Dive Computer No Deco Computations Question

[tursiops]

You may have misunderstood the reverse profile thing. Of course, as long as you stick to no decompression limits there is no evidence of additional risk by diving the first dive shallower.

There is no "penalty" involved in reverse profile diving, as long as you dive according to the tables or computer.

However, bear in mind that reverse profile may mean a shorter bottom time (again- if you stick to ND limits), and many divers think this is a penalty introduced by some shady algorithm or company lawyers who force to add penalties to mitigate risk and lawsuits.

Nope, the NDL calculated by Haldanean-based models (such as Buhlmann) doesn't care if you dive shallow or deeper first. At the end of the dive you surface with whatever nitrogen saturation levels, which gradually decrease during the surface interval. When you enter the second (or third or whatever) the new ND limits are updated. And you will see that reverse profile will result in shorter NDL for repetitive dives.​

Now, most poeple may have not noticed this because they use computers and multilevel dives. But if you try square profiles- like classical table dives- you will notice that reverse profile results in shorter bottom time. For a recreational diver who wants to have fun underwater, this can be considered a punishment. For me it is, at least. When I used to dive tables, we planned the diving sites according to depths and did square profiles. With computer, we do multilevel and don't care anymore if it means a couple of minutes less at max depth (e.g. 30m) you ascend a little bit and NDL is updated on the fly and no one almost cares you can still enjoy a prolonged dive.

Perhaps you should look a little deeper into reverse profiles and what Suunto does. The penalties show up on the third dive, according to Suunto.

"So if you first make a shallow dive and after that a deep dive, we penalize the third dive if the surface interval is short."​

Citation: p177 of this document: Info - Reverse Dive Profiles Workshop, 2000​

​

 

[rongoodman]

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.

 

[tursiops]

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.

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.

 

[boulderjohn]

And penalizing one for diving deeper on a repetitive dive than on the preceding dive....which was discredited a quarter-century ago.

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 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.

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.

 

[tursiops]

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.

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]

You may have misunderstood the reverse profile thing.

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.

 

[AsherN]

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 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]

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.

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]

More than you ever wanted to know about decompression models and algorythms.

https://www.divegearexpress.com/amfile/file/download/file/144/product/8927/

Covers all the models being discussed on this thread.

 

[tursiops]

More than you ever wanted to know about decompression models and algorythms.

https://www.divegearexpress.com/amfile/file/download/file/144/product/8927/

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."​