IF YOU ARE AWARE of the term “deep stops”, you might also be aware that they were becoming increasingly popular but over the past couple of years have been the subject of increasing controversy.
I’m going to explain what we mean by a deep stop, the ideas behind the concept and the controversies, and will also try to explain how deep stops are linked to gradient factors and, most importantly, what all of this means for us as divers.
The traditional view of deco theory goes back over 100 years to JS Haldane’s work for the Royal Navy at the start of the 1900s, and subsequently refined by the US Navy and Professor Buhlmann in Switzerland in the 1960s and ’70s. This view says that as we go deeper we absorb or on-gas nitrogen and eventually reach saturation, the point at which the tissues can take on no more nitrogen.
At the end of the dive, as we ascend, we become supersaturated – in other words, the tissues now have more nitrogen than the gas we’re breathing at ambient pressure. This is known as supersaturation.
Supersaturation, as indicated by the prefix “super”, is a good thing, because it allows off-gassing. The shallower we go, the more supersaturation we experience the better, because the higher the level of supersaturation the more off-gassing we get, and we will be off-gassing as efficiently and as quickly as possible.
Figure 1: Supersaturation and m-values.
This makes the decompression as efficient as possible (figure 1).
However, like most things in life we can have too much of a good thing. There is such a thing as too much supersaturation, and in fact there is a maximum value or m-value that represents the point at which there is too much.
Beyond this point, we enter what is known as critical supersaturation and as the word “critical” might suggest, this is no longer a good thing. This is the point at which bubbles form and decompression illness (DCI) occurs.
This is why the traditional approach to decompression has been to ascend as shallow as possible to allow as much off-gassing as possible but without breaching the critical supersaturation limits and causing DCI.
This was all very well until we started to realise that things were not quite as simple as this. The traditional view has the m-value as a firm boundary. Stay within that line and all is well; cross it and bubbles start to form and we get DCI.
Unfortunately, the body is not as black and white as this and it is impossible to say exactly where the m-value line actually lies. In addition, who is to say that your m-value line is in exactly the same place as my mine?
Even though the m-value is drawn as a distinct line, it is better thought of as a fuzzy line area in a very wide grey area (figure 2).
Figure 2: M-values are a solid black line through a fuzzy grey area.
A technology developed in the 1970s, it became clear that it was possible for bubbles to form even if we were well inside the m-value line. These bubbles, known as “silent bubbles” or “asymptomatic bubbles”, would form well within the traditional m-value limits.
This is a big problem for the traditional model, because it assumes that bubbles cause DCI, and if bubbles form then we will get DCI.
The reality is that we do get some, and often lots of, bubbles forming and yet do not get any traditional sign or symptoms of DCI. As a result, decompression researchers started to look at the implications of these bubbles, and how to manage them.
Bubble models were developed to try to control the formation and growth of these bubbles. This was achieved by stopping deeper than the traditional decompression stops, and this was the source of the deep stop concept.
We might not be able to stop bubbles forming, but by stopping deeper we can try to stop the bubbles from growing to a size at which they cause too many problems.
Pyle stops, deep stops and bubble models started to be introduced in the 1980s and 1990s and to be adopted by technical divers.
Pyle stops were among the first approaches to dealing with deep stops. The concept introduces a deep stop halfway between the maximum depth and the first traditional decompression stop.
So for a 40m dive with the first stop at 9m, we would introduce a Pyle stop halfway between 40m and 9m, which is approximately 24m.
Figure 3: Pyle stops.
Another stop would then be introduced halfway between the first deep stop and the first traditional stop, so halfway between 24m and 9m, at around 15m.
This is repeated until there is less than 3m between the last Pyle stop and the first traditional decompression stop.
So, in this case, we would put in another deep stop at 15m plus another at 12m before carrying on to the first traditional stop at 9m (figure 3).