Dive Tables vs. Computer Algos - repetitive rec dives

[Jay]

How conservative/aggressive are PADI (DSAT) dive tables versus various computer algorithms on repetitive rec dives? Do DSAT algorithms match the tables? (I would expect so, but don't know).

If your computer's plan mode / dive software can provide three NDLs for these three square ocean dives (no SS) using Dive Times below. Please let me know what algo and/or computer you're using.

Dive 1 - clean dive to 30m for 15min BT / 21min DT inc SS (tables 5min NDL - Group J)
Dive 2 - SI 40min (E), 20m for 24min BT / 29min DT inc SS (5min NDL - R)
Dive 3 - SI 59min (G), 18m for 24min BT / 26min DT no SS (10min NDL - R)

Adjustments: I'm assuming computers will insist upon inputting Dive Time (and I'm hoping they can input multi dives - my Geo 2 can't). If the software permits entering Bottom Time then that definitely would be more accurate for an apples/apples comparison. The Dive Tables require / assume a SS for the first two dives.

- Bottom Time definition (tables): time measured from the beginning of descent to beginning of the ascent.

- Dive Time (computers): time between leaving the surface to descend, and returning to the surface at the end of a dive.

Ascent times: @ recommended ascent rate: 30fpm / 9.144m/min:
Dive 1 - 3min 16sec -> round to 3min BT
Dive 2 - 2min 11sec -> 2min
Dive 3 - 1min 58sec -> 2min

Dive Times:
Dive 1 - 15+3+3=21
Dive 2 - 24+2+3=29
Dive 3 - 24+2=26

 

[JohnnyC]

I don't know any computer that will do that because the NDL the computer is displaying for repetitive dives is based on actual gas loading during the first dive. You're comparing apples to oranges any way you slice it. You're also giving two separate definitions of bottom time. Computers just don't work the same way tables do, and thus predictive algorithms when diving a computer will be an estimation at best. You're only going to get comparable data if your dive profile exactly matches your square table profile, and a computer WILL NOT calculate it the same because it's actually tracking in real time. Really the only way you could physically replicate it would be in a pressure pot, so I'm not sure what exactly you're trying to accomplish. Of course unless you're doing a chamber dive you're not going to replicate the profile, so any data you glean from this comparison is going to be inaccurate simply because it has zero correlation to actual use.

Even comparing DSAT tables to a computer running the DSAT algorithm will give you different results.

You can plan the dives in a program like MultiDeco if you want, but because of the way it works it doesn't show NDL's. As an example, for 30m for 15 minutes, a GF of 50/80 gives a stop of 1:20, whereas a GF of 95/95 doesn't call for any stops at all. It's way to much screwing around to try and find the gradient factors that won't give you an obligation. It also won't give you a safety stop. It will either tell you that you don't have to stop, or you have a required obligation. A required 5m 3 minute obligation in MultiDeco IS NOT the same as a safety stop.

Fundamentally you're talking about two different ways of dive planning, and the reason computers are much preferable to tables for recreational no-stop diving.

It also demonstrates the fallacy of "conservatism" in dive planning algorithms, where they are often compared in terms of what "gives" you more NDL time. The fact is, because it's just a mathematical model trying to approximate a physiological response, the term really only means, "how long before the computer gets pissed off and starts yelling at you," and doesn't correlate to your personal limit where a decompression obligation would be required.

 

[tbone1004]

GF's of 45/95 will roughly correlate to PADI's DSAT table per Shearwater. That to me is aggressive, but with PADI's tables it assumes square profiles which usually has some conservatism built in. The computer will not

 

[Diver0001]

How conservative/aggressive are PADI (DSAT) dive tables versus various computer algorithms on repetitive rec dives? Do DSAT algorithms match the tables? (I would expect so, but don't know).

If your computer's plan mode / dive software can provide three NDLs for these three square ocean dives (no SS) using Dive Times below. Please let me know what algo and/or computer you're using.

Dive 1 - clean dive to 30m for 15min BT / 21min DT inc SS (tables 5min NDL - Group J)
Dive 2 - SI 40min (E), 20m for 24min BT / 29min DT inc SS (5min NDL - R)
Dive 3 - SI 59min (G), 18m for 24min BT / 26min DT no SS (10min NDL - R)

Adjustments: I'm assuming computers will insist upon inputting Dive Time (and I'm hoping they can input multi dives - my Geo 2 can't). If the software permits entering Bottom Time then that definitely would be more accurate for an apples/apples comparison. The Dive Tables require / assume a SS for the first two dives.

- Bottom Time definition (tables): time measured from the beginning of descent to beginning of the ascent.

- Dive Time (computers): time between leaving the surface to descend, and returning to the surface at the end of a dive.

Ascent times: @ recommended ascent rate: 30fpm / 9.144m/min:
Dive 1 - 3min 16sec -> round to 3min BT
Dive 2 - 2min 11sec -> 2min
Dive 3 - 1min 58sec -> 2min

Dive Times:
Dive 1 - 15+3+3=21
Dive 2 - 24+2+3=29
Dive 3 - 24+2=26

You are comparing apples and oranges.

The short answer is that tables are generally more conservative because there are a lot of rounding errors. This results from the fact that you are almost never at your maximum depth during the entire dive.

The mathematical algorithm underpinning the tables, however, would be considered fairly aggressive if it were just put 1:1 into a computer that sampled your depth every few seconds and eliminated those rounding errors.

I think there is an article you can find on the internet from ... Baker called "Understanding M Values" that has a table in it showing the exact 1/2 times and M values used to calculate the tables. Most computers these days are tweaked to be considerably more conservative than that.

R..

 

[Diver0001]

GF's of 45/95 will roughly correlate to PADI's DSAT table per Shearwater. That to me is aggressive, but with PADI's tables it assumes square profiles which usually has some conservatism built in. The computer will not

Tables don't define a floor. Without crunching any numbers my gut feeling is that GF's of 95/95 would probably put you closer to what a table suggests. Not quite sure how to make a Shearwater calculate a manual safety stop.

R..

 

[tursiops]

Shearwater is the only computer I know of that allows you to look ahead at surface intervals, like having 30 mins now but wondering what 90 mins would do for your repetitive NDL. So, your request for comparisons can only be done with a Shearwater.

 

[RonR]

Shearwater is the only computer I know of that allows you to look ahead at surface intervals, like having 30 mins now but wondering what 90 mins would do for your repetitive NDL. So, your request for comparisons can only be done with a Shearwater.

Actually, we have had that function in the Atomic Cobalt all along, as well as the ability to calculate future NDL's with varying EAN values. Cobalt Guide:No-Stop Time Calculator

Also you could do this using the onboard dive simulator/ planner:
Cobalt Guide: Simulation Menu
The Cobalt's onboard dive simulator basically replicates the function of desktop dive planning software, and it will show your NDL (or ascent time in deco) at any point in a simulated dive. And it allows for saving multiple repetitive dive profiles and surface intervals. Using sea level and a standard risk setting and moderate exertion settings, I ran those profiles. I used the function on the Cobalt that automatically generates an ascent profile at 30 fpm (or a surfacing schedule if you have stops). Differences in safety stop times generate a slight difference in overall dive times.

At the 15 min. point of the 1st dive, starting ascent, the NDL was 4 min. That dive was 20 minutes on the Cobalt, probably because the safety stop generated was 2 minutes.
At the start of ascent on the 2nd dive, the NDL was 3 minutes, total dive time was 30 minutes.
Dive 3 showed 13 min NDL at the start of ascent, 26 min. overall dive time.
These dives would be within the "folded" RGBM algorithm- basically a Buhlmann model with bubble factors added. At 45 m we would be switching over to a fully iterative RGBM calculation.

I'm not sure what this would prove, however. Such square profiles don't exist in real life, only as abstractions. The great advantage of computers is that they track what you actually do in real time.

Ron

 

[dmaziuk]

Uhmm... what is it that you're actually trying to achieve?

Seriously, if that's your real diving schedule, then you don't need a computer. So why do you care what it might compute? You can be reasonably certain that "rec RGBM" computers won't be happy with 40 min SI, nor with direct ascents w/ bare minimum 3 min SS. But that shouldn't affect those first 3 dives, so now what.

If you add an afternoon dive or two, and do that schedule for a few days straight... then I'd strongly recommend reading the original DSAT paper first, esp. the bit right in the beginning where they bent their test diver on day 2.

 

[Jay]

On what it is I'm trying to achieve:

I'm trying to understand how conservative or aggressive PADI (DSAT) dive tables are versus various computer algorithms on repetitive rec dives. We generally know how various algos stack up versus each other on single / rep (rec) dives, but we (or at least I) don't know if my thought that tables are relatively aggressive (as are DSAT computers, rel to other computers / algos) is an accurate thought or not. Yes, this is quite a theoretical question.

Additionally (and not the main reason for opening the topic), on a recent shipwreck dive, one of the guides and I were chatting computers/algos and tables and he felt that the Dive Tables were too conservative. The core part of some of these dives is pretty square. My view on the tables was that they are relatively aggressive, they've been scaled back over the decades, and any computer on the most aggressive setting wouldn't be more aggressive than the tables (using their square table profile).

 

[Jay]

On apples/other fruits etc:

I've selected times and depths and times that don't involve (table) rounding, neat square dive profiles (as per tables), SS as required by tables, and adjusted for Bottom / Dive Time differences. I don't see where or why there's other comparison errors inherent or added to the question in the way I put it. If so, please do specifically say so.

Edit: obviously I've rounded the ascent times (minor error), and I guess that the NDL times generated by the computer or algo might be rounded?