Algorithm question (Perdix AI vs DSAT)

[MomZo]

Yes, depending on the Gradient Factors you set in the Perdix.

Even if I let the setting to Medium conservatism? Or should I set it to Low conservatism?

 

[tursiops]

Even if I let the setting to Medium conservatism? Or should I set it to Low conservatism?

I don't know; I don't use a Suunto so have no way to compare the details.
Low conservatism (GF 45/95) will be a similar to your Suunto for the first dive of a series (according to the Zoop manual). Repetitive dives (as you have discovered) will be less conservative than your Suunto.

 

[scubadada]

Hi @MomZo

1st dive NDLs don't tell much of the story regarding how dive computers are going to perform over multiple dives, multiple days. Unfortunately, there is not much information available on this topic. I would suggest taking a look at the 2017 ScubaLab testing results cited by @Jay_Antipodean in post #27. Both Buhlmann ZH-L16C 45/95 and 40/85 were tested, as implemented on a Perdix. If you want to see Suunto results, you can find the 2016 ScubaLab testing results on the Scuba Diving Magazine website. The Viper Novo and Zoop Novo were tested in 2016, same 4 dive protocol.

I have had some difficulty in mimicking DSAT with Buhlmann due to some differences regarding depths and repetitive dives, but I can get reasonably close. You might want to dive them in parallel for awhile to see how they behave

I think you will find that Buhlmann will give you more NDL on 2nd and subseqent dives. 45/95 and 40/85 might both meet some of your requirements, choice is yours

 

[DavidFL]

Hi @MomZo

My first DC was a Suunto Gekko (predecessor to the Zoop); my second DC was a Suunto Cobra 2. I dove them as primary/secondary for several years and they were always close enough for me to think their implementations of Suunto's RGBM decompression algorithm were about the same.

I bought a Perdix (before AI was available) and now dive with the Perdix as my primary and the Cobra 2 as the backup. After trying various combinations of settings, I have the Perdix in recreational "low" conservatism, and the Suunto Cobra in P0 (least conservative). I'm happy with using both computers for information with those settings. When I tried Suunto P0 (lowest) and Perdix recreational "Medium" the Perdix was showing shorter NDL's on some of my typical 80-90' drift dives in Jupiter/West Palm on 32-36% nitrox. I did two live-aboard trips this year (Exuma Cays and Belize) using those computers in the low conservatism settings, so multiple repetitive dives over multiple days, all within NDL limits and all on +/- 32% nitrox, and they were never wildly different.

Anecdotally, on repetitive dives on multiple days, you will get higher NDL times in my opinion, on the Shearwater. It won't be as much as you would intuitively think based on the notoriety of Suunto conservatism. It will be more noticeable if you've tripped one or more of Suunto's widely discussed/rumored adjustment factors, like reverse profiles, higher ascent rates, shorter surface intervals.

 

[Kevrumbo]

. . ..My question is about the Buhlmann ZHL-16c algorithm it uses versus the DSAT that I've been using all these years. I've read up on the basics of both algorithms but for my type of diving, I'm seeing a big disadvantage of using the Buhlmann even on the LOW conservative setting.
Here's an air dive plan on the perdix vs oceanic:

(sorry the column spacing isn't preserved when posting here)
Depth Oceanic Perdix
50 81 76
60 57 50
70 40 35
80 30 25
90 24 19
100 19 15
110 16 12
120 13 9
130 11 8

As you can see, the Oceanic DSAT times, which come very close to the PADI RDP NDLs, are significantly greater than the Buhlman numbers and that's on the LOW conservative setting.

It would seem I'm limiting my dive times by using a Perdix as long as I am comfortable using the higher limits that DSAT allows.

1st dive NDLs don't tell much of the story regarding how dive computers are going to perform over multiple dives. . . I have had some difficulty in mimicking DSAT with Buhlmann due to some differences regarding depths and repetitive dives, but I can get reasonably close. You might want to dive them in parallel for awhile to see how they behave

I think you will find that Buhlmann will give you more NDL on 2nd and subseqent dives. 45/95 and 40/85 might both meet some of your requirements. . .

It does appear that way just for first dive NDL's . . .that PADI RDP yields more NDL time.

Fundamentally though for an example of a table repetitive dive profile with SI and RNT's, in terms of total bottom time allowed within NDL for a first and second dive, the results between Buhlmann ZHL16 and PADI RDP should be similar.

It's all explained very well here, with the most graphic difference shown being between the IANTD Table (Buhlmann) and the older Navy Air Table:

One of the most common comments IANTD receives from new users of these tables is that they are too conservative. When comparing the IANTD Air Table with the US NAVY Air Table, a diver will get more no-decompression dive time using the US NAVY Air Table, but only for the first dive. If the diver plans a repetitive dive it is noticed that typically there is more no-decompression dive time using the IANTD Air Table.
IANTD Comparison of Dive Tables

 

[dmaziuk]

Fundamentally though for an example of a table repetitive dive profile with SI and RNT's, in terms of total bottom time allowed within NDL for a first and second dive, the results between Buhlmann ZHL16 and PADI RDP should be similar.

If the slowest compartment in model A is 60 minutes, then over an 8-hour sleep it will off-gas to 1/256th of its evening loading. If model B's slowest compartment is 650 minutes, it won't lose even half of its loading overnight. Whether it matters in practice depends on whether you can load your slow tissues, and how much, but in theory at least model B will put you in progressively more "loaded" starting pressure group every day.

I.e. the results should be similar on day one but diverge more and more over multiple days of diving.

 

[tursiops]

I.e. the results should be similar on day one but diverge more and more over multiple days of diving.

And, of course, they do, but IANTD's two dives on one day test is not able to show that.

 

[Kevrumbo]

If the slowest compartment in model A is 60 minutes, then over an 8-hour sleep it will off-gas to 1/256th of its evening loading. If model B's slowest compartment is 650 minutes, it won't lose even half of its loading overnight. Whether it matters in practice depends on whether you can load your slow tissues, and how much, but in theory at least model B will put you in progressively more "loaded" starting pressure group every day.

I.e. the results should be similar on day one but diverge more and more over multiple days of diving.

As a hypothetical, that is a valid premise -a gross example would be residual intermediate tissue along with the slow tissue loading after multiple mandatory staged decompression dives per day over consecutive dive days using a high FN2 bottom gas like Air or Nitrox.

Here's an interesting "side-bar comment in passing" about multiple dives per day over consecutive days:

. . .The second, I think, is simply part of a general recommendation to avoid diving right up to the no decompression limits (or computer ceiling), along with the observation that none of our predictive algorithms have been adequately tested over multiple consecutive days of multiple dives per day. Having a day off periodically in such a sequence is a common recommendation perceived to be likely to increase safety.

 

[MomZo]

Thank you all for your answers. It is all I wanted to know to convince me buying a Perdix as my primary computer. I will still continue to use the Zoop as a backup and will compare their NDLs on multiple consecutive diving days.