Which expert is right? re:Nitrox on air tables

[RonFrank]

I like this explanation...
that lack of fatigue or freshness that has been mentioned is the lack of "fatigue" as a symptom. Fatigue is a symptom of DCS as we all know, now in mild cases, fatigue is all you may feel. However, no matter how you look at it, if you feel fatigued, then you have DCS.

Wow, I must have got DCS from sking the other day, I could barely walk back to the Truck at the end of the day...

 

[jbd]

The fact is that the DCS rate from the Navy tables is about 0.06%,....
S

Thanks for posting the link to the article, its the one I was referring to.

 

[grf88]

Wow, I must have got DCS from sking the other day, I could barely walk back to the Truck at the end of the day...

Could be! what altitude were you skiing at.

 

[Charlie99]

The fact is that the DCS rate from the Navy tables is about 0.06%, or 1/100 the rate Seaduce and everybody else "knows."

0.06% is the rate of ALL dives done while using the tables, and undoubtably includes many dives that did not approach the limits of the tables, while other dives may have exceeded the limits and/or had rapid ascents. That report tells us little or nothing about the DCS rate that would occur for dives done to the limits of the table.

The equivalent gross DCS rate for PADI and NAUI is about 1/60th the quoted Navy rate: "PADI and NAUI have reported hat field risks (p] across all exposures ae less than 0.001%" ... Technical Diving in Depth, BR Wienke, p. 182.

The DCS risk of diving to the table limit is something quite different.

While the math in the TDID section on Decompresssion Risks and Statistics is pretty heavy, my understanding of it is that, in crunching the data from many thousands of dives to try and estimate the actual risks of diving to the limits of the tables, that the extrapolated DCS risk while diving to the limits of either the PADI or NAUI table is just under 0.02% per dive, and the Navy table has a risk about 50% higher than the PADI and NAUI tables.

 

[PaulChristenson]

Could be! what altitude were you skiing at.

Noooo....more importantly HOW DEEP DID HE GO IN THE POWDER??

1atm...2atm??



Paul in VT
no powder just hardpack

 

[DA Aquamaster]

We've been swattin' this fly for two decades and the sucker just won't die! The fact is that the DCS rate from the Navy tables is about 0.06%, or 1/100 the rate Seaduce and everybody else "knows." And that includes deco dives on the Navy tables.

Don't believe everything you see on the internet. The stats being cited and compared are apples and oranges. I started diving with US Navy tables in college, read lots of researhc on Navy tables at the time and I am pretty familiar with their limitations. Experiemental studies with navy tables were originally done with US Navy divers in excellent physical condition with an average age of 23 and hit rates were determined for single and repetitive dive profiles to the limits of the tables (including deco dives).

For a single dive to the limit of the table, (a square profile) the hit rate was acceptable but the hit rate approached 6% on repetitive dive profiles. The US Navy was concerned not only with diver safety but also operational neccesity and was inclined to accept a higher hit rate if it could get a diver out of the water and allow a vessel to get under way sooner. Lets not forget the US Navy is not inclined to risk an entire ship and the crew aboard to safegaurd the well being of one expendable diver. The hit rate is even less a consideration for naval vesel with an onboard recompression chamber.

The Navy created tables to accomodate repetitive dives but preferred not to use them and instead normally stuck to one dive per day per diver unless repetive dives were absolutely neccesary to accomplish the mission in a timely manner. And again, in those cases, a recompression chamber was normally available so a 6% hit rate was not an issue.

I don't know many civilian dive boats with a mix of middle aged, overweight males and females doing repetitive dives over multiple days and no recompression chamber would last long with that operational philosophy as lawsuits from bent divers would probably run well over 10%.

For that reason when US Navy tables were used by sport divers, numerous safety factors were applied. In my cert course this included always rounding up to the next greater time and depth, and further rounding up to the next greater time and depth for cold water or strenuous dives and doing a 5 minute safety stop if you were within 3 squares of a no deco limit. Divers were further discouraged from doing square profiles and if a square profile were needed, to again round up to the next greater depth and time. I still have my original US Navy Tables and they still have a red line that I added to conform to the lower limits determined by doppler ultrasound studies performed in the late 80's.

The stats cited on the website provided are based on the Navy's operational use of the 1987 version of the Standard US Navy Air Decompression Tables and reflect their safety when used the way the US Navy uses them. This means primarily a single dive per day within the NDL's, rounding up to the next greater time and depth and again rounding up for cold or strenuous dives. So in short, there is no contradiction between those numbers and the 6% hit rate when the tables are dove to their limits on repetitive profiles.

Those stats also do not include deeper dives and deco dives that are now more likely to made using mixed gases and tables other than the US Navy Standard Air Decompression Tables.

It bothers me that there is a resurgence of divers who feel the Navy Tables are acceptable for recreational diving given the number of far better tables now available. (The PADI RDP is however in my opinion not one of them.) I was not impressed when I took a TDI deco procedures course and found the whole course based on US Navy tables including repetitive dive planning.

Granted these were the updated 1987 US Navy Tables with 30 ft/min ascent rates as oppsed to 60 ft/min, but these tables still offer less safety margin than most other available tables and software and are at the same time much less efficient than nearly all accellerated deco tables and applications that have greater margins of safety.

 

[Seadeuce]

Like many other divers on this forum I too began my diving with the USN tables, and thankfully survived.

The stats quoted can be, and have been, the subject of much discussion over the years. Rather than get into a debate on such things, which will be inconclusive, I prefer to offer some basic logic where these tables are concerned.

USN tables (BTW still the basis for commercial diving) had, say, for a 30m (99ft) depth an NDL of 25 mins. This was followed by an ascent rate of 18m (60ft) per min. "Not faster than your smallest bubbles." No safety stop was required, nor recommended.

Buehlmann, on the other hand, for the same depth has a 17 min. NDL (16 min. on computer), an ascent rate of 10m (33ft) per min.
A safety stop of 3 mins. at 3m (10ft) is recommended.

With respect to all posters, I don't think you have to be a genius to work out which set of tables offer the safer profile.

As to the difference in safety being expressed in mathematical terms? Again, logic should conclude that with an NDL that is 32% less, and an ascent rate 44% less, there just has to be a significent % difference between the failure rates of the two sets of tables.

Seadeuce

 

[Charlie99]

For a single dive to the limit of the table, (a square profile) the hit rate was acceptable but the hit rate approached 6% on repetitive dive profiles.

Do you know whether that 6% hit rate was in a dry chamber or while divers were immersed? I have heard that a dry chamber run is much more likely to cause DCS than the identical profile done while in the water-supported state of a wet chamber or real dive.

It bothers me that there is a resurgence of divers who feel the Navy Tables are acceptable for recreational diving given the number of far better tables now available. (The PADI RDP is however in my opinion not one of them.)

Which one(s) do you prefer? Why?

 

[gasgirl]

My experience has been similar. Before I developed better control of my ascent rates and also started doing deep stops, I found that nitrox would reduce fatigue levels when doing multiple dives per day. Once I started spending more time in my ascent + deep stops + safety stop, the air vs. nitrox difference disappeared.

I just completed the Nitrox course and really enjoyed doing multiple days of multiple dives on EANx. Our bottom times were limited by our cold tolerance rather than NDL or air consumption. I also noticed there was a lack of the post-dive fatigue I have experienced on other dive trips.

Our normal dive plan starts with the max depth in the first half of the dive followed by a shallower depth for the last half of the dive. I also do a safety stop at 15 ft for 3 minutes. What constitutes a deep stop and when do you recommend doing one?

 

[DA Aquamaster]

I just completed the Nitrox course and really enjoyed doing multiple days of multiple dives on EANx. Our bottom times were limited by our cold tolerance rather than NDL or air consumption. I also noticed there was a lack of the post-dive fatigue I have experienced on other dive trips.

Our normal dive plan starts with the max depth in the first half of the dive followed by a shallower depth for the last half of the dive. I also do a safety stop at 15 ft for 3 minutes. What constitutes a deep stop and when do you recommend doing one?

The current trend if diving within normal recreational limits is to do a deep stop at 1/2 max depth.

An ideal recreational dive profile would have the max depth at the beginning of the dive with a long, slow gradual ascent to the surface throughout the rest of the dive. That is not normally practical unless you are diving a wall. But the plan you describe of doing half the dive at max depth and the other half at a shallower depth is an excellent compromise as long as the ascent rates are slow, the second half of the dive is shallow enough, you don't crowd the no deco limits and you still end it with a safety stop if the depth of the last half of the dive is deeper than 30 ft.