The Newry & Mourne Sub Aqua Club - Technical Issues

Technical Issues


A diver wishing to carry out deco-stop diving can choose from a number of methods. Their ease of deployment and effectiveness depends on the conditions in which you find yourself diving. These are some of the options, with their advantages and disadvantages:

Static-line decompression, as the name implies, involves the diver using a fixed shot or anchor line to carry out his decompression schedule. This provides a very solid line and one to which spare gas can be attached, and the boat cover knows exactly where the divers will surface.

The main disadvantage is that the diver must navigate back to the shotline. Many British divers are not keen on this system, diving as we do in moving water. I favour this type of decompression, even in moving water, and use a Jon line, which is attached to the shotline at one end and to the scooter ring on my harness via a karabiner at the other. I face into the current, lie horizontally and relax.

Delayed SMBs are very popular with UK divers, with the diver shooting a bag to the surface using a reel or dropline before he ascends or at a midwater level, and drifting under the DSMB. The diver is clearly marked for the surface cover to monitor his position in the water and this is a relatively small piece of equipment for the diver to carry.

A disadvantage is that the surface cover might have to follow a number of DSMBs, which could be spread over a wide distance. If one diver has a problem and the DSMB is a long way off, will the surface cover know he is in trouble?

Generally this system works well, and I use it as a back-up, but divers must be able to deploy the DSMB without difficulty. We have seen many incidents with divers getting snagged, dragged to the surface, etc. Take every opportunity to practise the skill and never attach the reel to yourself when shooting the bag.

Decompression trapeze systems are a combination of the above methods, providing one big SMB for a group of divers. The divers must ascend the shotline but cross over to the trapeze via a transfer line. Once they are all on the trapeze, the transfer line is released from the main shot and the group drifts while decompressing, with the boat cover following.

Most systems have two or three bars at 9, 6 and 3m. These bars are 2-3m long and about 5cm in diameter. Spare decompression gas can be attached to the trapeze and the divers can either hold onto the bars or drift alongside them.
The trapeze is rolled up from the bottom and deployed from the boat before the dive. With the transfer line attached to the main shot, this process can take less than a minute.

One advantage is that if a diver needs assistance under water during decompression, there are others around to lend a hand. The primary disadvantage is again the need to ascend the main shotline at the end of the dive.
It is easy to attach useful accessories to the trapeze for divers on long decompression stops, and we drink fluids using plastic containers with bite release valves.
I have also seen divers with reading matter and slates for games. Or, just chill out.

Deep stops and dive computers
Reading around decompression theory, you may have read that the pressure gradient should be minimised to reduce the chances of DCI, micro-bubble formation and fatigue after diving. Technical divers have observed that many ailments can be avoided by including "deep stops" in their profiles. The "deep stop" can be calculated using PC software. Could you combine this idea of deep stops in recreational decompression-stop diving while using your dive computer?

The answer is yes, but let me explain the thinking behind deep stops first, for those who might not have heard of this technique. The basic idea is to slow the ascent rate and allow the body to get used to pressure changes gradually during decompression, before the first compulsory decompression stop.

As you know from your reading, we have been using this technique for many years. I am a firm believer in it, as it has all the advantages listed.
You might also hear divers refer to these stops as "micro-bubble stops" or "Pyle stops", after the US deep diver Richard Pyle, who has used this method for many years.

Deep-stop depths are calculated by halving the distance between the maximum depth and the first compulsory stop. At this depth a short stop is carried out, usually of about one minute. The next deep stop is worked out in the same way, by calculating the halfway point between the depth of the first deep stop and the first compulsory stop. This process continues until the diver reaches that first compulsory stop.
This could be done using a dive computer to do the above calculation, though this might be problematic should you experience a little narcosis during the dive.
However, as a simple rule I generally put in at least one deep stop during the ascent when I'm diving with a computer.
For example, if my maximum depth was 50m and my computer stated that my first stop was at 9m, I would pull a deep stop at 30m for one minute.
If you wanted, you could do another one at 20m.

With regard to decompression, your computer will calculate the required compulsory stops taking into account the deep stops. It will not shorten your schedule, because you are still loading inert gas into some tissues, but it will give you the deep-stop advantage.

Even better would be to make a gas switch to a nitrox mix at the deep stops. This will shorten your stops if you use a two-mix nitrox computer, or add a safety factor if you are using an air computer.

Is your stage cylinder big enough?
The size of cylinder you require depends on the amount of time and depth your deco schedule demands. Another factor is your gas consumption rate, which can vary considerably from diver to diver and from dive to dive.

All divers should know their breathing rate and how to calculate the gas required for the whole dive. There is nothing worse than finding out that you have too little during decompression.

To find out how much you require, the calculation is RMV x P x T = litres.
* RMV stands for Respiratory Minute Volume, which means how much gas a diver breathes on the surface.
* P is the absolute pressure in bar at the depth at which you're decompressing.
* T is the time you spend at that depth.
Take a dive that requires you to carry out a deco stop at 6m for 20 minutes. You have an RMV of 18m per minute.
Our calculation would look like this:
RMV x P x T = litres
18 x 1.6 x 20 = 576 litres
For decompression gases we generally work on double the amount required to cover any emergency situations. So multiply the answer by 2 to obtain a final figure of 1152 litres.

This calculation is carried out for each decompression stop to work out the total amount of gas required. Then you can work out the size of the cylinders you will need to carry it.

In recent years I have seen divers using 7 litre cylinders for decompression. These were fine for the type of diving they were doing initially, but as their depths and times increase, they are simply not large enough for more serious decompression, taking into account the correct reserve levels.
I urge divers who need a larger-volume cylinder to use it. Carrying a 10 or 12 litre is no more difficult than a 7 litre.