Diving with gradient factors for a new recreational diver

[MrVegas]

I've published this table before, but it adds to your point about first-dive NDLs. The US Navy tables are way out of line with everything else, so perhaps not the best comparison to make.
View attachment 702154

Thanks for posting that again. The i300 uses PZ+, which gives an extra 6 minutes at 100 feet compared to the Perdix at 30/70. (The i300 manual does have an NDL table buried somewhere in the back of the document.) Obviously, that all depends on how conservative one wants to be. My kid's Mares Puck Pro actually gave him a minute or two more than the i300, which was a little surprising. (Mares uses a variant of RGBM.) I think these threads are actually pretty valuable for those that are recreational divers, but diving a little deeper or longer than the standard AL80 reef dive. I've certainly learned a lot from folks here.

 

[scubaozy]

It is also less likely to occur if someone is taught how to use a computer.

I agree to this.

Decompression theory tells us how nitrogen enters and leaves our tissues while diving, and it tells us why that can be dangerous and why we must use a safe diving protocol. Once you have learned that, you can learn how to manage your dives to avoid DCS. That management can be done via tables or a computer.

Not sure this is relevant, at least to what I am trying to communicate. Theory is surely important and is also quickly forgotten. I consider tables more practical training than theory. Others might have a different opinion on that.

I have not taught any OW classes for several years, but when I did, the computer version of the course, included an online simulator that allowed you to move a generic computer through a variety of circumstances and watch how it reacted and how it provided guidance. That was a much better illustration of tissue loading and unloading than a table.

A colleague of mine at the office is a novice diver and he was trying to reach to 50 dives to be able to participate to LoB diving. He had no own equipment, I recommended him to buy a simple no thrills dive computer and over lunch, showed him quickly on my wristcomputer/watch general concepts. He was off to Red Sea soon after, doing his first wall dive on Elphinstone reef. He was diving with one of the speedboats going there quickly and doing 2 dives with 1h interval and then back. It turned out that everyone else than him and his wife was on nitrox including the guide. By the second dive, they accumulated quite a bit of decompression, just following the guide. Once everyone else did their safety stop, they were signaled by the guide to surface, they noticed something was off, and tried to resist but eventually give in as everyone on board is waiting for them in the choppy seas. He estimates 8-15 minutes of missed decompression. Luckily, they walked away without any symptoms. I really felt very bad about this and even guilty but I did show him general concepts and he already had prior training on RDP some years ago (need to mention that he was never my student).
We do not teach decompression diving/procedures in owd, this is excluded. We call it emergency decompression if ndl is exceeded on tables. Same with computers, we just explain it as an emergency procedure. On the other hand if you look at this table:

https://www.taucherpedia.info/images/de/e/ee/DECO2000_Vorderseite.png

It is directly into your face that as soon as you exceed ndl, 3 meter stops added and longer 6meters and so on. There is no confusion on what to do, and absolutely identical to the behavior of a dive computer.

 

[dmaziuk]

I understand little about the onset factors of DCS, though I hypothesis I formed from my misc reading is that there may be some merit to being extra careful with the fast tissues first, which are more vital organs too, in DC algo design, after which to medium tissued based on current performance diving. Especially so for recreational divers.

In addition to what John said, Workman et al realized, doing experimental research, that fast tissues can tolerate mush greater overpressure than slower ones. Bubbles were believed to cause of DCS, then doppler ultrasound became common and it turned out there's bubbles -- so they blamed the fast tissues and quoted one Richard Pyle who was doing deeper decompression stops and felt better afterwards.

There was quite a bit of research done since, that showed rather convincingly that Workman was right, "they" were wrong, and bubbles are not a direct cause of DCS and are almost always there.

 

[tursiops]

bubbles are not a direct cause of DCS and are almost always there.

Isn't the point not the existence of bubbles as a cause of DCS, but rather as a diagnostic for DCS: more bubbles is worse than fewer bubbles?

 

[dmaziuk]

We do not teach decompression diving/procedures in owd, this is excluded. We call it emergency decompression if ndl is exceeded on tables. Same with computers, we just explain it as an emergency procedure.

It's "emergency" because you're on a no-stop dive. It says so right there, under "Emergency decompression". It doesn't say "it's emergency because the sky is falling", it says you planned your dive as a no-stop dive and now you have a stop. That makes it an emergency.

 

[tursiops]

It's "emergency" because you're on a no-stop dive. It says so right there, under "Emergency decompression". It doesn't say "it's emergency because the sky is falling", it says you planned your dive as a no-stop dive and now you have a stop. That makes it an emergency.

View attachment 702184

What you are showing -- from an earlier version of the PADI OW Manual -- is exactly what Ukarus said...violating NDL is an emergency procedure, it is not taught as decompressions stops. The latest manual says to do what your computer says, or if you are using tables do what the tables say.

 

[PfcAJ]

Well, a mathematical model can technically account for any variable you include in the algorithm. So if you could measure and quantify individual variables like hydration or right to left shunt fractions, you could have an algorithm that more accurately predicts DCS risk.

The ones in current dive computers take into account depth, time and mix. Then, they allow for fudge factors that people can modify to better match their own perceived or known risk factors and risk tolerance.

But the point is that ALL scuba algorithms do one thing - let a diver predict DCS risk and create a safe ascent profile. If the underlying clinical data is drawn from a population that has 25% PFOs, then that will be reflected in whatever the model generates.

Im not sure that’s really an accurate statement.

No commercially available models allow the diver to select their DCS risk in a discreet way. Furthermore, no commercially available algorithms are iso-risk.

As an example, an ascent profile generated for gf’s of 30/85 for one depth/time combo will not be the same as a 30/85 for another depth/time combo.

We might be able to say things like “x is less risky than y” but it’s kind of a shot in the dark. We don’t really know how much more or less risky it is.

 

[scubadada]

...My kid's Mares Puck Pro actually gave him a minute or two more than the i300, which was a little surprising...

First, clean dive NDLs do not always predict the computer behavior on repetitive dives. The RGBM based computers, Mares, Suunto, and Cressi, generally have a moderate/middle of the road NDL for the first dive. The NDLs for repetitive dives may appear considerably more conservative if penalties occur. Short SI (less than an hour) is fairly common, ascent violation or certain dive profiles are mentioned as others. From my own reading, these factors appear poorly outlined in manuals.

 

[scubadada]

...No commercially available models allow the diver to select their DCS risk in a discreet way...

Not commercially available, but you may in interested in this probabilistic decompression algorithm SAUL Recreational Dive Planner | Modern Decompression

 

[dmaziuk]

We might be able to say things like “x is less risky than y” but it’s kind of a shot in the dark. We don’t really know how much more or less risky it is.

Furthermore if the 100/100 has been tested and verified to be "X risky" and 30/85 was never formally studied & verified at all, even the claim that it's "less risky" is technically questionable. Common sense suggests that is should be "less risky" and anecdotally it seems to be so, but strictly speaking we can only claim that is "should be no more risky".