Feedback on recent two-tank and dive limits

[old frogman]

The copy I have (Australian version) which precedes yours is actually titled "The essentials of deep diving". Same book slightly earlier edition 1989.

What probabilistic modelling means is that the tables are tested by the US Navy in vivo. That is, many hundreds of dives are made by divers to see how many DCI cases occur. The US Navy is unique in this type of testing which is extremely expensive. A DCI percentage figure is arrived at, for example, 2%, 3 %, 5%, 7% etc. In relation to their current air tables 0-190ft the aim is to achieve a DCI of 3% or less across all depth and bottom durations.

While the first book by Lipmann may not use the word probabilistic. The US Navy have always tested their tables in vivo. However, the second book published later by Lipmann with Mitchell does make direct reference to the term probabilistic model. It also has a chapter on it.

If you want to delve deeper in US Navy probabilistic modelling, there is an unclassified document NEDU TR 04-40 "Graphical analysis: Decompression tables and dive outcome" Dec 2004.

Don't get too hung up on the DCI incident %. Look at it from the other perspective 97% or better chance of not getting DCI.

 

[NAUI Wowie]

One big reason computers are popular is they deal with multi-level profiles extremely well. If you had tried to plan your dive based on a square profile at max depth, your allowable bottom time would have been quite short. You could dive that reduced time, but why would you?

Personally, I'd say a better reminder would be to pay MORE attention to your dive computer, rather than to rely less upon it. It is a far better tool for the type of diving you're doing compared to the "basic planning" that you can realistically do.

Oh absolutely. And it was a situation where the dive guides rushed everyone in and I just didn’t clock the time. I have some questions about the guides in general, since when we arrived they were going to take some discover scuba folks on a 100’ wreck dive (which I learned later).

But definitely on me for 1) not keeping track of the SI, and 2) not insisting on more boat time when they rushed everyone back in for dive #2. Won’t do it again.

Apologies if this has been brought up on this thread but what Dive Op brings "discover scuba" folks on a 100' wreck dive? Does that mean 100 feet deep?????!!!!!!!

 

[NAUI Wowie]

Well that is a violation of the depth limits for a DSD for sure.

not just a violation, high chance of people panicking and dying if true. first dive drag someone to 100 feet? Sheesh

 

[NAUI Wowie]

No they don't. For a recreational dive, adjusting GFHi (only number relevant for NDL dives) lower shortens your NDL time. If your computer allows for 15 minutes at that depth, but you do 20 minutes anyway, then you screwed up. If you are a recreational diver only trained to NDL dives, then you shouldn't be planning to do any deco dives. If your computer says NDL is approaching 0, it's time to head up. If you do that, then the only stop is the safety stop.

for me, with my perdix and teric once i get close to 0 NDL and im down around 100 ft my safety stop automatically changes from 3 minutes to 5 minutes

 

[Belzelbub]

for me, with my perdix and teric once i get close to 0 NDL and im down around 100 ft my safety stop automatically changes from 3 minutes to 5 minutes

That means you have adaptive safety stops turned on. I do as well. But, it can be turned off so it’s only ever 3 minutes.

 

[Blackcrusader]

for me, with my perdix and teric once i get close to 0 NDL and im down around 100 ft my safety stop automatically changes from 3 minutes to 5 minutes

Yes if you have adaptive settings on. I do not on my Perdix and I dive it in Tec mode.
So safety stops are not expected in this mode you decide to do them yourself or not as the case may be.

 

[AsherN]

Interesting flyfishing exercise so far.

Let' look at a scenario. You have a son or daughter somewhere between 16 and 18 years of age and they have just completed their basic open water course, and you are going to give them an entry level Shearwater Peregrine dive computer or one similar with Buhlman and GF algorithm. What precautions are you likely to take to make him or her safe when using this dive computer? Or are you just going to give him or her the dive computer and just say read the user manual and figure it out.

Simple. I'll make sure they can read and understand all the numbers displayed on the DC. GF are not Voodoo. And for the average rec diver, not deep understanding is needed.

I certified in 1979. Logged a couple hundred dives right after that. Horse collar, tank with J-valve. Watch and depth gauge using tables. I took a break and started again about 7 years ago Bought a Cressi Leonardo. RGBM, no GF to worry about. I could choose between 3 conservatism settings. Dove that for 4 years. Then I dove my Apple Watch for a couple of years. The Oceanic+ app runs Buhlmann with GF. I've just switched to a Peregrine TX. Gues what, both the Apple watch and the Peregrine have 3 predefined conservatism settings. Yes, you can go and tweak the GF. But for the average rec diver, all they need to know is to set 1 of 3 levels, and I'll venture that most just leave the default x/85. More dangerous than that, a few years ago one of the diver was diving a Suunto and did not know what the meaning of the NDL number.

 

[AsherN]

I agree the Peregrine and Buhlmann GF is a great unit. But should it be in the hands of the newly certified recreational diver, as per the scenario question.

Absolutely it should. Comes out of the box with a mildly conservative GF model of 40/85. You don't need to have read all the books on the GF model to use it. Leave the default, like most do on any computer running any algorithm.

 

[AsherN]

A key issue with the Peregrine and others like it is they have a custom GF function so that you may vary the high and low GF to fine tune the stops. Imagine a new recreational basic level diver playing around with this function. The user manual does have a warning label on this function, but the custom function is there and very tempting to try it out. Hell, when I was young, I would have gone custom in an instant.

The GF setting cycles through High (35/75), Medium (40/85), Low (45/95) and custom. Default to medium. worse case, they change it to x/100. Not a big deal with a normal rec dive. If they go lower than 75, they just cut bottom time, again not a big deal other than lack of enjoyment. The vast majority of divers will just pick one of the 3 defined settings, and even then, most will just stay at medium.

 

[old frogman]

The GF setting cycles through High (35/75), Medium (40/85), Low (45/95) and custom. Default to medium. worse case, they change it to x/100. Not a big deal with a normal rec dive. If they go lower than 75, they just cut bottom time, again not a big deal other than lack of enjoyment. The vast majority of divers will just pick one of the 3 defined settings, and even then, most will just stay at medium.

So, if you are in the vast majority of divers (recreational no decompression diver) and no interest in technical diving why spend $500-$800 dollars on a Peregrine to just dive it on a default setting when for about $270 you can buy a Scubapro Aladin 1 Matrix which is simpler to use, has a user replaceable battery and is nitrox capable.

If you want to cater for adverse diving conditions just don't dive to the limit of the NDL and/or increase your safety stop. Take control of your situation.