Thailand – Australia – United Kingdom

Extend your diving

If you ask the majority of divers to define technical diving you will probably end up with a definition at least partly based on depth.

Koh Tao, Thailand

Although the ability to dive deeper than recreational limits has always been one of the major aspects of technical diving it is by no means the only aspect. Technical diving is about extending your diving and this extension can develop along a number of different dimensions.

One of the biggest advantages to using technical diving techniques is in extending the length of the dive rather than the depth. For example a recreational diver with a Deep Diver specialty is certified to dive to 40m. However, 40m recreational dives don’t make a lot of sense. The no stop limit for 40m is between 8 and 10 minutes depending on the tables you use. If you have spent three hours driving to the coast and another two hours on a boat to get out to a wreck then that is a round trip time of ten hours for a ten minute bottom time. That’s an hour traveling time for every minute on the wreck. Similarly if we add up the cost of the fuel for the drive, cylinder fills for the dive, charter fee for the day, possibly overnight accommodation and all the other costs then it results in a huge cost for each minute spent on the wreck.

Of course that ten minutes also includes the descent time so we often end up spending only five minutes on the wreck itself. If the tide is running or you are dropped on a part of the wreck where there are no interesting features then this can be reduced even further. If we know that we only have a few minutes on the wreck then we tend to rush more and as a result may miss interesting points. The temptation to rush the descent in order to get more time on the wreck combined with the overall time pressure also increases the risk of nitrogen narcosis. Thinking back to my own experience of 40m no stop dives I can remember lots of enthusiasm and excitement before the dive followed by a feeling after-wards that the dive didn’t quite live up to expectations. I always felt that I hadn’t really seen the wreck and had no feel for how it was lying and the structure of the wreck. There was always a feeling that although that dive was a bit of a disappointment the next dive would be better and I would get a better view of the wreck next time. On those dives where we were lucky and had fantastic visibility there was a huge feeling of regret that we couldn’t stay longer on the wreck.

Technical diving and specifically decompression diving offers the possibility of significantly increasing the time you can spend on the wreck with only marginal increases in cost, effort or risk. Once you are properly equipped and trained for decompression diving then the no stop time disappears as a limitation of your diving. If you are prepared to undertake a certain amount of decompression stops on the ascent then you can pass that no stop time and enjoy a significantly increased time on the bottom. For example, if we increase our bottom time from ten minutes to 20 minutes then we in effect more than double our actual time on the wreck as the descent time stays the same. This means that we are getting more than twice as much time on the wreck for the same cost, the same travelling time and almost the same effort. This extra ten minutes will give us much more time to explore the wreck and get a feel for the layout. We can certainly see more of a wreck in a single 20-minute dive than we will in two ten-minute dives. Now the price of this extra time is that we will have to perform decompression stops on the way back to the surface. In this case a 20-minute bottom time will give us 11 minutes of decompression stops. This seems to be well worth the effort. An additional 11 minutes of decompression at the end of the dive doesn’t seem to be an unreasonable cost to pay for the increased time on the bottom. After all getting time on the bottom is why we dive.

As we increase the time on the bottom the amount of decompression also increases. If we do a 30-minute bottom time then we will have to do 30 minutes of decompression on the way back up. While 30 minutes on a wreck is very appealing, 30 minutes of decompression is now starting to become a significant price to pay. For many divers 30 minutes of decompression is a significant amount and they start to think that the benefit to cost relationship is starting to swing too far towards the cost. In addition, as the decompression obligation increases the gas reserves that are required start to increase. However there are additional ways that technical diving techniques can start to help.

The above examples were all worked out assuming that the diver is breathing air. However, by changing our breathing gas we can gain a further advantage. For no stop dives at 40m the advantage of nitrox is minimal. The risk of oxygen toxicity means that we can only use relatively weak nitrox mixtures which have a minimal effect on no stop times. Using EAN28 at 40m increases our no stop time from eight minutes to 12 minutes. Although this is a 50 per cent increase the additional cost of a nitrox fill for four minutes extra on the bottom is often judged to be not worth it. However, for decompression dives the advantage becomes more noticeable. In the example above, a dive to 40m for a bottom time of 30 minutes resulted in 30 minutes of decompression if we are breathing air. On the other hand the same dive, using EAN28 rather than air, gives only 17 minutes of decompression.

Although this is less than the 50 per cent advantage on the no stop dive a saving of 13 minutes is much more noticeable and is often considered to be well worth the cost and effort of using nitrox rather than air. We have already established that we can see more of the wreck in a single 20-minute dive than in two ten-minute dives and this principle is even more applicable for a 30-minute dive. We have the time to properly explore the wreck, to investigate certain areas in detail and even to start to penetrate into the interior of the wreck. We can often swim the length of a wreck and see the bigger picture of the shape, condition and layout of the wreck. It’s not until you start to do longer dives on a 40m wreck that you can even start to get an overall feel for the wreck and really start to get to know it. For wreck researchers this length of bottom time is very valuable in providing time to be able to start identifying features and begin the process of identification.

In addition to using nitrox as our main breathing gas, technical diving techniques also allow us to switch to a second nitrox mix during the decompression in order to further optimise our decompression. Decompression stops allow time for dissolved nitrogen to come out of the body. We cannot ascend any higher until a certain amount of nitrogen has been allowed to escape. If we do ascend before the required amount of nitrogen has been allowed to escape then it will form bubbles which is the cause of decompression sickness (DCI). When we switch to a second nitrox mixture with a higher percentage of oxygen, and by implication a lower percentage of nitrogen, the nitrogen dissolved in our body comes out faster and so we don’t need to wait as long before we can move up in the water. This is why the technique is known as accelerated decompression, although some people prefer the term optimised decompression.

By using EAN50 as a decompression gas we can reduce our decompression to 13 minutes and by using EAN80 or even 100 per cent oxygen we can reduce it to just nine minutes. Alternatively we could further increase our bottom time to 35 or even 40 minutes until we reached the maximum amount of decompression time we were happy to do. Of course you don’t get anything for free and using this technique introduces a number of other considerations that the diver will need to take into account. In particular, much more detailed gas planning and the risk of oxygen toxicity must be considered. This is why specialised training is required in order to make use of these specific tools. In particular the decompression times and dive practices contained in these examples should not be taken as correct and are only provided for illustration (that should keep the lawyers happy).

It should now be starting to become clear why some qualifications limit the time as well as the depth of the dive. There is only so much trouble you can get into on a no stop dive whereas there is more potential for problems on a decompression dive. Recreational dive qualifications limit divers to no stop times to limit the risks they are exposed to. Some other courses limit the diver to 15 minutes of decompression and are sometimes referred to as advanced recreational courses rather than full technical courses. As the diver progresses to full technical courses then the skills they will learn and the planning techniques they will use means that there is no limit on the amount of decompression a diver can carry out as long as they stay within the safe limits of gas availability, CNS and all other planning variables. In many cases it will be the diver’s own personal limits rather than any external factor that determines the maximum amount of decompression they want to do.

The skills contained within technical diving courses that allow a diver to extend the time they can spend on the wreck are also a way to extend your diving. Many people undertake this training so that they can be safer and more skilled at the depths they are currently diving at rather than to progress any deeper. The skills required for technical diving can also make you a significantly better recreational diver. Having better buoyancy control, awareness of your situation and of your buddy will all help to make you a better diver. Together with better self rescue and buddy rescue skills they will also make you a safer diver no matter what depth you are at.


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