Info Why are tables not taught in OW classes anymore?

[steinbil]

This conversation also reminds me of something I learned from diving with tables that I think a lot of computer users miss out on:
As part of planning/executing a dive with the tables, we were taught to adjust our MDL limits based on other factors, like temperature or how good/comfortable you feel. Of course you can do this (and teach it) with computers, by changing the GF or padding the NDL, but I think the idea that the computer "does everything for you" can make some divers less aware and take less control over how they use the computer. While my experience using tables gave me a feeling of responsibility for calculating my own safety margins for a particular dive. I'm sure this can be achieved with good education also, using computers, but I think it puts the onus on the instructor to make the students dive their computers instead of the reverse...

 

[scubadada]

Hi @steinbil,

Do you do your dives with table, as you describe, or do you use a computer?

 

[steinbil]

Hi @steinbil,

Do you do your dives with table, as you describe, or do you use a computer?

As I've mentioned a couple of times in this thread, I now mostly dive my Perdix with a GFhi of 85, but I feel comfortable diving it in gauge mode as a bottom timer with tables. The computer is, of course, more convenient and flexible, especially for weird profiles or dive trips with several dives per day. As I mostly dive locally, single dives, from a boat, I could easily just use tables.

 

[tursiops]

GUE minimum deco table (for EAN32)

Most GUE divers would do the example dive on EAN32, but if Air was the only option, one shortcut for diving air on the nitrox table is to "use a deeper table" giving you a shorter bottom time. So instead of 40 minutes at 27m, you would have 30 minutes (equivalent to 30m on EAN32).

Thank you for the table. At first glance it is quite similar to the PADI RDP for 32%.

Depth	GUE NDL	PADI RDP NDL
40 ft/12 m	344 mins	not listed
50 ft/15 m	170	156 mins
60 ft/18 m	100	90
70 ft/21 m	60	60
80 ft/24 m	50	45
90 ft/27 m	40	35
100 ft/30 m	30	30

Can you confirm that "depth" on the GUE table means bottom depth? I understand the principle of averaging the variations in a non-flat bottom, but does the "depth" on the GUE table mean that only the bottom roughness has been averaged out to some "average bottom depth?"

The point of moving down one depth line (3m) on your table to compensate for the nitrogen uptake difference between 21% and 32% is an interesting approximation; it is equivalent to a depth change of 3m. But the actual depth change needed depends on the starting depth, and is more like 6m (two lines) than 3m (one line). For example, the EAD for 32% at 81 ft (24m) is 100 ft (30m).

 

[steinbil]

Can you confirm that "depth" on the GUE table means bottom depth? I understand the principle of averaging the variations in a non-flat bottom, but does the "depth" on the GUE table mean that only the bottom roughness has been averaged out to some "average bottom depth?"

I'm not 100% sure I understand your question, so I'll try to explain my understanding of the table:
The depth stated in the tables are bottom depths as max depth in a square profile. To supplement this, for practical use, they allow using weighted averages on certain dive profiles (continuous ascent profile, multi-level bottom phase with vertical ascent or the square profile with averaging variations in a non-flat bottom). The average should never include the ascent phase (min deco ascent). So on the continuous ascent profile -> average max depth and 50% depth. On a multi-level bottom phase, use a weighted average of the different depths (deepest first) before starting a vertical ascent.

The point of moving down one depth line (3m) on your table to compensate for the nitrogen uptake difference between 21% and 32% is an interesting approximation; it is equivalent to a depth change of 3m. But the actual depth change needed depends on the starting depth, and is more like 6m (two lines) than 3m (one line). For example, the EAD for 32% at 81 ft (24m) is 100 ft (30m).

I might be misrepresenting how to use these tables with air, since I very rarely dive air, so it might not be accurate. But I do know the practical memorized rule-of-thumb limits for doing this on the fly without a table, are 30 meters 30 minutes on EAN32 and 30 meters 20 minutes on air -> and add 10 minutes for each 3 meter shallower. (I guess this is the rule of 130 when using feet). This will be very conservative at shallow depths. Since that gives 30 minutes at 27m on air, I deduced that the difference would be 3 meters. I do agree that it seems slightly strange, considering the EADs, but maybe this only works with the 130 rule and not the table, since the added conservatism at shallower depths would make up for the difference?

 

[tursiops]

The depth stated in the tables are bottom depths as max depth in a square profile.

I'm good with this.

they allow using weighted averages on certain dive profiles

Not sure what "weighted" means...some depths are more important than others?

The average should never include the ascent phase (min deco ascent). So on the continuous ascent profile -> average max depth and 50% depth. On a multi-level bottom phase, use a weighted average of the different depths (deepest first) before starting a vertical ascent.

Seems a bit....arbitrary, frankly. "Multi-level bottom phase" means nothing to me. Is this GUE-speal for a multi-level dive?

I might be misrepresenting how to use these tables with air, since I very rarely dive air, so it might not be accurate. But I do know the practical memorized rule-of-thumb limits for doing this on the fly without a table, are 30 meters 30 minutes on EAN32 and 30 meters 20 minutes on air -> and add 10 minutes for each 3 meter shallower. (I guess this is the rule of 130 when using feet). This will be very conservative at shallow depths. Since that gives 30 minutes at 27m on air, I deduced that the difference would be 3 meters. I do agree that it seems slightly strange, considering the EADs, but maybe this only works with the 130 rule and not the table, since the added conservatism at shallower depths would make up for the difference?

The rule of 130 is pretty aggressive; the old US Navy rule was 120. the RDP is 110 (all on air). See Info - The Rule of 120 for a discussion.

 

[steinbil]

Not sure what "weighted" means...some depths are more important than others?

Weighted by time spent. If you spend 20 minutes at 30m and 10 minutes at 18m, you can't average it to 24m.

Seems a bit....arbitrary, frankly. "Multi-level bottom phase" means nothing to me. Is this GUE-speal for a multi-level dive?

I gave an example earlier the thread: You descend on a wreck and spend part of the dive on the deeper part of the wreck at 30m and part of the dive on a shallower part at 18m, then do a vertical ascent. As long as you start with the deeper part, GUE allows this profile for depth averaging.

The rule of 130 is pretty aggressive; the old US Navy rule was 120. the RDP is 110 (all on air). See Info - The Rule of 120 for a discussion.

With the 130 rule I was referring to EAN32, as you can see by my numbers (30 minutes at 30 meters = 30 minutes at 100ft).
This seems to perfectly align with the PADI RDP for EAN 32.

It seems the GUE rule of thumb for on the fly MDL dives on air would be the 120 rule, which seems slightly more aggressive at some depths (gives 4 minute longer NDL at 18m).

 

[tursiops]

Weighted by time spent. If you spend 20 minutes at 30m and 10 minutes at 18m, you can't average it to 24m.

Got it.

I gave an example earlier the thread: You descend on a wreck and spend part of the dive on the deeper part of the wreck at 30m and part of the dive on a shallower part at 18m, then do a vertical ascent. As long as you start with the deeper part, GUE allows this profile for depth averaging.

So the depth averaging is just over the part of the dive not including the ascent to the surface. It is an average of the various levels, time-weighted at each. That underestimates the nitrogen that has been on-gassed, since it is on-gassed more effectively at the greater depth.

This seems to perfectly align with the PADI RDP for EAN 32.

I don't think so. The PADI 32% RDP has a Rule of 125, not 130. You have to find the time-depth combo that gives the smallest sum; that provides the Rule. All other depths are then more conservative. For the RDP it is at 80 and 90 feet. For the GUE table it is at 70-100 feet, giving a Rule of 130.

 

[JoeTPhilly]

This is all getting confusing with these tables and bickering. This thread itself probably shows why tables aren't taught anymore. LOL

 

[steinbil]

So the depth averaging is just over the part of the dive not including the ascent to the surface. It is an average of the various levels, time-weighted at each. That underestimates the nitrogen that has been on-gassed, since it is on-gassed more effectively at the greater depth.

Correct. Yes, in theory it will underestimate the nitrogen loading. I'm guessing the added conservatism of always rounding to a deeper depth and generally erring on the conservative side makes up for this in practical diving. They also limit this to certain dive profiles, as I said. I'm no expert on this, so don't quote me on that, but it seems to make sense. And in practice it seems to be working, giving safe dive profiles that usually mimics the typical buhlmann computers with some conservatism. I'm open to the fact that it might have weaknesses, and I would be interested in seeing whether there are certain conditions that would make this practice unsafe, which is why I'm participating in this discussion.

I don't think so. The PADI 32% RDP has a Rule of 125, not 130. You have to find the time-depth combo that gives the smallest sum; that provides the Rule. All other depths are then more conservative. For the RDP it is at 80 and 90 feet. For the GUE table it is at 70-100 feet, giving a Rule of 130.

I couldn't find the imperial version online, so I looked at the metric one, and it seems I was tricked by the strange uneven distribution of depths. For whatever reason (probably to make it easier to memorize?) they go from 30m to 26m to 24m to 22m instead 30-27-24-21m. In that sense, the GUE table is slightly more aggressive at those depths. It would be interesting to see how the PDI RDP and the GUE MDL stack up against Buhlmann, for reference.

Is there any way to simulate dive profiles in a deco app? I guess for square profiles to mimic/compare the tables you can just input different bottom times/depths until you hit deco... There must be a better way?