Bailout

Twin set. Link to copyright statement. 2_66_10For many years of diving I had not really thought about bailout; 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. If it was a non-decompression dive I would have a single cylinder with a 3 litre pony. More than enough for an emergency ascent without decompression stops.

Then I got a rebreather. Now, this review of bailout strategies is not all about rebreathers, but it was using a rebreather that made me start thinking more about bailout options. So if you are not into rebreathers just bear with me, because the planning issues apply equally to open circuit.

The trouble with a rebreather is that on-board bailout is just a 3 litre pony, often only half full when you really need it because it has been used as the diluent. So the question arises, just how much bailout time can you get from this?

I won't go into the mathematics, save to say that it is en embellishment of basic gas planning calculations.

Time available ascending on a 3l ponyThe chart shows the time available for decompression stops at 6 metres or 3 metres for a given depth of dive. The bailout supply is a 3 litre pony cylinder, either full (232 bar) or half full (100 bar), and an average RMV (Respiratory Minute Volume) of 25 litres per minute.

A diver carrying a 3 litre pony cylinder for bailout could ascend from 50 metres and complete either 6 minutes of stops at 6 metres or 8 minutes at 3 metres. However, if the pony was only half full at the time of the emergency, they wouldn't even get close to the surface (the time available is negative), and that is all provided their RMV does not shoot up above 25 litres per minute with the stress of the situation.

It also shows that a 3 litre pony cylinder can be a viable bailout option for dives involving short amounts of decompression. The corollary is that for dives involving more than a few minutes decompression a better bailout strategy is necessary, which is pretty much back where I started, using a twin set and rule of thirds.

But again, with a rebreather in mind but equally applicable to a large single cylinder, a twin set and rule of thirds is not the only solution. A diver could carry a larger pony, or a second 3 litre pony, or maybe a 7 litre side mount for bailout.

Time available with a second 3l ponyAn 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 bailout cylinder.

Some divers prefer this to using a twin set. A single large cylinder on the back with the main gas supply, a pony of bailout gas and a pony of decompression mix mounted on either side of it.

Pick the right size cylinders for the dive plan and it can be quite a compact and practical rig.

Personally 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 over half the diluent and nearly all the oxygen available for open circuit bailout, 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. A 15 or 18 litre cylinder of bottom gas, with perhaps a 7 litre cylinder on either side. One of these carries bottom gas for bailout and the other one carries the decompression gas.

H-Twin takeoff. Link to copyright statement. 2_66_03A 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 comfortably back mounted, but raises the need to be able to shutdown taps on the main cylinder in case of regulator failure.

A twin take off on a single cylinder can also be used for bailout with more simple equipment configurations. Dive in the Mediterranean or Canaries and 12, 15 or 18 litre cylinders with twin take offs are much 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 bailout regulator.

So if you don't want to carry larger amounts of bailout, 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 get to their buddy for assistance, to get back to the shot line and a hang tank closer to the surface, or to summon assistance.

It is the third of these strategies that I personally work with. I leave a 10 litre cylinder rigged to 9 metres of rope and a buoy on the boat. The skipper is briefed that if I send up a second delayed SMB then please drop the bailout cylinder as close to it as possible and I will be waiting.

Y-Twin takeoff. Link to copyright statement. 2_66_05I have never needed to use it in anger, but practice runs and one impromptu test have all shown it to work well enough.

The impromptu test happened by accident. I was diving from Len Hurdiss' boat Autumn Dream. Aware that other divers could be above me, I swam well clear of the wreck to shoot my delayed SMB. I got to 9 metres and felt a tugging on my line. I looked up to see another diver above me and gesturing beside me, where I then noticed my emergency bailout bottle dangling beneath it's 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 bailout bottle in.

Carl admits 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 bailout 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 everything was ok.

I don't habitually carry a lifting bag, but maybe I should leave one permanently tied to my bailout bottle for just this eventuality. Anyway, I can vouch for Len being a spot-on skipper when it came to a surprise exercise.

tripple rig, 1 main cylinder, deco and bailout. Link to copyright statement. 2_67_02Taking a more philosophical approach to bailout, should it be viewed as almost compulsory to carry a bailout supply?

When I first started diving, rule of thirds was exclusively for cave divers. Twin sets were only used 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 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 bailout were side effects rather than the primary motivation. But again I soon felt naked when diving without the added safety margin.

As I mentioned above, most of my UK diving is now on a rebreather and I have re-thought my bailout strategies again.

Taking a view that properly prepared equipment is pretty reliable, and that risk should be a personal decision, Richard Stevenson is credited with coining the term "Alpinism". 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 mountain guide in the Alps. I had 2 friends who completed every 4000 metre peak in the Alps 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 that would be hurt would be the climbers themselves."

"No body 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 bailout a personal choice then?"

"My bailout 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 your own mind up."

With Rich's comments in mind, I would probably describe my own diving as Alpinist with a safety rope. Overall I think Alpinist diving has many followers, but not as many as those who like to carry full bailout.

On the kind of extreme exploratory diving that rebreathers are occasionally used for, sometimes the only type of bailout practical is another rebreather. Cave diver Oliver Isler built a triple redundant rebreather for his own cave explorations. A few Inspiration users have engineered doubled up systems known as "Twinspirations".

It is now several years since Rich first posted his Alpinist thoughts, so I asked for his current thoughts on the matter. "I haven't used OC bailout for years now" said Rich. "All last year we dived below 100 metres and to a maximum depth of 153 metres with no bailout and no problems."

"I can understand why people take OC and I would always urge divers to carry bailout as the chances of someone else needing it are far greater than me! Also you can't argue the safety aspect of diving with OC bailout. 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 bailout strategy by generalising this sentiment: You can't argue the safety aspect of carrying bailout, but what you actually do has to be an informed personal choice.

Floppy pony. Link to copyright statement. 2_66_13Rule 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 1/3 in, 1/3 out, and 1/3 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 cylinder 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.

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 are pathetic. It is virtually impossible to get the pony cylinder rigid against the main cylinder and the pony consequently flops about and amplifies any off-centre effect.

Offsetting the pony cylinder for balance. Link to copyright statement. 2_66_16For 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 back pack.


Related pages: