One of the features I’ve been emphasizing about the SAUL algorithm (besides its greater accuracy), is that it allows the diver to choose a level of risk that he/she feels is appropriate. Since this is quite different from the way we initially learned to think about dive safety, now is a good time to review the differences and to go over some basic ideas and facts about probability.
When you were first learning to dive, you would have been told that, in diving, there is always some small probability of decompression sickness (DCS), but that you can minimize your risk by diving safely – all of which is true. But, once you were introduced to dive tables and/or dive computers, there was no further discussion of probability in relation to DCS. Instead, you went on to look at dive tables to work out no-decompression limits (NDL’s), and then proceeded to dive computers which, in effect, automated the working out of NDL’s.
The problem with NDL’s (and with current dive computers) is that they are deterministic. That is, they have an absolute dividing line: below it you’re okay, above it you’re in trouble. As with speed limits on highways, there may be a tendency to continually push the limit – until you get caught or “bent”. (Although, to be fair, PADI dive tables, do have a disclaimer of sorts on them advising that you shouldn’t actually plan to dive to the limits, that the limits are something of an absolute maximum. But, then, you’re left wondering – “How close to the limits is too close?”)
While some current dive computers offer riskier or less risky options, how can anyone make a sensible choice without a clear idea of how much risk each option involves?
So let’s look at the probability of DCS in diving. In the higher ranges, if you were to dive each of the listed profiles on the PADI recreational diving tables, right at their no-decompression limits (on air, including a 3 minute safety stop at 15 fsw), your risk of DCS would average out to .00264, which is 264 chances in 100 thousand or, roughly, just over a 1 in 400 chance of getting bent. (I averaged over all the listed profiles because the risks at the limits of the different profiles are not equal.) To see the comparative risks of PADI’s NDL’s for different profiles, check out my post on How Saul Relates to the PADI Dive Tables.
On the other hand, most recreational diving typically involves much lower risks. Project Dive Exploration, which solicits and collects actual dive data from large numbers of divers found (in one subset of their data, that roughly corresponds to typical recreational diving) a DCS rate in the neighbourhood of .0007, which is about 70 chances in 100 thousand. Or, to put it another way, only about 1 chance in 1,400 of getting bent.
What factors should you consider when choosing an acceptable level of risk? Obviously, less risk is preferable to greater risk. No one wants to get bent. (On the other hand, the least risk of all is to stay out of the water. Not a serious option.) But less risk generally does mean less bottom time. So what do you do?
Regardless of the level of risk you choose, there is some possibility (even if very small) of getting DCS on any single dive. As you increase the number of dives, you’re more likely to experience the consequences of a higher level of risk. Using the (not recommended) 1 in 400 level of risk mentioned above, if you were to dive 400 times at that level of risk you’re quite likely to get bent.
It’s probably unnecessary to remind you (but I’ll do it anyhow) that a 1 in 400 risk doesn’t mean exactly 1 incident of DCS in every 400 dives, but that, over a large enough number of dives, or a large enough number of divers, the number of cases of DCS will average out to 1 in 400. Meaning that, while you might manage those 400 dives without getting bent at all, you’re about twice as likely to get bent one or more times. And, if you were to continue at that same level of risk through 1000 dives…. Well, just don’t. (To see the probability calculations, click here. Continue reading