Which expert is right? re:Nitrox on air tables

[grf88]

gasgirl I would recommend that you do a search on the internet for Pyle stops. I think you will find the information very informative. Richard Pyle discovered the value of deep stops by accident and then researched the reasons behind them.

Here is one link: http://www.bishopmuseum.org/research/treks/palautz97/deepstops.html

 

[Rick Murchison]

Do you know whether that 6% hit rate was in a dry chamber or while divers were immersed?

I say again...
The "6% hit rate" is a myth, doesn't exist, isn't true, and wasn't done, dry or wet.
Protestations to the contrary always fail to cite a specific study because there isn't one. What there is, is a trail of unsubstantiated papers that refer to each other over and over year after year, with, at the bottom of the pile, nothing in the way of substantiated fact.
Goebbels was right.
S

 

[Mako Mark]

The way I look at it, if I haven't been diving in a month feeling fatigued doesn't automatically imply DCS. Since that's true, feeling fatigued when I have been diving recently doesn't automatically imply DCS either. It's a possible symptom, but not rock-hard evidence.

OK smart alec, insert "post dive" and repeat after me:

"I will not quote mark out of context"
"I will not quote mark out of context"
"I will not quote mark out of context"
"I will not quote mark out of context"
"I will not quote mark out of context"

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...

You too, go to your room.

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

OTOH,

How fast was the ski lift ?

 

[DA Aquamaster]

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.

I don't know.

Which one(s) do you prefer?

There are several tables that are more conservative than the US Navy tables in terms of deco required including the DCIEM Tables, tables based on the Buehlmann ZHL-16 algorithm like the IANTD tables as well as several software applications based on either Buehlmann/gradient factors or VPM models. My normal practice is to cut tables with DPlan.

Why?

DPlan is a Palm based version of DecoPlanner that can be run on an inexpensive monochrome PDA. If it falls overboard or gets crushed by a tank, I will not cry nearly as long as I would over a laptop. And a Palm is a lot more portable. DPlan uses a gradient factor model and produces much more conservative deco schedules than US Navy tables and incorporates much deeper stops. For example a 150' dive for 20 minutes on air with air for a decompression gas:

DPlan......USN
70' 1
60' 1
50' 1
40' 2
30' 4
20' 7..........2
10' 12........7

With accellerated deco using 50% for a deco gas however, the difference shrinks to 8 minutes compared with the straight USN air tables.

DPlan
70' 1
60' 1
50' 1
40' 1
30' 2
20' 4
10' 7

DPlan and most other deco software applications are also very flexible in terms of bottom gases, deco gases, altitude compensation and conservatism factors. So given a choice, I probably won't use a conventional table again at all.

 

[Charlie99]

I say again...
The "6% hit rate" is a myth, doesn't exist, isn't true, and wasn't done, dry or wet.
Protestations to the contrary always fail to cite a specific study because there isn't one. What there is, is a trail of unsubstantiated papers that refer to each other over and over year after year, with, at the bottom of the pile, nothing in the way of substantiated fact.
Goebbels was right.
S

I haven't seen any definitive data on any testing of the USN tables.Protestations to the contrary always fail to cite a specific study because there isn't one. What there is, is a trail of unsubstantiated papers that refer to each other over and over year after year, with, at the bottom of the pile, nothing in the way of substantiated fact. Goebbels is dead.

Joking aside, perhaps the reason this "myth" continues is because of lack of data of any sort. The sort of gross DCS per # of dives data provided Harris Taylor is not at all definitive since most of those dives were NOT to the table limits.

Does anyone have any real info on the tests back in the early 1960's that resulted in the USN/Workmann limits?

Bruce Wienke provides quite a bit of statistics in his TDID book, but I find it rather confusing in that there is a lot of jumping back and forth between risk probabilities and confidence factors for a multiple dive trial.

 

[Walter]

Any time you decrease your bottom time or increase your surface interval (all other factors staying the same) you've decreased your chances of DCS. Have you decreased them significantly? That would depend on the individual plans. If you simply look at NDLs, the PADI RDP is more conservative than the US Navy tables, but less conservative than most other agency tables that were derived from the US Navy tables (NAUI, YMCA are some examples) at most depths. Looking at them objectively, one comes to the conclusion that no table is always more conservative or always more liberal with respect to NDLs. If you look at surface intervals and repetitive dives, the PADI RDP stands out as being much more liberal than the others. Will this difference get you bent? Not likely, but the remote possibility does exist.

The same concept exists for using nitrox with air tables. Yes, you reduce your risk of DCS, but not significantly. Is it worth it? That depends on how conservative you'd like to be.

 

[del_mo]

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.

While FAR from an expert, I have been involved in two DAN/USN research studies (as a guinea pig). The conversations suggested dry chamber "diving" is much less likely to cause DCS because you are not exercising, etc. Future studies will help determine the fudge factor to go from dry diving to wet diving.

If you are interested in the two tests I have been involved in, there were "flying after diving," the first was a 120 minute 60 foot dive with a decompression stop at 10 feet for 26 minutes. Then a surface interval of 14 hours before flying for four hours at 8000 feet. The second test had a SI of 11 hours. So far no hits, although the 11 hours SI just started.

 

[DA Aquamaster]

I haven't seen any definitive data on any testing of the USN tables.Protestations to the contrary always fail to cite a specific study because there isn't one. What there is, is a trail of unsubstantiated papers that refer to each other over and over year after year, with, at the bottom of the pile, nothing in the way of substantiated fact. Goebbels is dead.

Joking aside, perhaps the reason this "myth" continues is because of lack of data of any sort. The sort of gross DCS per # of dives data provided Harris Taylor is not at all definitive since most of those dives were NOT to the table limits.

Does anyone have any real info on the tests back in the early 1960's that resulted in the USN/Workmann limits?

Bruce Wienke provides quite a bit of statistics in his TDID book, but I find it rather confusing in that there is a lot of jumping back and forth between risk probabilities and confidence factors for a multiple dive trial.

I can't cite anything from the 1960's, but Karl Huggins from the University of Michigan did doppler ultrasound research (in the early 80s I think) and reworked the US Navy tables to account for that data and recalculated critical nitrogen limits for all 6 compartments to confrom to the observations of those studies. The result was the Huggins table with shorter no deco limits.

The short story is that diving the US Navy tables to their limits produces "silent" bubbles whether a case of DCS results or not. Personally, I'd prefer to use a table that did not do that to me. But hey, to each his own. If someone wants to belive US Navy tables are both safe and suitable for recreational diving then they, Santa Claus and the Easter Bunny can have at it and dive to their heart's content.

 

[padiscubapro]

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.
Sheeeesh!
S

Not TOTALLY correct, but overall I do concur with you.. The US navy had problems with very specific schedules.. The one that has a high DCS rate (compared to all the other profiles) is the 150ft air table with repetitive 150ft dives (I dont remember the specific run times), I dont remember the exact figures but its more than a few percent of those dives...

When looking at statistics you also have to be aware that there are several versions of the tables in use, some even are doppler modified.

 

[DA Aquamaster]

When looking at statistics you also have to be aware that there are several versions of the tables in use, some even are doppler modified.

Right. For example the US Navy made a significant change to the table in 1984 when they went with a 30 ft/min ascent rate. As indicated above Karl Huggins applied doppler research to the US Navy tables in the early to mid 80's and developed new limits to the US Navy table to provide a safer table designed to greatly reduce the occurrence of silent bubbles. Simply stated and overly simplified, current bubble models are based on the concept that small extant bubbles from initial dives will form nuclei for more and larger bubbles on subsequent dives. The presence of silent bubbles following a significant number of US Navy table profiles, in light of currnet theory, makes that table far less than ideal for use by a recreational diver doing repetitive dives. We should also note that the the US Navy never adopted any doppler based limits and again point out that is not a surprise as they are only minimally interested in repetitive dives.

If you generate a series of repetitive dives and then work that series with a variety of tables, you will get a variety of answers as to the lenght of time allowable on the last dive or the surface interval required to allow a given bottom time on the last dive. In fact, the number of "correct" answers is probably going to exactly equal the number of tables you use. In short, dive tables are theoretical and largely a matter of faith.

So like any other theoretical approach you need to select a dive table based on a survey of what is available and careful consideration of how each one will apply to your specific needs and practices to find one that will allow you to safely do the diving that you do.

If a diver is only planning on doing one dive a day, is in good physical shape and is willing to accept a slightly elevated risk of DCS, the US Navy tables may be ideal for that particular diver. But anyone doing repetitive dives, in less than ideal physcial condition and wanting a higher margin of safety, will be far better served by another table.

In my case, I dove US Navy tables for years and while I never got a clinically significant case DCS, I also had many many dives where I did not feel particularly well even when staying well within the limits, especially after diving multiple dives per day over multiple days. Newer tables leave me feeling much better after several repetetive dives. So for me claims by a minority that the US Navy tables are safe based on their perception of a lack of research specifically indicating that US Navy tables are unsafe, has no relevence as my personal experience is a much more compelling argument.

It is a personal choice, but choose very carefully.