For many years I went diving without any real thought of "bail-out", or how to get back to the surface if something went wrong with my air supply.
     If I was making a decompression dive, I would have a twin-set and follow the rule of thirds (see panel). If it was a non-decompression dive, I would have a single cylinder with a 3 litre pony bottle, more than enough for an emergency ascent without deco stops.
     Then I bought a semi-closed-circuit rebreather. Now this review of bail-out strategies is not all about rebreathers, but it was using one that made me start thinking harder about bail-out options. So if you are not into rebreathers, please bear with me, because the planning issues apply equally to open circuit.
     The trouble with a rebreather is that on-board bail-out consists of a mere 3 litre pony, often only half-full when you need it because its contents have been used as the diluent. So the question arises, just how much bail-out time does it provide?
     I won't go into the mathematics, except to say that it is an embellishment of basic gas-planning calculations, which I have covered in these pages before (Breathe Easy, June 2001).
     Table 1 overleaf shows the time available for decompression stops at 6m or 3m for a given depth of dive. The bail-out supply is a 3 litre pony cylinder, either full (232 bar) or half-full (100 bar), and an average breathing rate or RMV (Respiratory Minute Volume) of 25 litres per minute.
     The table shows that a diver carrying a 3 litre pony for bail-out could ascend from 50m and complete either 6 minutes of stops at 6m or 8 minutes at 3m. However, if the pony was only half-full at the time, the diver wouldn't even get close to the surface (the time available is negative). The situation could be even worse, as the diver's RMV may well shoot above 25 litres per minute in a stressful situation.
     So a 3 litre pony can be a viable bail-out option for dives involving short amounts of decompression, but for dives involving more deco, a better bail-out strategy is necessary. Which brings me pretty much back to where I started, using a twin-set and rule of thirds.

But whether with a rebreather or a large single cylinder in mind, this is not the only solution. A diver could carry a larger pony, or a second 3 litre pony, or perhaps a 7 litre side-mount for bail-out.
     Table 2 shows how much the situation improves with a second 3 litre pony. An advantage of a second pony is that it could be filled with a gas more suited to decompression, such as 50%, 80% or pure oxygen, with the proviso that the mix must be compatible with whatever depth you can get back to on the first bail-out cylinder.
     Some divers prefer this to using a twin-set: a single large cylinder on the back carrying the main gas supply, with a pony of bail-out gas and a pony of decompression mix mounted on either side of the main tank. Pick the right-sized cylinders for the dive plan and it can be quite a compact and practical rig.

I like to mount any cylinder of deco mix where I can get to the tap and leave it turned off until I need it. The conventional option is to side-mount it.
     Alternatively, to keep everything on the back, either one or both pony cylinders can be carried inverted.
     Funnily enough, this configuration is not that different to the configuration used with a rebreather. Start a dive with full cylinders of oxygen and diluent and there will typically be more than half the diluent and nearly all the oxygen available for open-circuit bail-out, as long as open-circuit regulators are attached to both supplies.
     Unfortunately, this isn't always the case. Rebreather-users rarely top up the diluent cylinder between dives during the day, and at the end of a weekend's diving the oxygen cylinder could easily be less than one-quarter full.
     A larger version of a triple back-mounted rig has gained some popularity for use with intermediate-depth trimix where a travel mix is not required. This consists of a 15 or 18 litre cylinder of bottom gas, with perhaps a 7 litre cylinder on either side, one carrying bottom gas for bail-out, the other the decompression gas.
     A variation which extends the capability slightly is to have a twin take-off from the cylinder of bottom gas, with one of the smaller cylinders carrying a travel mix. Again, it can all be back-mounted comfortably, but raises the need to be able to shut down taps on the main cylinder in case of regulator failure.

A twin take-off on a single cylinder can also be used for bail-out with more simple equipment configurations. Dive in the Mediterranean or Canary Islands and 12, 15 or 18 litre cylinders with twin take-offs are far more common than manifolded twin-sets. Getting away from open circuit, some semi-closed-circuit rebreather users are fitting twin take-off taps to their main supply, one to feed the loop and one for an independent bail-out regulator.
     So if you don't want to carry larger amounts of bail-out, what other strategies are possible?
     A 3 litre pony may not be enough to get a diver to the surface, but it could be enough to get a diver to another gas supply. It could provide enough time for a diver to reach a buddy for assistance, to get back to the shotline and a hang-tank closer to the surface, or to summon assistance.
    
     It is the third of these strategies that appeals to me. I leave a 10 litre cylinder rigged to 9m of rope and a buoy on the boat. The skipper is briefed that if I send up a second delayed SMB, please drop the bail-out cylinder as close to it as possible and I will be waiting.
     I have never needed to use it in anger, but practice runs and one impromptu test have shown it to work well enough.
     The impromptu test happened by accident. I was diving from Len Hurdiss's boat Autumn Dream. Aware that other divers could be above me, I swam well clear of the wreck to shoot my delayed SMB. I reached 9m and felt a tugging on my line. I looked up to see another diver above me and gesturing to the side of me, where I saw my emergency bail-out bottle dangling beneath its buoy.
     It took me a few seconds to work out what had happened. Seeing Carl's delayed SMB next to mine, Len had thought I had sent up a second delayed SMB, assumed the worst and dropped my bail-out bottle in.
     Carl admits that he had a bit of a shock when my delayed SMB shot up through his legs, then a second shock a minute later when my bail-out bottle dropped right in front of him. He soon realised what was happening, that it was a false alarm, and helpfully sent it back up on a lifting bag to let Len know that everything was OK.

I don't habitually carry a lifting bag, but perhaps I should leave one permanently tied to my bail-out bottle for just this eventuality! Anyway, I can vouch for Len being a spot-on skipper when it came to a surprise exercise.
     Taking a more philosophical approach to bail-out, should it be viewed as almost compulsory to carry a bail-out supply?
     When I started diving, the rule of thirds was exclusively for cave-divers. Twin-sets were used only in open water for increased gas supply, not for redundancy. Manifolds were designed to provide only a single take-off from a pair of cylinders, effectively making them into one large cylinder.
     I did many decompression dives with a single cylinder and miserly air consumption and felt perfectly safe. Then I got a pony cylinder and felt naked without it. Then I started using twin cylinders and regulators as a means of carrying more air.
     Redundancy and bail-out were side-effects rather than the primary motivation. But again I soon felt naked when diving without the added safety margin.
     Now, with most of my UK diving on a rebreather, I have re-thought my bail-out strategies again. Taking a view that properly prepared equipment is pretty reliable, and that risk should be a personal decision, Rich Stevenson is credited with coining the term "Alpinism". This is an approach he applies to rebreather diving, but can equally be applied to open-circuit diving.

To precis Rich: "Before I became interested in diving, I worked as a mountain guide in the Alps. I had two friends who completed every 4000 metre peak over a few weeks in succession, using nothing other than what they carried on their backs. This type of climbing was called Alpine Style.
     "Climbers took this style of climbing to the Himalayas, and naturally more of them died, but this was regarded by the climbing community as acceptable, as they knew the risks, and the only ones who would be hurt would be the climbers themselves.
     "Nobody wants to die diving, but we seem to place a lot more emphasis on returning safely after a dive than with other adventurous pursuits. Is taking bail-out a personal choice, then?
     "My bail-out cylinders have been used more times by other divers than by me. This doesn't mean that it will never happen, but you have to examine your own statistics and then make up your own mind."
     With Rich's comments in mind, I would probably describe my own diving as Alpinist with a safety rope. I think Alpinist diving has many followers, but not as many as those who like to carry full bail-out.

On the kind of extreme exploratory diving for which rebreathers are occasionally used, sometimes the only type of bail-out practical is another rebreather. Cave-diver Oliver Isler built a triple-redundant rebreather for his own cave explorations, and a few AP Inspiration-users have engineered doubled-up systems known as "Twinspirations".
     It is now several years since Rich Stevenson first posted his Alpinist thoughts on the Inspiration-users' e-mail list, so I asked for his current thoughts on the matter.
     "I haven't used open-circuit bail-out for years now," he said. "All last year we dived below 100m and to a maximum depth of 153m with no bail-out and no problems.
     "I can understand why people take open-circuit and I would always urge divers to carry bail-out, as the chances of someone else needing it are far greater than me!
     "Also, you can't argue against the safety aspect of diving with OC bail-out. The point I was trying to make on the Inspiration list was that it really is a personal choice."
     So I will conclude this discussion of bail-out strategy by generalising this sentiment: You can't argue against the safety aspect of carrying bail-out, but whatever you decide to do has to be based on an informed personal choice.

twin take-offs as an approach to bail-out - using an H-valve


and with a Y-valve


How not to do it - a loose pony bag flopping about and pulling everything off centre


a pony cylinder held rigid with a quick-release clamp


using an extra-long camband to hold the main and pony cylinder slightly off-centre

RULE OF THIRDS The "rule of thirds" comes from cave-diving, where a diver does not have the option of simply surfacing if something goes wrong. It is based on planning gas usage as a third in, a third out and a third in reserve. Spread across two cylinders, this means that at any point in the dive, should the regulator on one cylinder fail, there should be enough gas in the other one to get back out of the cave. It applies equally well to open-water dives involving decompression with gas supply, where the ceiling is a decompression ceiling rather than the roof of a cave.

RIGGING A PONY Some divers don't like pony cylinders because, mounted out to the side of a main cylinder, it pulls them off balance (see My Little Pony, Deep Breath, October). There are a few considerations here. The bags and straps supplied with most pony cylinders are fine for protection against knocks, but as a mounting system they are pathetic. It is virtually impossible to get the pony cylinder rigid against the main cylinder, so the pony flops about and amplifies any off-centre effect. For comfort, a pony cylinder has to be mounted rigidly against the main cylinder and as close-in to the backpack as possible. The best systems are sets of cam-bands specially designed to hold the pony cylinder, or quick-release clamps attached to each cylinder with stainless jubilee bands. It is no use having a rigid clamp if the bands holding the parts to either cylinder are floppy. Finally, if you still feel that a single pony cylinder pulls you off balance, try mounting the assembled main cylinder and pony slightly off-centre to compensate. I like to use an extra-long cam-band to go round both main and pony cylinders and hold them to the backpack. This keeps the weight of the entire assembly central on my back and pulls the pony in close to the backpack.

Table 1: Decompression minutes possible on 3 litre pony
RMV:  25 lpm
Full, 696 litres free at 232 bar
Half-full, 300 litres free at 100 bar
Max depth	Time at 6m	Time at 3m	Time at 6m (half-full)	Time at 3m (half-full)
10m	16.5 min	20.3 min	6.6 min	8.1 min
20m	14.9 min	18.3 min	5 min	6.2 min
30m	12.7 min	15.6 min	2.8 min	3.5 min
40m	9.9 min	12.2 min	0 min	0 min
50m	6.5 min	8 min	-3.4 min	- 4.2 min
Table 2: Decompression minutes possible with second 3 litre pony
RMV:  25 lpm
Full, 696 litres free at 232 bar (Pony 1 and 2)
Half-full, 300 litres free at 100 bar (Pony 1)
Max depth	Time at 6m	Time at 3m	Time at 6m (half-full)	Time at 3m (half-full)
10m	33.9 min	41.7 min	24 min	29.5 min
20m	32.3 min	39.8 min	22.4 min	27.6 min
30m	30.1 min	37.1min	20.2 min	24.9 min
40m	27.3 min	33.6 min	17.4 min	21.4 min
50m	23.9 min	29.4 min	14 min	17.2 min