I was quite surprised on my recent trip on the T&C Aggressor to hear the DMs making a big deal about avoiding reverse profiles. I didn't think that was a thing anymore.
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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.I was quite surprised on my recent trip on the T&C Aggressor to hear the DMs making a big deal about avoiding reverse profiles. I didn't think that was a thing anymore.
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)And penalizing one for diving deeper on a repetitive dive than on the preceding dive....which was discredited a quarter-century ago.
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 was quite surprised on my recent trip on the T&C Aggressor to hear the DMs making a big deal about avoiding reverse profiles. I didn't think that was a thing anymore.
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: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, the captain always selected a 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.
You may have misunderstood the reverse profile thing.
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.I am aware of the Apple Watch issue and what happens if you exceed the maximum depth. I also agree with you that no matter what it should still provide info to safely get you to the surface in the event that happens. I rarely dive beyond 100 feet so fortunately this is not an issue.
From what I gather all computers seem to have shortcomings of some sort then. Is there a particular brand or model that is known to not have any shortcomings?
That’s your opinion. I don’t think it’s overblown at all.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.
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:More than you ever wanted to know about decompression models and algorythms.
Covers all the models being discussed on this thread.
When you say the Shearwater and Garmin run the same algorithm do you find the NCL times the same on both computers all the time?