Deco for Beginners
How best to shed that unwanted nitrogen we accumulate while we’re under pressure is a question we face on every single dive. But mastery of buoyancy control, some knowledge of different gas mixes and the right computer can dispel the uncertainty, says John Bantin
The human body has a great capacity to take change in its stride, provided you give it time to do so. We have all been taught that coming up from one depth to a lesser depth when there is a great pressure difference can cause decompression sickness (DCS), so we take it easy. Give our bodies time, and we can accommodate the change without ill-effect. The human body is a wonderful thing. In 30 years of diving, including 14 as a professional with thousands of dives with almost as many different divers under my belt, I have never knowingly seen anyone suffering from DCS. This must say something for both diver training worldwide and the assumptions of the physiologists who calculate our decompression tables and diving computer algorithms. Every dive is a decompression dive. On every dive you put your body under hydrostatic pressure and then take the pressure off again. Even if you are doing a dive that requires no formal deco stop, the speed at which you come up is part of your decompression schedule. I am not a physiologist but I do know that my body is extremely complicated. It’s certainly more complicated than that bottle of soda-water so often used to demonstrate the ill-effects of DCS in basic diving theory. However, I have come to trust my diving computer (and I have used a lot of different ones in my time) because it seems to work.
I know that my body will absorb inert gas. I know, as I sit here and write these words, that it is saturated with nitrogen from the air at 1 bar of atmospheric pressure. I know that if I go up the Matterhorn I should take things easy until my body has produced extra red blood cells to cope with the reduced oxygen levels in the air and I have adjusted to the reduced pressure.I know that I can subject myself to the much greater pressures found under water, provided I give myself time and take the pressure off carefully afterwards. Under water, it is the inert-gas element of the air that I breathe that gets soaked up, rather than the oxygen that I can absorb or metabolise.
So what is the secret of this controlled ascent? Buoyancy control! A diver should be able to make himself neutrally buoyant at any time. In other words, he never sinks or floats upwards unless he wants to. This is done firstly by wearing the correct amount of weight applicable to the rest of his diving equipment, especially his suit; and secondly, by judicious use of the direct-feed control and dump-valves of either BC or drysuit. You can ascend using only the ascent-rate indicator on your computer, but an easier and more satisfying way is to give yourself some visual datum. If you are diving on a coral reef that comes up close to, or breaks, the surface, it’s easy. Just slowly follow the reef up and enjoy what you may see on the way. Remember to stay away from the reef once you reach the shallows, however, or you may get pushed over onto the top by the breaking waves. This will make picking you up by boat a little awkward, or even dangerous for you. If there is no natural datum, use the boat’s anchorline or a shotline that has been put in to mark the dive site.You should have planned your method of ascent before you went near the water, so if none of these options is available you may have to use your delayed surface-marker buoy. This open-ended bag, often sausage-shaped, can be filled with air at depth and sent to the surface, attached by a line deployed from a winder-reel or a shorter length of narrow webbing with a small weight on one end. Naturally you should not deploy it from a depth greater than the length of line or webbing available. Webbing is nicer to hold on to but can be carried only in short lengths. If you are diving in an area subject to currents, you can deploy your buoy from depth with a reel and this will clearly mark your position to your boat or surface cover from the moment you start your ascent. Seventeen years ago the British Sub-Aqua Club introduced its 88 Decompression Tables, with a mandatory stop at 6m. Many British divers complained at the time that it was impossible to stop at that depth, thus revealing a history of poor buoyancy control. When it was suggested that it was a good idea to be slightly heavy and hang on a buoy line, one group of divers from my local club were heard to say that it would not be effective because you would be going up and down with the waves! Always remember, depth is the vertical distance measured from the surface. Whatever system you use as a datum, it is nearly always best to be neutrally buoyant. Only very experienced divers have the control and discipline to come up carefully in bluewater conditions.
Nitrox and MOD
Give your body time to adjust. It’s clear that if you can reduce the amount of inert gas (the nitrogen) you breathe, you will absorb less in the first place. This is where nitrox mixes come in. By breathing less nitrogen, you subject your body to less stress. The convenient way to do this is to substitute more oxygen for nitrogen in the mix we call “air” (21% oxygen, 79% nitrogen). After all, oxygen is readily available. Nitrox is air enriched with extra O2. The only problem is that oxygen can be a hazardous gas when breathed under pressure, so as we increase the proportion of oxygen in the mix we reduce the pressure and therefore the depth at which it is safe to breathe it. This gives us the “maximum operating depth” of the mix. Some older divers like me can remember going to great depths breathing nothing other than air, but currently air is thought to have an MOD of 56m. Training agencies, ever with a mind to litigation, now usually opt for a 40m limit or even shallower, depending on a diver’s certification level. We should not lose sight of the fact that the human body has been designed or evolved primarily to breathe air with 21% O2 (nitrox 21) at sea-level. We can also damage ourselves by exposing ourselves to high levels of oxygen and higher-than-normal pressures for too long. All this is covered in the basic nitrox course. No-one should use nitrox unless they have been trained to do so. As the diver who has been breathing air at depth ascends, the pressure on him is reduced and the nitrogen absorbed by his body is passed from his tissues back into his bloodstream. It then turns back to gas in his lungs and is exhaled. He must allow sufficient time for this to happen or it will come out of solution within his tissues, causing damage. As he approaches the surface, the pressure changes get more acute. The pressure at 40m is 5 bar and the pressure at 30m is 4 bar, a ratio of 5:4 over 10m of ascent. The pressure at 10m is 2 bar but the pressure at only 5m above is still 1.5 bar, a ratio of 4:3 over only 5m. So you should be even more careful as you get nearer to the surface. Most contemporary computers now have a safety stop designed-in, and this will be displayed in the 5m to 3m zone. It’s a way for computer manufacturers to add some safety and deters users from making an impatient dash to the surface. Even though you’re off-gassing, you still have to breathe. And every inhalation during the ascent carries with it more nitrogen for you to absorb while trying to off-gas that which you have already absorbed. It’s a complex process. So what if you could find some way of reducing the amount of nitrogen going in during this stage of the dive? That would speed up the process of getting it out from your body.
This is where some divers will use a richer nitrox mix as a “decompression gas”. By switching to a rich nitrox mix (one with a greater percentage of oxygen and therefore less inert gas, one that would have been dangerous to use at depth)in the shallows, the pressure gradient at the lungs between the nitrogen in the body and that of the gas being inhaled is increased. This means that the offending nitrogen already absorbed is off-gassed far more quickly. Of course, the diver must have the discipline to use this gas only when he is sufficiently shallow to do so safely. Switching to the wrong gas at depth could be fatal. In fact, if a diver is prepared to carry the required number of tanks during a dive, he could break his ascent up into three parts, He could breathe air at a maximum depth of say 50m, swap to nitrox 32 at 30m, and then swap again to nitrox 50 at 18m. This would give him a lot more time at depth, combined with more manageable deco-stop times than he could otherwise safely take, and give his body time to adjust to the changes.
Many of the recently qualified technical divers reading this will howl with derision at the idea of breathing air at 50m. They think everyone should use trimix. This is a breathing gas where both the percentage of nitrogen and oxygen have been reduced by the inclusion of a third inert gas, helium.Helium reduces the effects of that other problem many divers encounter, nitrogen narcosis, but it adds to decompression times because it is more readily absorbed by the body. If you live in an industrialised country where helium is available and you can afford it, that’s all very well. But trimix diving is a whole new ball game. Some professional divers will complain that you should not be breathing any nitrogen in the mix, that you should breathe a mix of oxygen and helium only, but then things get even more complicated, and much more expensive. Alas, if you look at a globe of our world you will notice two things. The first is that a great deal of it is blue, and the second that most of the best diving areas within the blue line lie a long way from the shores of industrialised countries. This is where most of the best diving is. Unless you think this sort of diving should be limited to an elite few who can afford to mount expensive expeditions to these places, taking all their helium with them, you are left with the option of breathing oxygen and nitrogen in the proportions best-suited to the job.These gases are available in our natural atmosphere and pure oxygen can be relatively easily generated.
Air as the bottom gas may not be ideal, but it is readily available. If we break up our dive into three sections, we can speed up our decompression, have more time available at depth, and spend less time hanging about on the way up. But how can we calculate the decompression times for this?
Enter the physiologist with his algorithm and the electronics expert with his computer. What we need is a nitrox-compatible computer that can be adjusted during the dive to calculate the exact decompression requirement needed for a particular nitrox mix at a given time.
When you change regulators, you change the pre-selected computer setting to match. Examples of computers capable of doing this are the DiveRite Nitek3 and Nitek Duo, the Apeks Quantum and Pulse, Delta P VR2, Cochran Commander and the Suunto Vytec and D9.
Examples of computers that can manage trimix dives are the Delta P VR3 and Dive Rite Nitek He.