Depth averaging tables.

[Basking Ridge Diver]

On your NAUI tables, which do not have the gray areas, your calculation is actually as I gave it originally: 80-13=67. Since the 'correct" answer is 57 minutes, not 67 minutes, use of either the NAUI or the PADI tables for this dive would be dangerous. Sorry, I do not think you get a "better understanding." I think you get an incorrect and dangerous understanding.

Actually for NAUI tables it is a bit different than PADI tables -
It is true your 80 mins at 50 feet but where you missed it is at 4 mins @ 130 feet your in Pressure Group C. You are in PG C because 5 mins is the lowest value for 130 ft with NAUI Tables so you must go to 5 mins.
At Pressure Group C you run down the table and your new Pressure Group is C.
At PG C you run over to 50 feet and where you missed it your residual time is 21 not 13.
Your Adjusted Maximum Dive time is 59 minutes.

If you use 67 you don't understand how to read the NAUI tables and you are treading into dangerous territory - which is why I stick to the tables I know.

But the difference between 57 and 59 mins is pretty darn close for me while staying in NDL...

Which is what I said in post #55.

 

[tursiops]

Please, I never said PADI and NAUI tables were the same. I said the NDL for 50 feet was the same. I've been trying to explain what the Wheel or eRDPML would give for the multilevel dive you proffered.

Your use of the NAUI tables violates the NAUI rules for using their dive tables. The use of Table 2 to get from the Table 1 output to the Table 3 repetitive dive REQUIRES the surface interval (at 1 ATA) to be at least 10 minutes, otherwise the tables are not valid. You have a zero surface interval, so you cannot use those tables. They are not designed for what you are doing. Any results you get from your procedure that compare to the actual multilevel calculations you should be making (but cannot with NAUL tables) are coincidental. I'm sorry, you cannot make up your own rules to use those tables.

 

[Basking Ridge Diver]

I'm sorry, you cannot make up your own rules to use those tables.

Know your risks, mitigate them as best you can, you and only you are responsible for you. For any one reading this thread - I am not advocating this approach for you. As well I do not know the impact of multiple dives per day using this method. Please note my disclaimer.

That said - I can because they are close enough for me - and I have said before help me plan my dives. I am comfortable using this approach because I understand the use of tables and how they might be slightly off is ok with my type of diving. I do not push NDLs - I dont need to I enjoy my diving - YMMV.

 

[tursiops]

I understand the use of tables

Actually, you've demonstrated you don't.
But I appreciate your saying to readers, don't do this.

 

[Basking Ridge Diver]

So your going to quibble over 2 mins? I even showed you were mistaken on the Naui tables - I think I get it now - it is not the tables...

 

[ScubaInChicago]

Simple 130 rule works awesome for average depth on 32% or 30/30 between the depths of 40-100'. Anything beyond 100' is taunting safety in a single tank configuration. NDLs for subsequent dives are 50% under 2hrs, and 75% after. I've done many 3hr plus dives using this method and adjusted on the fly for deco as needed. No special computer needed other than one that shows avg. depth.

 

[stuartv]

Has anyone here ever depth averaged regular tables?

Well, this thread sure went OT quickly. But anyway, it seems to me that the response to what you're asking about it pretty straightforward.

Depth averaging works for calculating gas consumption. That is because gas consumption is linear. If you descend to double the ambient pressure, you will use twice as much gas.

Depth averaging simply does not work for deco (even in it's simplest form - NDL calculation). On-gassing and off-gassing is not linear. In the popular model, on-gassing has a half-time. Actually, a bunch of half-times. If you stay at a certain depth for twice as long, you don't end up with twice as much gas dissolved absorbed into your body. Not being linear means depth averaging just doesn't give correct results - well, except for maybe in the "a broken clock is right twice a day" sense.

Some simple thought experiments should help make this clear. What if I get in the water and stay just below the surface for 30 minutes then descend to 100' for 30 minutes. After this 1 hour, will I have the same tissue loading as if I just went to 50' immediately and stayed there for 1 hour? Would it be the same as if I went to 100' first for 30 minutes and then ascended to just below the surface for 30 minutes?

Those all have the same average depth and the same amount of time and yet would all have VERY different results for tissue loading at the end of 1 hour.

Depth averaging for deco calculations does not work. You might get lucky and come up with specific examples that work out to close to the results you would get from, say, Multi-deco or The Wheel, but that's luck.

 

[Neilwood]

This discussion is interesting to me as doing a shore dive is a similar issue and one that makes it almost impossible to form an accurate plan (or knowing your Nitrogen loading for surface interval and subsequent dive purposes). Actually doing the dive is easy with a PDC.

Simple profile (depths picked for simplicity) for a dive of approx 40 minutes with equal length legs- dive at buoy A to 5m, follow the bottom the first corner B which will be at the deepest point (15m), swim parallel to the shore to the next turn C which will also be around 15m, turn to swim up to point D at 5m and swim parallel to the shore back to point A.

The only way I can figure it is averaging the depths of the legs A-B=8, B-C=15 and C-D=8 with D-A=5 (effectively a safety stop).

 

[tursiops]

This discussion is interesting to me as doing a shore dive is a similar issue and one that makes it almost impossible to form an accurate plan (or knowing your Nitrogen loading for surface interval and subsequent dive purposes). Actually doing the dive is easy with a PDC.

Simple profile (depths picked for simplicity) for a dive of approx 40 minutes with equal length legs- dive at buoy A to 5m, follow the bottom the first corner B which will be at the deepest point (15m), swim parallel to the shore to the next turn C which will also be around 15m, turn to swim up to point D at 5m and swim parallel to the shore back to point A.

The only way I can figure it is averaging the depths of the legs A-B=8, B-C=15 and C-D=8 with D-A=5 (effectively a safety stop).

Why bother with all that "depth averaging?" It gives a decent gas calculation, but can provide a misleading N2 calculation. In your example, just call it a square profile to 15m for 40 mins. That leaves you with only a small N2 buildup and no real problem for a SI or repetitive dive. Better yet, use your PDC!

 

[Basking Ridge Diver]

Using a PDC seems to promote Flying by your PDC as opposed to putting in some thought and coming up with a preplan with the intent to Plan your dive and Dive your plan. I guess that is what I was thinking with a few of the threads that have been floating around.
But what is becoming apparent is there is no standard format or approach that I see on how to plan a generic recreational dive other than square profiles or some home made plan using excel. Some folks have said their PDC can plan multilevel diving - mine does not... And to be honest I don't want to pay for some dive software since I am not planning a Tech dive...