Thanks to SDI/TDI and Mark Manthey for this article.
Diving with redundant air source for recreational divers
By Mark Manthey
Diving with an alternate air source (octopus) is something all divers do. However, not many recreational divers use a REDUNDANT air source. That is a completely independent air source (second cylinder, first stage, and second stage regulator set). For most recreational diving, your buddy is your redundant air source. However, in the Great Lakes region, where divers often dive below 100 feet in cold water, redundant air sources such as a pony cylinder are common, almost standard equipment for recreational divers.
So, where, when, and why dive with a redundant air source?
Should these dives be considered a technical dive? Now, that’s a great question, but we won’t address that here. Clearly, the ability to bailout of a dive, and to be self-reliant, gives divers in a catastrophic gas loss situation (such as second stage freeflow) more, better and safer options to surface safely without panic, without the assistance of a buddy.
· Depth, due to the longer ascent time. At 130ft or, 40m, assuming a 30ft, or 10m per minute ascent rate, and a 3-minute safety stop, the total ascent time would be about 7 minutes. Depending on the gas loss rate, there may not be enough gas in the cylinder for the diver to safely surface.
· Overhead environments such as a LIMITED penetration wreck dive, or ice diving. (This kind of diving may not require decompression)
Many options are available for diving with a redundant air source;
· Back mounted doubles with a manifold. This configuration is favored by many technical divers. The manifold allows the diver to access the gas in both cylinders from a single regulator, and each cylinder has a first and second stage regulator. It may sound like overkill, but it’s a great setup, and I dive with them as a recreational diver often, on deep dives.
· Sidemount. In recent years, sidemount configurations have gained popularity, and for good reason. Sidemount offers two independent cylinders, and easily accessed valves. Many sidemount divers use two 80cf aluminum cylinders, as they are widely available on nearly any dive boat, offering the diver 160 cubic feet of gas.
· Single cylinder with H-valve. The H valve offers the redundancy of doubles, on a single cylinder. It’s compact and lightweight.
· Pony cylinder. Pony cylinders come in a wide variety of sizes and mounting options. The key is to find the one right for your planned dive. Pony cylinders range from just two or three cubic feet, to 40 cubic feet. A small pony cylinder may be only good for a minute or two for deep dives but should be ok for bailout on shallow dives. In my estimation, its best to get larger pony cylinders. If you’re going to make the investment, you should give yourself the largest margin of safety for the money you spend. Pony cylinders have a variety of mounting options.
Again, you should choose the one right for you. Generally, they are either slung on the left side, or mounted on the right side of the divers’ main cylinder by a mounting bracket. I prefer to sling them. This allows the diver to easily manipulate the valve or hand off the cylinder to another diver if needed. A back mounted pony keeps the cylinder neatly tucked out of the way, and is favored by public safety divers, because it keeps it from dragging through the mud and muck they commonly find themselves in. However, this setup makes it difficult to manipulate the valve if needed.
Diving with redundant air sources offers safer and better options in an emergency, however, there are some issues that should be considered.
1.Plan your dive, dive your plan
First, there is no good reason for running out of air under normal diving conditions. Plan your dive, dive your plan. Use the rule of thirds and calculate a turn pressure that allows you to safely reach your exit point with adequate air pressure remaining. All divers should be in a position to surface with roughly one-third of their remaining gas pressure. Redundant air sources are for emergency use only and should not be used in calculating the depth and duration of your dive. If you need to use your redundant air source, the dive is over, and you’re in bailout mode.
2. Practice using it
Second, if you’re going to dive with a redundant air source, make sure you and your dive buddies are familiar with how to use it, and practice, practice, practice deploying it. Did I mention you should practice deploying, and using, a redundant air source? Ok, good. It is spooky how quickly learning how to deploy a pony becomes second nature. When diving with a pony, for example, I dive with the regulator charged and the valve shut off, to prevent an accidental gas loss. I make sure all my buddies know that if they need to use it, it will only have two or three breaths in the line until I can open the valve. I know some divers that always dive with their pony valves open. It’s not right or wrong, just a preference.
3. Valve shut off
Valve shut down is not a skill generally learned at the recreational level of diving. However, in a situation where a diver is having a catastrophic gas loss, I firmly believe it to be appropriate for the affected valve to be shut down. Shutting down the valve will save the remaining gas in the cylinder, and it could then be used later if needed. This also needs to be planned and practiced, practiced, practiced with buddies. I have had second stage freeflows myself, on occasion, and have assisted several divers with them. The freeflow is almost always caused by second stage icing due to cold water. Once the valve is shut off for a minute or so, the regulator has a chance to thaw, and will function normally again once the valve is turned back on. In my personal preference, I always position the cylinder on my BCD high enough so that I can shut down my own valve. A buddy may also assist with valve shutdown and opening. If the gas in the cylinder is simply allowed to deplete, the diver will lose the ability to power inflate their BCD or drysuit, once they reach the surface. They would then have to remember to orally inflate. This one simple thing could cause an already distressed diver to panic.
In conclusion, diving with redundant air sources allows a diver to mitigate some risk, however, they also come with risks of their own. I believe it is impossible to mitigate one risk without accepting a risk of one sort or another. Every diver must make that decision for themselves.