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.

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6 replies
  1. Ted Reitsma
    Ted Reitsma says:

    My very first checkout dive, the high pressure hose bust by the first stage. In my first 20 dives, a second stage malfunctioned and I have had 2 free flows over my first 180 dives (one was fist stage issue, other was second stage issue). So I have been diving with pony bottles for years. I am also a self-reliant diver. I love sidemount and use that when I can (back mounts- you need to be strong to lift those suckers). My comment on pony bottle: get one that fits in your luggage without paying extra duty on the plane. Don’t buy the ‘spare air’- its not a lot of air and strapping to your leg creates drag. Lastly my pony bottle is slung on, but I do NOT recommend the straps as seen on the cover picture. hook it up much like a side mount, using a bungie attached like a sidemount at the top to keep it streamlined under or near your armpit. On the bottom, clip like a sidemount, but use a bungie cord that is long and you can this way pull it so it gets snug up to your body (learned this from Sidemounting.com) This is NICE, no dangling, good stream line ( I use cameras a lot and no issues this way). Easy to get on/off boat/short this way too. You only live once, so be safe. It is worth it.

  2. Mark
    Mark says:

    OK, let’s talk about a few things in here that are at best inconsistent and at worst just wrong.
    You list manifolded doubles, two tank sidemount, and H-valve on a single tank as options for redundant air supply, but then later say that your redundant air supply is for emergency use only. For these 3 methods, that is not true at all, as both supplies will be used for regular dive functions throughout the dive (i.e. suit inflation off one reg and BC off the other, for sidemount you will alternate second stages throughout the dive. Your statement about “If you need to use your redundant air source, the dive is over, and you’re in bailout mode.” only applies to the pony bottle option.
    Also, H-valve or Y-valve is not really a redundant gas supply, as you still only have 1 tank, but it is only a redundant regulator and half valve that can be operated separately from the other one.
    A “line” is what you use to moor your boat. That thing that connects your first and second stage regulator components is called a hose.
    Using the rule of thirds for open water diving can really be overkill, depending on the type of dive. For example, most wreck dives are not an out and back type situation, so first of all when you turn the dive after the first third you still have half your bottom time left, so that would be the same amount of laps you’ve already taken, but then when you reach the end of that second third as you get on the boat you have really cut you dive unnecessarily short (I know I’d be upset to come up with 1000 psi in an AL80 having cut 20 minutes off the dive time I could have had using a more a more appropriate open water gas management plan like rock bottom. Thirds is designed for overhead penetration like cave or linear wreck penetration.
    OK, I’d like to end on a positive note: I appreciate your writing this article to encourage divers to always use an alternate gas supply for their own safety. I know I feel a lot safer knowing I don’t need to rely on a buddy for basic life support in the event of any kind of failure on my own gear, so I always have at least 2 tanks, 2 first stages, 2 second stages, and 2 sources of inflation. Often this is sidemount or doubles, but on that rare occasion when I dive single backmount, I always take a pony (AL30) for safety. I recommend this for everyone, as you have here.

  3. Chris Basham
    Chris Basham says:

    I dive as a rule with a 5 or 7 liter bottle. Many think I am nuts, but I point out that I carry the redundant air with me for practice as it is the same configuration when diving my rebreather. Currently, I will drag with me a 11 liter bottle with 32% Nitrox. I will use that down to 40 meter recreational dives or for me in a 40 meter deco dive. since we are discussing recreational dive no deco maximum depth 40 meters a 5 or 7 liter bottle is sufficient. Just this weekend the bailout was actually used. at 22 meters my dive buddy signaled out of air and showed his SPG reading zero. I pulled out the second stage and gave it to him. We did not use my octopus and returned to the boat using about 110 bar of the 5 liter tank. If we used the octopus we both would have had to surface earlier as my air would have been consumed. My view is redundancy is always better.

  4. Ken
    Ken says:

    I and most of my regular buddies dive with redundant air—usually it is a 40CF pony, particularly with the rebreather divers. I prefer sidemount and some prefer backmounted doubles. This style of recreational diving has evolved over years of experience and has not been mandated by agencies or boat captains. Some of us have technical and/or solo certifications. I think it is sound practice for all diving.

  5. aaron
    aaron says:

    i now keep a 40cf with me as a pony as i have come across a few cowboys in the last year on dives where we where close to end of the dive when thay ran out or had a problem i dont want them near me as we come back up so for me i now keep a 40cf on every dive for me it dosent matter if its a 30ft dive or a 130ft ill always have a space 40cf


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