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SOMETIMES I AM AMAZED THAT CERTAIN DIVERS DON'T GET BENT. Take my first dive at Bikini Atoll, for instance. The boat was rigged with a three-level decompression trapeze, parallel bars at 9, 6 and 3m and surface-supplied regulators with a hot decompression mix to the 6m and 3m bars.
All in all, you could hardly get a better set-up for making extended deco stops. So what am I amazed about?
Well, some of the divers had never made a real deco-dive before. They all said they had. They may have crept just past the caution zone on their American-made dive computers and watched the graph move back from red to yellow to green as they made a multi-level dive on a Caribbean wall, but had they ever made a real deco stop? They were not inexperienced divers, but the idea of ascending at a steady rate and then holding level at each stop depth was alien to them.
It wasn't in their training. They had never watched anyone do it. They had never mixed with divers who were into that sort of thing, and they had certainly never done it themselves.
The trapeze became a playground. They got to the 9m stop and started doing loop-the-loops and vertical figure-eights between the bars. Fabio the manager and dive guide must have been having kittens. Funny thing is, they all survived with no symptoms. After some pointed advice, they behaved more appropriately on subsequent dives.
On other occasions I have watched divers do everything right with a suitably cautious profile and end up with a bend. Some may have had a predisposition such as a PFO [hole in the heart], but some didn't. That's the main thing about decompression sickness. There ain't no justice. No matter how well-behaved you are, there's always a chance it could happen to you. It's just one of those risks we all take on every single dive.
I won't be telling you about signs, symptoms and treatment. That's a job better suited to a medical expert. I won't claim that any procedure will eliminate risk completely; that would be blatantly misleading. What these pages are about is good practice in the planning and execution of decompression dives.
It's easy to get into the habit of clocking up a couple of minutes of deco on a dive computer without any real planning. Then the habit gradually stretches, until before long you are making half-hour or longer decompression stops with the same casual approach.
At some point the limit of acceptable risk has been passed. Aborting to the surface having missed a minute of deco is unlikely to have serious consequences, but missing 60 minutes is a much more scary prospect.
The main thing to remember on any decompression dive is that it's a virtual overhead environment. Inside a cave or a wreck it is the rock or the steel that prevents you aborting directly to the surface. On a decompression dive it is the diver's physiology that prevents him from doing the same thing when something goes badly wrong.
It's all about risk-assessment and contingency-planning, something I seem to mention every time I write one of these articles. Which risks are acceptable and which contingencies need to be considered are personal decisions, but whatever your conclusion, at least you will have thought about what you're letting yourself in for.
Not having enough gas to complete the required decompression is something that has cropped up more than once in incident reports. So start with gas-planning. Measure your Respiratory Minute Volume (RMV) and use that to calculate how much gas is needed for a dive.
I won't go into details of the calculations here. If you want a refresher, then paw through your old copies of Diver or look on www.divernet.com for Breathe Easy (June 2001) and Bail-out in the Balance (January 2003).
With a basic requirement, start thinking about various contingencies. What happens if the dive is a little deeper or longer? What happens if a regulator fails? What happens if the work is harder than anticipated and a diver breathes more gas?
The cave-diving rule of thirds is a reasonable starting point for all of these, but is not necessarily enough. If you start calculating extended dive plans and contingencies you will come across situations where a one-third reserve leaves a diver still under water when the last gasp is sucked from the cylinder.
So having come up with a basic plan, the next step is to work backwards and calculate at which cylinder pressure the ascent needs to begin, allowing for the various contingencies.
The ascent should then begin at whichever of two conditions arises first: the planned bottom time is reached, or cylinder pressure approaches the critical level.
When using a dive computer with tables as a backup, a similar calculation leads to how many minutes of deco the computer can safely be allowed to accumulate.
Some divers would argue against this, but it does allow bottom time to be extended beyond the basic plan if reserves are still OK and accumulated stops do not exceed the critical number of minutes. There is of course an added risk - if a dive computer fails after the planned bottom time has been extended, the table schedule is no longer valid.
One of the funny things about contingency-planning is that some scenarios such as regulator failure are habitually planned for, while other scenarios are virtually ignored. How many divers carry a spare mask in case the one they are wearing gets broken or lost? I admit that I don't, but some divers do.
Then there are some things for which it is much more difficult to allow. What happens if a drysuit zip fails? A diver's BC or wing should have enough buoyancy to compensate, but that does nothing for warmth. The HSE Dive Safely video features just such an incident and its unfortunate consequences. If you haven't seen it yet, get a copy.
Some things are so commonplace that we hardly think of them as the consequence of a risk-analysis. Making a bluewater ascent with no reference point carries two risks: the difficulty of maintaining precise control over ascent rate and depth, and the danger of drifting far enough to be lost on reaching the surface.
A simple delayed surface marker buoy solves both problems and is now the preferred method of ascent for most dives involving moderate amounts of decompression - see The Dividends Of Delay (April 2000) or on Divernet.
There are exceptions. Where divers could drift into danger on a DSMB, it may be preferable to ascend and decompress on the shotline. And when deco-stops become too long, it may be easier for a skipper to maintain contact with divers on a line or deco trapeze than on a slowly dispersing shoal of DSMBs.
Both options require reliable navigation back to the shot, perhaps involving a reel and distance-line on the seabed.
Hanging on to a shotline in a current and with waves shaking the line about can aggravate decompression problems in a diver's arms. Last time I found myself in this situation (it was the preferred local system), my buddy got a shoulder bend.
Rather than hang on tightly with one arm, I prefer to hang on loosely with both hands linked over the line, allowing my arms to flex rather than staying tense and rigid. I often use a similar technique to balance on my DSMB reel during stops.
More than a couple of divers on a shotline and it can get crowded. A solution from across the Atlantic is the Jon line, a 2m line with a loop at one end and a clip at the other. At the stop depth, the loop is wrapped round the shotline and the other end passed through the loop so that the Jon line grips the shotline. The clip is then attached to the diver's harness, so he can hang away from the shotline "hands-free", and there is no risk of aggravating a shoulder bend.
Jon lines give divers some room to spread out, but it can still get crowded. A decompression trapeze consists of one or more horizontal bars hanging at the stop depths between a pair of buoys. At the start of a dive, one end of the trapeze is clipped onto the shotline with a weighted transfer line, the purpose of the weight being to make sure that it stays deeper than the trapeze.
In a current the trapeze can be disconnected from the shotline and allowed to drift freely, although this introduces another problem. How does the last diver coming up the line know he is last and that the trapeze can be set adrift?
The last pair of divers could begin their dive sufficiently late that everyone else is already on the trapeze by the time they get to the transfer line. Fine most of the time, but what happens if they ascend earlier than planned?
Another method is for each diver to tick his name off on a slate at the bottom of the transfer line, or to have a distinctive tag that he clips to the bottom of the transfer line on the way down and removes on the way up. But occasionally a diver will forget to unclip his tag.
Divers could agree a time after which they will unclip the transfer line anyway. As they will already be shallower than the main clip connecting it to the shotline, and would not want to descend again, the solution is to have a second clip that can disconnect the transfer line from the bottom rung of the trapeze.
In practice, a combination of all these methods copes with most contingencies, though on a really serious decompression schedule disconnecting the transfer line could be a job for a support diver.
A trapeze offers further advantages. With all the divers in one place it is easier for them to help each other out in the event of problems. The boat can stay close to the trapeze and "fend off" other boats that could otherwise approach too close.
It is also easier to put additional safety measures such as spare cylinders on the trapeze, or even decompression gas supplied from the surface.
Including some deeper stops before getting to the regular stops in a decompression schedule can greatly reduce the risk of DCI.
Most decompression models are based on a series of tissue compartments into and out of which gases diffuse. One method of generating deep stops is to reduce the gradient at which gas is released with "gradient factors". Unless your name is Einstein this can't be worked out in your head, but can be done by a dive computer or PC planning program.
Another method of generating deep stops that also requires a computer is to follow the growth of bubbles, either as the sole means of modelling decompression or in addition to a more conventional tissue-compartment model.
The only method that divers can realistically do in their heads is generally referred to as "Pyle" stops, named after the diver who came up with this rule of thumb: ascend halfway to the first conventionally scheduled stop and wait two minutes; repeat until the distance to the first conventionally scheduled stop is less than 10m.
It's completely arbitrary, but has an established track record and the advantage of being a system that can be worked out in your head and on the fly.
While it works fine with any dive computer, it will take a diver outside any set of tables that were not generated with deep stops specifically in mind. So when used with tables, the safe option is to count the deep stops as part of the bottom time.
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Diver hanging onto a delayed SMB
On the deco trapeze - the diver on the right is using the surface-supplied deco gas
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.
Open-water divers typically allow 2/3 of their gas for the dive and hold 1/3 in reserve. For dives involving extensive decompression stops, this would not necessarily allow enough gas for some contingency scenarios.
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PREPARATION
Make sure you are correctly weighted. You should be neutrally buoyant at 3m with cylinders nearly empty.
Practise in shallow water. The time to test a new equipment configuration is not on a deco dive.
Plan your deco schedule, even when diving on a computer. Include deep stops.
Plan for contingencies: deeper, longer, higher workload, equipment failures.
Plan your gas requirements. Make sure there is enough for each contingency. Rule of thirds is a good starting point, but not necessarily a solution.
BEFORE THE DIVE
Be properly hydrated. Whatever you do, don't start with a hangover.
Check that everything works.
Check all your gas against the plan and that you have the correct mixes.
DURING THE DIVE
Monitor gas, depth and time against the plan. Be prepared to ascend early if anything exceeds the plan.
Any diver can decide to abort the dive.
If anything goes wrong: STOP, THINK, ACT. Don't let a hasty panic decision lead you into inappropriate response. Then assess the success of your actions.
Allow plenty of time to begin your ascent.
ON THE ASCENT
Ascend at the prescribed rates. If you are late arriving at a decompression stop, count the delay as part of your bottom time.
Make deep stops, even if your computer does not require them.
Stay neutrally buoyant or just a little negative at each stop.
Watch your stop depth. Don't go shallower. But beware that being deeper may not decompress you adequately.
Stay horizontal in the water.
Maintain a very gentle exercise. You need to keep up blood circulation without aggravating bubbles.
AFTER THE DIVE
If you have a rich deco mix, stay on it while waiting for the boat to pick you up, while climbing on board and for a few minutes after.
Stay properly hydrated.
Avoid strenuous work.
Monitor yourself and your buddy for any DCI symptoms.
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