in my humble opinion where divers can barely stand up, get up, off or on a boat without assistance from others in part because of all "extra" gear they carrying. I am heading in the other direction on this trend.

Hope this helps and as noted my comfort using a 6 is only in the environments I described. If you are in dark cold deep dives, deco or close, wreck penetrations, overhead issues etc I would not use only a 6 nor recommend a bottle that small.

 

[tbone1004]

part of the reason for 40 over 30 for deco bottles is they hang more like an AL80. An AL30 is neutral without a first stage at 500psi, an AL40 is neutral with a first stage, so if you have to carry it, the al40 is less negative in the water so it actually trims the best of all of the small aluminum bottles. An al30 has enough gas for 20 minutes of deco, which is good for a 90 minute bottom time at 100ft on EAN32. Quite a bit of time considering for that dive in a cave you would have to carry about 350cf of gas. Significant amount of bottom time where capacity isn't the concern, but trim is.
A 63 is actually the same buoyancy characteristics, but at 4lbs negative when full with a first stage is too heavy for stages, so al80's are used. On top of their obvious abundance in the scuba world, they really do make the best stage bottles if you have to carry them.

It has been mentioned briefly, but why do you want redundant air? Are you concerned about regulator malfunctions, or actually running out of air? Either one requires appropriate rock bottom calculations, but if you own your own tanks, an H-valve may be a much easier, and imho better solution to a pony bottle for recreational diving if your concerns stem from equipment failure over running out of gas.

 

[redacted]

I have seen dive ops that limit pony bottles for recreational dives to 30 cu ft or less.

 

[DA Aquamaster]

I have seen dive ops that limit pony bottles for recreational dives to 30 cu ft or less.

So have I.

I suspect their concern is that with an AL 40, the diver will be tempted to use it to do decompression diving. I've had similar concerns expressed when doing recreational dives in doubles off a boat. Most boats don't encourage decompression diving, due to liability concerns, due to greater space taken up on the boat, and due to concerns that it might delay departure to the second dive site, and to the dock after the diving is done.

Once you explain the plan to them - using only half the gas for each dive, with no need to swap tanks between dives, most of them are ok with it. We may still do 5-7 minutes of deco on a dive, but if we do, we'll be the first divers in the water so that we can be done with deco in time to be back aboard before, or right on the fins of the last recreational diver out of the water.

 

[Kharon]

Just my 2 cents. First, if you don't lose your head 6cf will get you up from 130 feet. Let's do the math. 130 to 70 - one minute at 60 feet/minute. 70 to 40 - another minute at 30 feet/minute. 40 to safety stop depth, another minute. If you are sucking down air - even at 2 cf/min - you will be fine. Sans panic, at a SAC of 1.0 (conservative) you will still have time for a short safety stop.

I (very recently) downsized from a 13cf to a 6cf for just that reason. It's a last resort, get me up safely resort. Too many over complicate the issue. Too many think you need more air than just to surface. Nonsense. If your equipment is well cared for a pony is very good insurance. However, you don't need to carry vastly more than you need.

As an aside, if you want a 13 cf pony i have one that just had hydro & vis and set up with sling. PM me if interested.

 

[Eric Sedletzky]

I've been on boats where the rules are "No deco diving" and "Be back on board in an hour". Other than that they don't care what you have on as long as you respect the first two rules. Many times there are divers practicing with all their tech gear and getting used to it for some bigger dive somewhere else, so they use any opportunity they can to use all the stuff.

For a while when I was into using the pony system, I just used a nice little set of back mounted twin 72's. I didn't care for slung bottles for recreational dives, and twin 72's were so elegant and balanced well that they were a joy to dive, even better than a single monster water heater tank.

 

[Dive Right In Scuba.]

I'm with those who advocate for a 30 or 40.

The old cliché about a spare air is that it allows you to experience running out of air twice on the same dive. Will it get you from 100 feet to the surface? Sure it will, but so will a CESA, and a CESA is much cheaper. In my mind, if you are going to carry a pony, you want to be able to do a proper ascent, with a comfortable safety stop and all, rather than just barely make it and hope you don't have a DCS problem. If you are going to carry enough air for that, then you might as well be really safe and go at least with the 30.

 

[redacted]

Unless you need to pack it for air travel.

 

[Nemrod]

A 19 is more than adequate for me and I find it much easier to pack. If there was a 13 that was similar in length but smaller diameter I would go to that. But the typical 13 is a fat little thing and simply does not sling well. Therefore I find the 19 perfect for my uses. N

 

[dumpsterDiver]

Just my 2 cents. First, if you don't lose your head 6cf will get you up from 130 feet. Let's do the math. 130 to 70 - one minute at 60 feet/minute. 70 to 40 - another minute at 30 feet/minute. 40 to safety stop depth, another minute. If you are sucking down air - even at 2 cf/min - you will be fine. Sans panic, at a SAC of 1.0 (conservative) you will still have time for a short safety stop.

I (very recently) downsized from a 13cf to a 6cf for just that reason. It's a last resort, get me up safely resort. Too many over complicate the issue. Too many think you need more air than just to surface. Nonsense. If your equipment is well cared for a pony is very good insurance. However, you don't need to carry vastly more than you need.

As an aside, if you want a 13 cf pony i have one that just had hydro & vis and set up with sling. PM me if interested.

Are you assuming a SAC of 1 cu-ft/min in the above referenced ascent scenario? If so, I'm not so sure your numbers work for an ascent from 130 ft using a 6-cu-ft pony bottle? Can you clarify your assumptions/calculations?