Info Deeply Safe Labs: A website for dive computer testing

[-JD-]

[ ETA Holy necro-quote! @dmaziuk pulled @boulderjohn 's quote from a 2012 thread ...

I have been the source of some of those postings. A couple of years ago a representative of DAN did a presentation at a social gathering at our dive shop. She said that over 80% of DCS accidents happened on the first day of a dive trip, and over 80% happened on the first dive of that day. I have repeated that information several times on ScubaBoard. Unfortunately, there is no way to link to her presentation.

]

If true, that would suggest the exact opposite of DCS risk increasing with repetitive dives.

My "lay" take on that (assuming true) is that some divers are DCS time-bombs on arrival. That may be due to (non-exclusively) specifics of travel (fatigue, stress, dehydration), being "rusty" (i.e. they screw up and blow NDL, skip deco, ascend too fast ...), or being physically predisposed to DCS.

 

[dmaziuk]

True but if "DCS stress accumulated" during multi-dive multi-day trips, you'd have a bathtub distribution with peaks on both ends of the trip.

I mean, we know it does if you keep doing 6 dives/day to the no-stop limit on air, and yet somehow lots of LOBs are not returning to port full of bent divers.

 

[tursiops]

somehow lots of LOBs are not returning to port full of bent divers.

Nor do they promote diving to the NDL limits (1h is typical), and they make Nitrox readily available, and typically only the morning dive(s) are deepish, and the DMs in the water do multi-level dives with a lot of time spent shallowish, and safety stops tend to be mandatory on all dives.

 

[-JD-]

True but if "DCS stress accumulated" during multi-dive multi-day trips, you'd have a bathtub distribution with peaks on both ends of the trip.

I mean, we know it does if you keep doing 6 dives/day to the no-stop limit on air, and yet somehow lots of LOBs are not returning to port full of bent divers.

I would imagine that the stress in multiple days of multiple dives reaches a flat mean pretty early on, like by the end of a full day. I can see that that mean level of stress has it's own degrading effect which could also lead negative outcomes, but like undertaking other ongoing multi-day "athletic" endeavors the body may somewhat acclimatize to the stresses.

 

[scubadada]

I would imagine that the stress in multiple days of multiple dives reaches a flat mean pretty early on, like by the end of a full day. I can see that that mean level of stress has it's own degrading effect which could also lead negative outcomes, but like undertaking other ongoing multi-day "athletic" endeavors the body may somewhat acclimatize to the stresses.

Each day on a LOB is pretty similar, same number of dives, same exposure. Of course, you continue to load your slow compartments. It is interesting to examine your tissue loading graph during a week on a LOB.

 

[boulderjohn]

If true, that would suggest the exact opposite of DCS risk increasing with repetitive dives.

That was a long time ago, and my memory is fuzzy. My memory of it includes a SB thread on the topic, and IIRC, John Chatterton argued that repetitive diving over a number of days essentially gets your body used to the off-gassing process and builds a resistance to DCS. His statement was quite controversial. (It is possible I am thinking of two different threads here.)

I also remember doing some math of my own. There is a common joke pulled here in ski country in which we warn skiers that 100% of serious ski accidents happen on the skier's last run. It takes them a while to figure that out--the skier stops skiing after a serious injury. Applying that to DCS over repetitive dives of a number of days, if DCS chances were 100% random, they would be more likely to happen on the first dive of the trip, with decreasing likelihood as days go by, because the divers would stop diving after getting DCS. I remember applying that math to the DAN statistics and seeing that they were far beyond that probability.

 

[elmo]

Hi @Dr Simon Mitchell

I suppose you will agree that all the dive tables that include a repetitive dive procedure increase the decompression time of repetitive dives according to the time spent at the surface. In the beginning, I seem to remember that Haldane recommend adding the time of both dives. The US Navy used this method, but it was very restrictive. All the navies around the world started using the 120min compartment to calculate decompression of the repetitive dive. Some commercial diving companies preferred to artificially fill all compartments at the end of the first dive. The French company COMEX company used the three methods and selected the most conservative. On the recreational side, DSAT selected the 60min instead of 120, as it seemed to be sufficient in that field.

These methods imply a longer decompression than the one you can calculate if you use the load of inert gas in the leading tissue of the second dive. I’m aware that you will not learn anything reading this, but you can eventually correct me, for the benefit of other readers and mine.

Our tests demonstrate that the dive computers that are implementing an algorithm based on the ZH-L16 C parameter set only use the inert gas load to calculate the decompression of repetitive dives. This point was very easy to confirm by a simple calculation based on the same parameters.

We used a first protocol which can be seen as “heavy” but showed significant differences. We compared the behaviour of some “old” computers to ZH-L16 C dive computers. As we used square profiles for these tests, we could compare to some dive tables, but the result will be very similar to that of “old” computers.

The point is: some ZHL16C dive computers gave a decompression time divided by 2 for the second dive. Maybe claiming that computer Y is not as safe as computer X because computer Y prescribes shorter decompressions is also simplistic, but I would be scared should it be true. I’m not the only one, in my country, some hyperbaric chambers representatives are worried as well.

I have no doubt that technical divers can easily counteract this with GF. But the marketing target has changed with the arrival of Garmin, Mares and Seac. Even Shearwater now clearly aims at the recreational diver. That’s what we can be worried about. What do you think @Duke Dive Medicine ?

Best regards,
Eric Frasquet,
Deeply Safe Labs.

Firstly, I do believe a comparison of different dive computers, from different manufacturers, produced across different years/decades is useful. Particularly looking at repetitive dives, where differences in algorithms become most pronounced. Thank you!
Any computer or software claiming to use ZHL16C (or A or B for that matter) should be tracking all 16 tissues. Not just the leading tissue, as a fast leading tissue at the end of the first dive is likely to wash out sufficiently that a slower tissue could be leading at the start of the subsequent dive. But that slower tissue may or may not control since the faster tissues will be loaded faster during that dive. If a computer is doing anything other than tracking all 16 tissues according to Bühlmann it isn’t using ZHL16c truthfully.
But tracking 16 tissues isn’t practicable for repetitive dive tables. So they must pick one. US Navy divers would be expected to do relatively long dives, frequently with decompression and typically longish surface intervals. DSAT recreational dives are relatively short, potentially shortish surface intervals, and by definition no-decompression. It is logical that Navy tables use a slower tissue (120 min) than DSAT recreational tables (60 min) for estimating tissue loading for repetitive diving.
I don’t know what commercial diving protocols are for repetitive diving but in saturation diving the tissues are saturated regardless so I expect it’s a moot point. Certainly moot with respect to dive computers used by us non-commercial divers.
None of that is to say that the ‘penalty’ for repetitive diving shouldn’t be greater than calculated by ZHL16c. It is quite plausible that off gassing is slower, or slower tissues are less tolerant of decompression stress than assumed by the theory. But if a dive computer manufacturer wants to include a penalty they should make it clear (and public) how they are assessing it. Part of the reason ZHL16c is popular is because it is predictable (by the computer, by software) and for group/buddy diving all computers can be set to calculate close to the same thing.

Ed: I should add if a dive computer does anything other than follow a published algorithm, it should say not just what it is doing (as I mentioned above) but on what basis it is doing it. E.g. study xyz demonstrated there needs to be a greater penalty for repetitive diving, or it seemed like a good idea at the time, or my lawyers made me do it.

 

[dmaziuk]

There is a common joke pulled here in ski country in which we warn skiers that 100% of serious ski accidents happen on the skier's last run.

That would work in this case: DCS hit on the last dive of the trip would add to the "last day" peak, QED.

 

[Deeply Safe Labs]

Hi @elmo ,

Your appreciation is most welcomed.

You are absolutely correct about tracking all compartments. In fact, I believe this is what has always been done in all dive computers, as it is necessary in order to identify the leading compartment. All algorithms use the leading compartment, the most restrictive one by definition, to compute the current decompression ceiling. This works fine for a single dive. Diving tables use a similar procedure for the single dive table.

For repetitive dives, Haldane, the US Navy, the French Navy, Bühlmann ZH-L12 and ZH-L16, DSAT, BSAC... all considered that a longer period tissue should be taken into account. Perhaps those who established these procedures considered that using only the leading tissue was inappropriate.

Up until "pure" ZH-L16 C computers, all dive computers have been computing repetitive stops to roughly match the increase that happens with dive tables. ZH-L16 C computers manufacturers, using only the inert gas load for repetitive dives, are stepping away from these "old" procedures. One can argue about the cautiousness of these old procedures, but "pure" ZH-L16 C is the first and only algorithm that ignores the state of the art, which leads to some unreasonable results.

For example, considering that for a 30m (100ft) dive of 16 min, the leading compartment is 12.5 minutes, it means that with such a calculation, with 75 minutes of surface interval (6 half times), you can repeat this dive with no decompression stops again, and again, and again... infinitely. Our tests confirm this, whatever the GF settings, the NDL (and/or decompression schedule) is identical for all dives in the "repetitive dives" protocol.

If I understand Dr Mitchell correctly, in message #60, §5, then tables and computers manufacturers have artificially increased decompression times for repetitive dives in order to, at the very least, approach a risk level as low as for an isolated dive. This would only corroborate our point about the increase in risk. Still, I see no reason to accept a higher risk for a repetitive dive than I would for a single dive.

Best regards,
Eric Frasquet,
Deeply Safe Labs.

 

[rongoodman]

For example, considering that for a 30m (100ft) dive of 16 min, the leading compartment is 12.5 minutes, it means that with such a calculation, with 75 minutes of surface interval (6 half times), you can repeat this dive with no decompression stops again, and again, and again... infinitely. Our tests confirm this, whatever the GF settings, the NDL (and/or decompression schedule) is identical for all dives in the "repetitive dives" protocol.

You could repeat that dive again and again and always have the 12.5 minute and faster compartments clear after 75 minutes, but that doesn't mean that the model is ignoring the inert gas build up in the slower compartments, just that it takes more repetitions for them to come into play. Eventually the 18.5 minute compartment would lead, but divers have to eat and sleep too and the slower compartments will continue to off-gass during those times. I dive with a Shearwater and have watched the tissue loading graph over the course of a number of liveaboard trips doing 5 dives daily. As you mention, the fastest compartments clear during the surface intervals. The medium ones usually clear overnight and the slowest ones never even approach the M value line.