Decompression Modules

[The Kraken]

First and foremost:
1 - I'm not trolling
2 - I'm not going to be doing decompression dives without proper training.

Now, all of that said, I was comparing two decompression profiles, one generated by "V-Planner" and the other taken from the U.S. Navy air decompression tables.

The two seem to be siginificantly different.

As specified: 100' dive on air for a period of 40 minutes.

Without getting into the exact bottom time, the two decompression profiles are listed as follows:

V-Planner;
50' for 00:20 min.
40' for 4:00 min.
30' for 7:00 min.
20' for 10:00 min.
10' for 21:00 min.

U.S. Navy
10' for 15 min. (Resulting Pressure Group "K")

I really don't know how to ask this question, but given the comparison, is the Navy table a safe table to use, or is it more for an emergency scenario?

 

[The Kraken]

Perhaps I should have named this "decompression models" instead of "modules".

 

[IslandHopper]

Just look at the two schedules .... now which one do you think is LEAST likely to bend you ?

There's a ton of good reading regarding deco at www.wkpp.org ... while you don't need to do thier type of diving, it may give you a lot of insight into how decompression works. (just don't try that at home kids, thier procedures are designed for He mixes and staged deco ... )

Another good place to start, is the first two chapters of PADI's "Encyclopedia of recreational diving" ... don't just read it, but UNDERSTAND it.

 

[ScubaDadMiami]

Navy tables were designed for 20 year olds who run miles everyday, work out constantly, are of the proper weight, swim more miles than a fish, etc. They were also designed (speaking of the more recent versions) at a time when certain aspects of physiology could not be examined using high tech equipment and before wide spread use of doppler technology.

Up until about a year and a half ago, I dove only the Navy tables, because they were the tables that had the most dives by more divers around. However, about ten years ago or so, as more information came out concerning their somewhat aggressive profiles (a la "bend and mend"), I started modifying them with my own personal fudge factors. For example, I would typically include all of my time up to my first deco stop as the bottom time for calculating my decompression obligation, I used a thirty foot per minute ascent rate rather than sixty, I would pull my ten foot stops at twenty feet, and I would throw in some additional stops along the way (depending on the profile). Also, don't forget that most dives are not exactly square profiles. So there was even more conservatism thrown in by considering all dives as square profiles.

Overall, this worked pretty well for me. However, I can say that I did have post dive fatigue frequently enough, leading me to conclude that I was undergoing "subclinical" DCS at times.

Now, I use V-Planner with the "plus one" conservatism factor (as opposed to nominal). This has worked well for me, and I would also say that the only fatigue I ever feel post dive is from lack of sleep the night before the dive.

I still carry my Navy tables with me in my pocket as a backup. After a while of using V-Planner, you get to pick up on the decompression algorithm patterns. So you can see what kind of adjustments to make to the Navy tables if you have to operate on the fly. Throw in a little ratio fudge factor here and there, and you are pretty close to what you would get by generating a table through V-Planner. Nice to know if you should ever need it in an emergency. I would not advise planning your diving this way.

 

[Abratty]

VPlanner uses a bubble model for its deco, the navy tables I believe use Workman m-values.

Basically, the idea originally is that different tissues absorbed inert gases at different rates and off-gased at similar rates. Without getting technical on a technical question, these rates were called M-Values and were the basis of Buhlman and most modern dive computers.

What was discovered empirically by tech divers was that they felt better after deeper stops than recommended by the workman or buhlman numbers. This lead to the development of various bubble theories, one of which is called the VPM (Vplanner uses this or a derivative of it, I dont recall). Bubble theory takes into account the dissolved gases in your body that Haldene, Workman, and Buhlman worked on, as well as free gas.

Hope that helped without being too technical.

If you want more in depth detail I can give you many MANY equations.

Allen

 

[jbd]

First and foremost:
1 - I'm not trolling
2 - I'm not going to be doing decompression dives without proper training.

Now, all of that said, I was comparing two decompression profiles, one generated by "V-Planner" and the other taken from the U.S. Navy air decompression tables.

The two seem to be siginificantly different.

As specified: 100' dive on air for a period of 40 minutes.

Without getting into the exact bottom time, the two decompression profiles are listed as follows:

V-Planner;
50' for 00:20 min.
40' for 4:00 min.
30' for 7:00 min.
20' for 10:00 min.
10' for 21:00 min.

U.S. Navy
10' for 15 min. (Resulting Pressure Group "K")

I really don't know how to ask this question, but given the comparison, is the Navy table a safe table to use, or is it more for an emergency scenario?

The Navy tables are not just for emergencies. As noted above they are based on dissolved gas in tissues. The newer models consider bubbles and RGBM is a dual phase model looking at both bubbles and dissolved gas. Deep, short stops reduce bubble growth and the other longer shallower stops allow more more nitrogen to be off gassed in dissolved form.

As noted above the Navy tables are more in line with lean healthy young folks instead fat old long time smokers. Nitrogen is 5 times more soluble in fat than in lean tissues. Another thing is that Navy dive operations have on site chambers and all the support crew needed to man them.

BRW's book Basic Decompression; Theory and Application 2nd edition has lots of information on this. He talks about the history and development of many of the different models. Best Publishing carries his books.

The NAUI dive table give a mandatory stop of 15 minutes for a 40 minute dive to 100 feet. Ascent rates are taught to be 30 fpm or slower.

 

[The Kraken]

Thank all of you for some excellent responses and posts.
My question has been answered.

D

 

[DA Aquamaster]

The average age for most of the USN studies was 23 and as indicated above they were in excellent shape with less adipose fat than the average civilian diver.

If I remember my 1985 training correctly, the US Navy tables had a hit rate of approximately 1 percent when used with a square profile on the first dive to no deco limits and approximately a 4% hit rate when used for square profiles on repetetive dives to the NDL's and a similar hit rate for single deco dives.

For the US Navy this was not a problem as repetetive dives were extremely rare and any deco dives were done with an on board recompression chamber. If the diver was synptomoatic, they could be immeditately recompressed on the spot. In many cases, on board recompression was used from the 40 ft stop to the surface. Basically with on board recompression, they would pull the diver up from 40 ft, quickly undress them and put them in the recompression chamber and then blow them back down to 50 ft and have them breathe O2 during the decompression. Oxtox was not a major issue as the diver was both attended and held the O2 mask to his face so that it would be/could dropped in an O2 hit or if the diver began to feel oxtox symptoms.

Due to the hit rate of the US Navy tables and the lack of an on board recompression chamber on 99.9% of dive boats, the navy tables are not dafe for use by recreational or technical divers unless a lot of fudge factors are utilized. For example, when I started diving, US Navy tables were the industry standard but we never dove square profiles or cmae close to diving a square profile, used the next deeper time or depth for hard working or cold water dives, and used both the next deeper time and depth for hard working cold water dives. An unofficial saftey stop was also often incorporated.

 

[msandler]

FWIW there are a number of table format deco resources that integrate current deco theories and algorithms, or at least are more conservative. DCIEM and BSAC are just a couple.

 

[The Kraken]

Hey, M . .
Do you have any links for them?