Decompression sickness (DCS) and dive computers
I was made aware of a recent incident where a diver experienced decompression sickness (DCS) after a dive (resulting in severe pain in the joints along with skin rashes), they spent around 10 hours in a hyperbaric chamber and made a full recovery.
This sort of incident is relatively common (arising for numerous reasons, including rapid ascents), and of course it's great that no one was seriously hurt. What was particularly interesting was that the diver publicly stated in a post + video that they were "100% not to blame".
I don't want to partake in unnecesary finger-pointing and criticism (and my feeling is, perhaps cynically, that based on their post they were fishing for engagement), but I think it's important to reflect on how the incident could have been avoided as well as the importance of taking responsibility for our own safety when diving.
As far as I understand it, the incident played out as follows (this is a summary base on the post that they made, and subsequently deleted - I wasn't going to link to it directly in any case so as to not finger-point nor provide further views / engagement, as already mentioned):
- The diver was (probably) on holiday and using a commercial dive centre
- They stated that they were an experienced diver with around 1000 logged dives (this is quite substantial for a recreational diver)
- They were diving on air rather than nitrox
- Their computer malfunctioned during, or after a previous dive
- They rented a replacement computer from the dive centre
- The first dive computer offered was set to nitrox mode and no one knew how to change it to air
- They were offered a replacement computer and assured that it was set to air mode (but it was in fact still on nitrox mode)
- They dived assuming that they were on air, but the computer was calculating their NDL (no-decompression limit) based on nitrox
- They unknowingly exceeded their NDL, didn't make the required decompression stop, and shortly after experienced symptoms of DCS (requiring recompression)
- They blamed the dive centre, rather than themselves, for the incident
In their explanation, they also stated that "nitrox is used to extend bottom time" (hence why the computer's NDL was longer then they would have expected on air).
I think that there are a few points worthy of reflection here:
- Responsibility - ultimately, we are responsible for our own safety when diving, unless undergoing specific training beyond our current abilities (whilst under supervision of a suitably experienced instructor). There is simply no other excuse here, particularly for someone claiming to have significant experience
- Redundancy - never rely on a single piece of equipment, particularly when it's as critical as a dive computer. Utilising a backup computer on every dive is absolutely essential (especially as it should be logging the exact same dive profiles as the primary computer, more on that in a moment)
- Don't rush - if unsure about any new equipment, take a moment to understand it before diving with it. Don't assume it is set up correctly; user manuals are trivial to find online and it needn't take more than a few minutes to check the settings (ie. to verify that the computer is set to air mode)
- Be conservative - assuming that the individual had dived the day before the incident (and that this was when their computer failed), diving the next day leaves insufficient surface interval for the residual nitrogen to off-gas. In addition to this:
- A new computer will not have been aware of the previous day of diving, and thus could not fundamentally be trusted to provide accurate modelling and calculations
- It would indeed have been more prudent to not dive until a sufficient surface interval had passed (eg. >24 hours), but at the very least the dive should have been conducted with a more conservative profile and not pushed the limits of NDL (on either air or nitrox)
- As for point 2, use a backup computer - after the initial failure this could have been switched out to be the primary (since it would have logged the previous day's dives and thus have an accurate model of the diver's nitrogen loading)
- Nitrox and safety margins - this one's more of a criticism of PADI training* and a particular framing of nitrox. The diver was on air, so the specific point is moot, but their statement that nitrox is a means to extend bottom time is a little concerning. Its primary purpose should instead be to increase safety margins** (and in this particular case, simply increasing safety margins and being more conservative would have potentially prevented the incident from occuring in the first place)
- If in any doubt, don't dive
* "Breathing less nitrogen means you can enjoy longer dives and shorter surface intervals" (source)
** I'm more of a fan of BSAC's approach to nitrox, which is firstly about increasing safety margins, and then optionally (and crucially, with appropriate planning) used to extend bottom time (source)
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