Myths and no-longer-truths live forever. Like turning your tank valve back a quarter turn, and a mask on the forehead means you are panicked, and Valsalva will always clear your ears.
Dive Computer No Deco Computations Question
- Thread starter butterwm
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The proceedings of the workshop that discredited it a quarter century ago are worth a deeper dive. There was no clear scientific evidence presented in favor of the rule. It was discovered that the rule originated with a 1972 PADI OW course manual, which suggested that in planning deeper dives, one should plan the deepest dive first. No one from PADI knew why that suggestion was made, nor did they know the individual source of the suggestion. (More on that later)
The "deeper dive first" concept had one real supporter at the workshop, and he was fanatic about it. His insistence was the reason that the workshop kept the recommendation for technical dives. His name was Dr. Bruce R. Wienke--the creator of the RGBM algorithm used by Suunto.
If you do some table work, you can easily see the likely reason for the 1972 suggestion. That was back in the days of the US Navy tables, which had huge surface intervals to begin with. If you do the deeper dives after the first dives using tables, the surface intervals are even longer--much longer. The typical 2-tank dive we know today would be impossible.
I dived the Spirit of Freedom liveaboard in Australia a few years ago, and the guy in charge of the diving told us that they had no choice but to follow the PADI rules requiring that deepest dives be done first. I challenged the guy about it privately, and he admitted that he knew there was no such PADI rule--it was just company policy. On the trip, each day the captain always selected a first site with a a relatively shallow bottom, and we were thus forced to do all the rest of the dives even shallower. The reason was obvious. They wanted to keep everyone as shallow as possible so they didn't have to deal with potential DCS issues, and they wanted people to blame someone else (PADI) for that requirement.
I have seen more and more and more of this as the years go by. Dive operators are getting more and more cautious in what they allow their divers to do, and it is really beginning to frustrate the heck out of me.
Page 171 of the Reverse Dive Profiles report gives some insight into where PADI got the idea of discouraging reverse profiles. Apparently the US Navy (old) tables were "unsafer" (in the sense of more likelihood of DCS) for reverse profiles than for forward profiels, due probably to their aggressive nature and slow off-gassing rates. This reportedly came from the early years of using those tables, which were copied by PADI. Here is an early PADI dive table (side 1 only), which is pre RDP:
A quick way to tell if a particular table is based on the old Navy tables is to look at the NDL for 60 ft; if it is 60 mins (thus the Rule of 120), it is probably based on those old Navy tables.
Blackcrusader
Contributor
No I didn't. I would come up to 5m depth the sunnto assumes you are doing a safety stop and not just continuing the dive. If clearing the safety stop time I went back down to say 10m and came back up to 5m depth again the sunnto would show another safety stop but with added time to clear it.
That issue with the Apple Watch, while true, is somewhat overblown. Especially on diving focused boards and FB groups. The watch is very firmly aimed at the OW diver, with a focus on the vacation diver. Those folks who will not flirt with 40m all the time. I dive the AWU2. My diving is generally concentrated in one month in the winter and 2 months in the summer. I just take the month subscription. If I do a one-of dive outside those times, I'll do a daily. I also use the Oceanic phone case, so that is my backup computer, running the exact same algorithm.
Belzelbub
Contributor
That’s your opinion. I don’t think it’s overblown at all.
The watch itself is rated for much deeper than the cutoff imposed by Apple, and I can’t really understand the reasoning behind it. While I disagree with a post dive lockout, I do understand where the manufacturer is coming from. Locking out while the diver is still in the water is just inexcusable in my mind. The rarity of the occurrence doesn’t make it OK.
ancient diver
Registered
More than you ever wanted to know about decompression models and algorythms.
Covers all the models being discussed on this thread.
Covers all the models being discussed on this thread.
Nice link, but the publication is 6 years old so the writing is older than that. It therefore does not contain many more recent innovations that are hugely popular, like SurfGF. The article is quite thorough on Buhlman models and Gradient Factors, but somewhat thin and even dismissive of bubble Models, i.e. VPM and RGBM One important paragraph (p54) is:
"On the dissolved gas side, a prominent contribution is that of Baker (1998). With the introduction of the concept of gradient factors, Baker provided the ultimate transparency in adapting a model. On the bubble model side, RGBM provides some predefined levels of conservatism, while in VPM, the parameters that can be tweaked are available, yet are all but intuitive. But what is interesting is that whether simple or complex, these models owe their functioning not so much to an underlying theory, but to data fitting."
They won't be exactly the same due to differences in sampling & recalculation intervals as well as precision and round-offs. E.g. if one recalculates every 20 seconds and another: every 15 seconds, they can only "exactly match" once a minute (assuming their clocks are perfectly synchronized). That would be more pronounced during depth changes and should start converging once you level off.
That has not been my experience. About a decade ago, I got a Suunto DX for testing and also had a Oceanic Atom I was using.
I was on a deco dive and the Suunto had me doing a lot of deeper stops which I was obeying. At one of the stops, I could see my richer deco gas that I had stashed about 3 feet shallower and my Oceanic didn't have me requiring a stop at my current depth so I figured I would get to the gas and accelerate my deco.
The Suunto had about a minute left at the current stop I was at so I just went to the gas.
That was a mistake. The Suunto went into violation mode (under water on a dive no less) and no longer gave me any stop information. It was insane. I luckily had my Oceanic with me.
I personally only dive Shearwater at this point. For the diving I do, I really don't think any of the other mainstream dive computers are suitable for me.
I understand that a lot of computers go into a 24 or 48 hour lockdown after surfacing and after a violation. I won't argue if that is right or wrong (but I do dive Shearwater computers which don't do that).
However, to go into lockout mode while on a deco dive is absolutely unacceptable IMO. Go ahead and penalize me or alert me or whatever, but to lock me out during a dive is ludicrous.
I will never dive a Suunto again.
- brett
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