Decompression Myths: Part 2
By: Mark Powell
In the last article I looked at some of the reasons why we have myths in general and also at some of the broad myths relating to decompression. In this second article I am going to be much more specific and focus on one particular area. I am going to consider two common myths about safety stops.
MYTH – It doesn’t matter if you miss a safety stop
The first myth is that safety stops don’t really matter. I’m sure you have all heard divers on the boat saying that they missed their safety stop but it doesn’t matter as they are optional. After all, in the last article I stated that a no-stop dive is one where you can ascend directly to the surface without needing to stop. Safety stops are often described as optional rather than mandatory stops. This is all true but that doesn’t mean that safety stops don’t matter. The first reason why this is the case is that the traditional approach assumes that there is a hard and fast line between having bubbles and not having bubbles. This limit represents the maximum amount of supersaturation that the tissues can tolerate. This maximum value (or M-value) is the dividing line between having decompression sickness (bubbles) and not having decompression sickness (no bubbles).
Unfortunately, it’s not as simple as that. M-Values have been represented by a solid black line drawn through a fuzzy grey area. This is illustrated, in colour rather than in shades of grey, in the diagram below. At the bottom, with low levels of supersaturation, the diagram is green, indicating no risk of DCS. At the top the diagram is red showing a high risk of DCS. The M-Value is represented as a solid black line drawn across the middle of the area. However, as you can see in the diagram, the black line does not really represent the exact point where the colour switches from green and red as the colour slowly changes from one to the other over a much broader area than that represented by the M-value line. As a result, an ascent from a no-deco dive that goes up to the M-value line or even just close to it risks generating bubbles even though we have not exceeded the M-value.