No Decompression Limit question.

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PADI tables and Navy tables are based on Haldane theory and are empirically derived (i.e. probability calculated from real world data). Only the HUGI tables are non-Navy based. They were done using Doppler ultrasound technology to research silent bubbles in the blood stream after exposure to a hyperbarric environment.

I think the SSI tables are based off of the doppler ultrasound technology, but I was under the impression that all that stuff was done by the navy too?

 

[haha49]

everyone is diffrent so you can potential get teh bends from diveing 10 feet for 10 mins.. bvery unlikly but in theroy it can happen

 

[TimAZ]

I think the SSI tables are based off of the doppler ultrasound technology, but I was under the impression that all that stuff was done by the navy too?

I'm holding an SSI table.

It says "Doppler no-decompression limits based on Navy dive tables"

 

[TimAZ]

Somehow, 13 years ago, I got through OW thinking that as long as you are in your NDL, you have no risk of getting DCS or the bends.

Now, it seems like there is more to NDL than just shooting up to the surface as fast as you want from any depth.

Apparently, from what I seem to understand, the NDL was created with the assumption that divers will ascend at no faster than 60 ft per minute. And that more rapid ascent will null the NDL calculations. Apparently it has to do with how rapid the bubbles are formed, and how rapid you are able to exhale and decrease the partial pressure of nitrogen in your blood stream.... Kinda like letting the air out of a shook up soda pop bottle slowly, instead of rapidly.

And since some of us are not under the bell curve, the additional safety margin is obtained by the slower than 30 fps ascent rate, the 15 ft safety stop, and some even advocates a extra safety stop at 1/2 of your deepest depth.

Am I correct??

Yeah, the tables/ascent procedures have changed. Decompression theory is still a theory. We talk about fast and slow tissue compartments and such, but all we are ding is guessing at nitrogen absorbtion based on the information we have. Another theory is that we should ascend from depths deeper than 60' at 60' per minute, then slow to 30 above that depth. Expect more changes to come.

 

[SkullDeformity]

I suspect in another 10 years they'll be telling us we've been doing it wrong all along, and how are we alive?

 

[fisherdvm]

The entries for the 80' row are: 5% DCI probability at 60 minutes bottom time, 1 percent at 15 minutes, USN table 40 minutes. (Ignore for the moment that the actual risk is about 100 to 1000 times lower, the point is that analysis of thousands of dives indicates that going from 15 minute to 60 minute bottom time at 80' increases the DCI risk by only a factor of 5).


Just for completeness, here are the entries for some more depths. Each line is
DEPTH / 5% bend probablity / 1% bends probability / USN Table NDL limit
Depth 5% 1% USN
40' / 170 / 100 / 200 minutes
50' / 120 / 70 / 100 minutes
60' / 80 / 40 / 60 minutes
70' / 80 / 25 / 50 minutes
80' / 60 / 15 / 40 minutes
100' / 50 / 8 / 25 minutes
120' / 40 / 5 / 15 minutes

These are all for a straight 60fpm ascent, no safety stop. It is my personal guess that the very wide variance in 1% vs 5% times at 80' and deeper would be less if the table was showing data for a slower ascent profile, or one that included a 3 minute safety stop. But in any case, even at the shallower depths, you need to extend the bottom time by about a factor of 2 to get 5 times higher DCI risk. Very fuzzy indeed.

Charlie Allen

I will bet that eventually, they will have a separate setting on your dive computer for 1. Age (over 40). 2. Sex. 3. Smoker vs. Non-smoker. 4. Diabetes - especially if ongoing for years. 5. History of arterial disease. 6. History of heart attack. 7. Chronic hypertension.

I think common sense would say you have to add a minimum of at least 5 minutes at the 15 ft safety stop, and ascent at no faster than 30 ft with the mentioned conditions (with the exception of gender, which might be more liberal with women and less with men).

 

[oly5050user]

PADI tables and Navy tables are based on Haldane theory and are empirically derived (i.e. probability calculated from real world data). Only the HUGI tables are non-Navy based. They were done using Doppler ultrasound technology to research silent bubbles in the blood stream after exposure to a hyperbarric environment.

Navy tables and PADI tables 2 different animals..Navy table based on 120 minute half times,so this gives you the 12 hr residual nitrogen between dives..
PADI based on 60 minute half time,gives you 6 hour residual nitrogen times.
First dive on Navy tables are longer than PADI's but repetitive dives are shorter than PADI tables.

 

[Charlie99]

I will bet that eventually, they will have a separate setting on your dive computer for 1. Age (over 40). 2. Sex. 3. Smoker vs. Non-smoker. 4. Diabetes - especially if ongoing for years. 5. History of arterial disease. 6. History of heart attack. 7. Chronic hypertension.

No point in trying to modify the dive computer to try to take into account even more uncertainties. What bends you is the profile you dive, not what your dive computer says. Nothing says you have to surface after every dive with your computer showing you right on the edge of deco. As I pointed out in my post above, there is no real difference between being a few minutes into deco and having a couple minutes of NDL left.

I prefer to have a computer which, in a relatively clear unabiguous fashion shows my relative degree of N2 loading. That's why I'm such a fan of the tissue-loading bargraph of the Pelagic computers. It give me info. I decide how to dive.

I think common sense would say you have to add a minimum of at least 5 minutes at the 15 ft safety stop, and ascent at no faster than 30 ft.

Agreed. Whether or not we like it, you are entering the DCS lottery each time you dive. Good ascents and stops further stack the odds in your favor.

 

[Charlie99]

PADI tables and Navy tables are based on Haldane theory and are empirically derived (i.e. probability calculated from real world data). Only the HUGI tables are non-Navy based. They were done using Doppler ultrasound technology to research silent bubbles in the blood stream after exposure to a hyperbarric environment.

What is the source of your info Muddiver?

On the HUGI table itself the first sentence is "These tables have been developed mathematically and have not been subjected to testing to validate them". Taking a short look at them, it appears that they are based upon the doppler adjusted USN/Workmann table, just like many other tables such as NAUI, SSI, and YMCA. The difference is that they have arranged the repetitive letter groups differently, so that that going up to the NDL will always put you in letter group N. Offgassing is assumed to occur at a 120 minute halftime rate, just like all of the other USN-derived tables. Expressed in a different way, the Hugi tables are based upon the USN Doppler limits, with the faster ongassing compartments assumed to have a 120 minute halftime for offgassing. This is similar to the "surface credit control" feature of some dive computers, such as the Pelagic/Oceanic series where compartments faster than 60 minutes are restricted to offgassing at a 60 minute halftime once you have surfaced.

The PADI/DSAT tables were based upon real life testing, including doppler monitoring. As Oly5050user mentioned, the PADI table repetitive dive calculations are based upon a 1 hour compartment, while the USN-derived (and Hugi) tables are based upon the 2 hour compartment. The 2 hour compartment is important only in very long, shallow dives such as spending hours working on the bottom of a 35' draft ship using surface supplied air. Normal single tank recreational dives are normally limited by much faster compartment. As a rule of thumb, the limiting compartment for a square profile dive has a halftime about 2/3 of the NDL for that depth. More specifically, it is only for NDL dives at 40' and shallower that even the 1 hour compartment becomes the limiting one, and only for even shallower dives dives does the 2 hour compartment become a limit.

The net effect is that the USN-derived (and Hugi) tables are poor choices for tracking recreational dives, in the sense that the actual risk from diving to a table limit varies dramatically. To put it another way, the USN tables (and USN-derived tables) are very conservative on 2nd dives compared to dive computers that fully implement the underlying model. The same is true for the PADI table when compared to dive computers running the same model, but the variance between table and computer is not as dramatic.

===========================

The PADI tables, the USN tables, the Hugi table, the tables at www.dir-diver.com are all based on a neo-Haldane model of mutiple dissolved gas compartments of varying halftimes. While not perfect, this model has been a very useful tool for 100+ years. The Canadian DCIEM tables are also based upon a dissolved gas model, but one which assumes a serial transfer from faster to slower compartments rather than the all parallel model of the neo-Haldanian models. NAUI has issued in the last year or so a set of RGBM tables based upon a dual phase, aka "bubble", model. There are several decompression programs that implement the VPM model, a well documented dual phase/bubble model. The Buhlmann model is yet another popular neo-Haldanian model. There are both tables, dive computers, and decompression programs that use this model.

For NDL diving, particularly table-based diving, the choice of model isn't all that important.

On a practical basis, most diving is done using old fashioned dissolved gas / neo-Haldane models while shaping the ascent and stops based experience that show that deep stops and safety stops improve safety and reduce post-dive fatigue. The thermodynamic and bubble models came along later and provided some theoretical confirmation of what divers had already discovered empirically.

Tables and computers only set limits. The diver determines what is actually done in terms of ascent and stops.

 

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I'm holding an SSI table.

It says "Doppler no-decompression limits based on Navy dive tables"

Well ok then, there's no better way to confuse me then to throw in 20 terms that are all similar. Thanks.