By Mark Powell
In the [first] and [second] articles in this series I covered some myths that may have made you think about things in a slightly different way but on the whole were fairly uncontentious. This article may be a bit more contentious as the subject we are going to discuss is a topic that is regularly taught by some agencies and instructors yet has been shown to be a myth.
When I learnt to dive, I was taught there were three golden rules;
1. Never hold your breath while ascending
2. Never dive alone
3. Never dive deeper than your previous dive.
This was in the late 80s and things have moved on since then then in many ways. Equipment has changed beyond recognition and so have many training practices. SDI was launched in 1998 and just one of many firsts was the launch of a Solo Diver program which has shown the diving industry that it is possible to solo dive if you are properly trained, equipped and have the right mindset. For more detail on how the approach to solo diving has changed take a look at this [https://vimeo.com/49259855].
Equally, the idea that you should never dive deeper than your previous dive (also known as a reverse profile) has also been revisited. If you were taught that you should never dive deeper than your previous dive for safety reason in the last 5, 10 or even 20 years then that information is based on an outdated view. The reason I say up to 20 years is that the ‘revisited’ information is based on the Reverse Profile workshop that was held in 1999. In other words, this information has been available for over 20 years.
The information below is not just my view or the view of an individual training agency. The reverse diving workshop was held at the Smithsonian Institute and was organised by DAN, AAUS, DEMA and Dive Training magazine. In other words, it was a genuine scientific workshop and so its findings should be taken seriously.
These workshops are held regularly and the intention is to review key principles and to see if the scientific recommendations are still valid, or whether they need to be updated based on more recent evidence. They follow the same overall format. First the original evidence that led to the current guidance is reviewed, then any more recent evidence is reviewed and finally a recommendation is made based on the total evidence. In this case they followed the same procedure but when they came to review the evidence upon which the original recommendation was based, they made an interesting discovery. There was no scientific evidence that was originally used to create this golden rule. In tracing the background to this rule, they found the first mention of it was in an agency training manual from 1974. This manual stated that if you do the deeper dive first, and a shallower dive second, then you will get more overall dive time according to the tables. This is absolutely true, almost all tables will give the same result. However, there was no indication at this point that it was for any safety reason. Subsequent iterations of this and other training manuals gradually strengthened this statement until, eventually, it became the absolute rule that “though shall never diver deeper than your previous dive”. However, as the workshop discovered, this was never based on any scientific evidence that this was because reverse profiles were more risky than ‘normal’ profiles.
MYTH: Never dive deeper than your previous dive
Furthermore, the workshop confirmed that since the introduction of this rule there had been no subsequent research that showed an increased risk of DCS with reverse profile diving. This should not really be a surprise as many commercial, scientific and even large numbers of rogue recreational divers have been doing reverse profile dives with no apparent increased risk for many years.
In order to remove any doubt, the findings and conclusions of the workshop are reproduced below in full.
– Historically, neither the U.S. Navy nor the commercial sector has prohibited reverse dive profiles.
– Reverse dive profiles are being performed in recreational, scientific, commercial and military diving.
– The prohibition of reverse dive profiles by recreational training organizations cannot be traced to any definite diving experience that indicates an increased risk of DCS.
– No convincing evidence that reverse dive profiles within the no decompression limits lead to a measurable increase in the risk of DCS was presented.
– We find no reason for the diving communities to prohibit reverse dive profiles for no-decompression dive profiles less than 40 m/130fsw and depth differentials less than 12 m/40fsw.
If you want even more information and want to read the full details of the workshop the final report is available in full [https://repository.si.edu/handle/10088/2724].
From this we can see that there is no evidence for banning reverse profiles based on safety reasons. Of course, there may be other reasons why doing the deeper dive first may be a good idea. We have already discussed that you will get more overall No Stop Time on the tables if you do the deeper dive first. In addition, if you do a deeper dive first and use up some of you gas you can potentially do a shallower second dive while staying within safe gas limits whereas this may not be possible the other way around. Finally for divers that suffer from ear problems sometimes a deeper second dive may cause difficulties with equalisation.
However, the key point is that none of these relate to DCS risk. This is an important point because, if you remember from the first article, I said that one of my intentions is to promote a better understanding of DCS risk. If we are telling divers that there is an increased risk of DCS with reverse profile diving, when the scientific evidence says otherwise, we are reducing understanding of DCS risk rather than increasing them. You might not think this is important but I think if we have a key principle of decompression theory that instructors teach as a fundamental safety rule but science says is not true then divers may also question what other key rules are not true.
Sticking to a rule just because that is what we were first taught when the scientific evidence indicates it is not true just serves to spread misunderstanding and is exactly why myths develop. So next time you hear someone saying it’s safer to do your deepest dive first ask them what evidence they have for that.
Just in case you were wondering; the rule that you should never hold your breath while ascending is still good advice.