Deco Tables

[Rick Murchison]

Then who is right and who is wrong. NOAA tables require a 29 min stop at 10 feet and if you do the EAD conversion from 36% you would need to a 21+ min stop on the air tables.

If that is the case then you might as well do 90mins on air at 70 ft and ignore the 23min stop at 10ft as well

I am not following the logic.

They are both "right" within the parameters and rules of the tables used. The NOAA and Navy air table procedures are based on 10 foot incremental steps. If you look at the NOAA Air table (which is the Navy Table without Doppler correction) you'll see that at 40' you have a NDL of 200 minutes while at 50' you drop to 100 minutes, and the table procedures dictate that as soon as you go the tiniest bit deeper than 40' you immediately use the next step - 100 minutes. This obviously adds a considerable additional margin of safety in the low 40's to the safety margin that's already there at 50' and 100 minutes. So, a table that normalizes its depths to the nearest 10 ft of EAD (like the SSI combined air/Nitrox does, for example) will be considerably different than one that calculates NDLs based on 10 ft increments on the mix involved.
Let's use the 60' example.
If I use the SSI combined Air/Nitrox table then I come down under 36% to the first depth equal to or more than 60'. In this case that's 69' and I am on the 50' EAD line. That yields a NDL of 70 minutes on the doppler or 100 minutes on the non-doppler table.
On your table, however, you go to 60' at EAN 36 (not 69') and your table makes its calculations at the 42' EAD rather than the 50' EAD mine uses - and Voila! 170 minutes NDL.
Let's look at one where your table will likely be more conservative... look at a dive to 81', which will force you to the 90' line on your table, where the EAD is 67'. Using EAD and the Navy table I can use an EAD of 60', with a NDL of 60 minutes. My bet is yours is in the 50-55 minute range, right?
Rick

 

[Bigcape]

They are both "right" within the parameters and rules of the tables used. The NOAA and Navy air table procedures are based on 10 foot incremental steps. If you look at the NOAA Air table (which is the Navy Table without Doppler correction) you'll see that at 40' you have a NDL of 200 minutes while at 50' you drop to 100 minutes, and the table procedures dictate that as soon as you go the tiniest bit deeper than 40' you immediately use the next step - 100 minutes. This obviously adds a considerable additional margin of safety in the low 40's to the safety margin that's already there at 50' and 100 minutes. So, a table that normalizes its depths to the nearest 10 ft of EAD (like the SSI combined air/Nitrox does, for example) will be considerably different than one that calculates NDLs based on 10 ft increments on the mix involved.
Let's use the 60' example.
If I use the SSI combined Air/Nitrox table then I come down under 36% to the first depth equal to or more than 60'. In this case that's 69' and I am on the 50' EAD line. That yields a NDL of 70 minutes on the doppler or 100 minutes on the non-doppler table.
On your table, however, you go to 60' at EAN 36 (not 69') and your table makes its calculations at the 42' EAD rather than the 50' EAD mine uses - and Voila! 170 minutes NDL.
Let's look at one where your table will likely be more conservative... look at a dive to 81', which will force you to the 90' line on your table, where the EAD is 67'. Using EAD and the Navy table I can use an EAD of 60', with a NDL of 60 minutes. My bet is yours is in the 50-55 minute range, right?
Rick

Yep 53 mins

So it appears that even though on the NOAA tables Ean36% the NDL limit is 100mins at 60fsw then the 24min deco stop is overkill?

I understand that the tables are in 10fsw incriment and of course they can be split up into 1fsw incriments if one chose to do the painstaking math.

I just am having a hard time with 70min difference. Is the 60fsw and less range the range of the most flexibility as compared to deeper dives?

for example:

Has anybody published a 1fsw incriment table starting at 40fsw? The NDL gap is 100 mins between 40 and 50fsw. However it is only 5min gap between 90 and 100fsw Yet we have the same 10fsw difference

 

[Bigcape]

With so many different variances how does one really plan a dive.

I mean on one table I need to do a deco stop for 29 mins and another I can stay down longer with NO deco stop!!

Sheeeeeeez

 

[Rick Murchison]

Has anybody published a 1fsw incriment table starting at 40fsw? The NDL gap is 100 mins between 40 and 50fsw. However it is only 5min gap between 90 and 100fsw Yet we have the same 10fsw difference

Now you're starting to get into some of the really sloppy areas of the tables. Why, for example, would you concern yourself with one foot increments when you're more than a foot thick in every direction? What's your depth? Where your heart is? Or your head? or your feet, or your hands? Where do you clip off your depth gauge???
The tables tighten with depth because bubble growth is a function of "Delta P over P" - that is, the change in pressure as you ascend over the total pressure (at around 100', it takes 4' of vertical movement to get the same bubble growth that you'd get in one foot at the surface). Bubble formation in the first place (or bubble growth threshold differential in more modern deco theories) is a function of a threshold ratio between dissolved inert (ok, hydrogen ain't inert but we ain't talkin' 'bout *that* technical here) gas pressure and ambient pressure. At some point on the shallow end your theoretical NDL is infinite because that threshold ratio is greater than the possible ratio based on the gas you're breathing. Both these factors in the theory mean that the curve starts at infinity somewhere around 20 to 30 feet (EAD), depending on the table. The Navy table is only truncated at 30 ft and shallower because the next repetitive group would exceed the 12 hr table limit in NDL time.
Bottom line - pick a table and use it. The rules for its use may give some startlingly different results along the (particularly shallow) edges compared to other tables, but those are structural rather than substantive, and you'll find all of 'em are reasonable and safe in the end - and a whole lot closer together in the "normal" diving envelope.
Rick

 

[Bigcape]

Fantastic response, very informative.

The shallower, closer to infinity makes sense to me. So now does the sloppyness in 60fsw and less in diving.

When i was referring to 60fsw to 60fsw, not 3 or so feet in either direction the number 100 vs 170 seemed like a dangerous swing in NDL times.

The tables haved not changed since my first table dated 1983. So, when I got one that said 170mins vs 100 mins for the exact same depth it concerned me at first that it was a typo in the table.

Thank you for your time in explaining it.


Oh, I wasnt concerning myself with one foot incriments, it was the fact that as I move into doubles with stages for working at the 60fsw range i was preparing myself for being down longer than 100mins and was thinking I was planning a DECO dive. (29min stop at 10fsw) Then I see this 170min NO DECO limit and I thought, "what the ****, this can't be right"

 

[Meister481]

Quickest hijack in history..... 2nd post.....

 

[PvilleStang]

If I may here, NDL vs. Depth is by no means a linear graph, thus the reason there's a 70 minute gap between a 10 foot increment shallow in the water column, and a 5 minute gap deeper in the water column. I would be interested if anyone had an algorithm off hand that fit the "best fit" for NDL vs. Depth.

 

[Charlie99]

If I may here, NDL vs. Depth is by no means a linear graph, thus the reason there's a 70 minute gap between a 10 foot increment shallow in the water column, and a 5 minute gap deeper in the water column. I would be interested if anyone had an algorithm off hand that fit the "best fit" for NDL vs. Depth.

Hempleman's (sp?) formula. Square root of NDL is proportional to depth.

Or to phrase it in reverse, NDL is the square of (440/depth). (NDL in minutes, depth in feet.)

Unlike the "120 Rule", this one also works for metric ..... NDL = (130/depth meters) ^ 2.

Obviously, this rule also deviates from the tables at depths less than 35', but is a pretty good approximation for 40' on through 130'.

Charlie Allen

 

[limeyx]

With so many different variances how does one really plan a dive.

I mean on one table I need to do a deco stop for 29 mins and another I can stay down longer with NO deco stop!!

Sheeeeeeez

Simple (well, maybe not "simple" but ...)

1) In your classes that covered nitrox/advanced decompression, your instructor should have covered multiple decompression algorithms (RGBM, buhlman, VPM) etc. and given an overview of how they are different

2) In the same class, the insrtructor should also cover ways to calculate deco schedules (computer, tables, some other vodoo methods that get people here all heated up)

Now, obviously what is covered could depend on the instructor and/or which agency you take the class at.

There are two realities:
(1) For most of the diving most of us do (say down to 160 feet), the differences in "most" methods are small enough that they are pretty safe
(2) As you have discovered, deco is something more than an "Educated guess" but definitely not foolproof (esp. outside rec limits) -- this makes some people extremely uncomfortable but that's the way it goes.

So what you do is try to work with your instructor (you did take a deco class, right?) to come up with something that's workable. Then you start off conservatively and (if you like) you can experiment with adding/removing or shaping your deco from that start point.

Decompression is also affected by other things such as hydration, fitness, water temps, current etc. so be prepared to dial back from there.

Some of the differences are down to different "philosophies" behind the deco model (dissolved gas, bubble model etc.)

so the best way really is to start with something someone you trust is using that works for them, and work from there.

 

[Bigcape]

Quickest hijack in history..... 2nd post.....

Thank you.