Nitrox for shallow water artifact diving??

[happy-diver]

I learnt to round up 45 yrs ago, must come in cycles and anyhow
who cares what was taught when your brain says “Hey dude round
up to give yourself a buffer dude and it's far more relaxing to come up
a bit sooner dude, and you can always see that thing that is, exactly the
same as all the other things you saw the next time you go diving there dude"

 

[Centrals]

I do understand the "round up" business on calculating ndl but what is the point when that is NOT necessary.

So some divers like the idea of "safety factor"?
We should kick the "DSAT" algorithm to touch and stick with RGBM.
No reverse profile, no short service interval etc etc.

 

[AfterDark]

So some divers like the idea of "safety factor"!

Using 35FSW tables for 25FSW dives will give a diver a VERY BIG safety factor!

 

[Centrals]

Using 35FSW tables for 25FSW dives will give a diver a VERY BIG safety factor!

That is for table. How about for computer?
RGBM all the way with added "safety factor"!
Both my non RGBM dc do not have that function and I must be very lucky! Touch wood.

 

[boulderjohn]

Using 35FSW tables for 25FSW dives will give a diver a VERY BIG safety factor!

Yep, it does--as I said. It is in fact, about the same as using the 50 foot table for a 41 foot dive, the 60 foot table for a 51 foot dive, the 70 foot table for a 61 foot dive, etc. It is all standard procedure.

Here are rules 2 and 3 for using the PADI tables, as written on page 6 of the Recreational Dive Planner: Instructions for Use.

2. Any dive planned to 35 feet or less should be calculated as a dive to 35 feet.
3. Use the exact or next greater depth shown for the depths of all dives.
​

That's how the tables are designed. I explained the limitations of that design earlier. I explained that it would be different using a computer earlier.

I received a private message warning me that getting through to you guys was pretty hopeless--"tilting at windmills." I am not writing for you. I am writing for the other readers who might might be confused and can learn something from this.

 

[tursiops]

Yep, it does--as I said. It is in fact, about the same as using the 50 foot table for a 41 foot dive, the 60 foot table for a 51 foot dive, the 70 foot table for a 61 foot dive, etc. It is all standard procedure.

Here are rules 2 and 3 for using the PADI tables, as written on page 6 of the Recreational Dive Planner: Instructions for Use.

2. Any dive planned to 35 feet or less should be calculated as a dive to 35 feet.
3. Use the exact or next greater depth shown for the depths of all dives.
​

That's how the tables are designed. I explained the limitations of that design earlier. I explained that it would be different using a computer earlier.

I received a private message warning me that getting through to you guys was pretty hopeless--"tilting at windmills." I am not writing for you. I am writing for the other readers who might might be confused and can learn something from this.

The USN tables have a special table for "shallow water." They begin at 30 ft, and go deeper in 1 ft increments.
At 30 ft, the NDL is 371 minutes; at 35 ft the NDL is 232 minutes, at 40 ft 163 minutes.
The normal tables (Table 9-7) increment in 5 ft intervals, beginning at 10 ft. For 10, 15, 20 ft the NDL is unlimited, but you still accumulate N2 until your reach about 400 minutes, attaining a (Navy) repetitive group of E to K for 10 to 20 ft, respectively.
At 25 ft the NDL is 1102 minutes.
The Navy answer to the previous discussion is that NDL is NOT unlimited above 35 ft, but it is above 20 ft.

Repetitive 25 ft dives? (Dive 1) 25 ft for 100 mins give Group G. An SI of 10 to 52 minutes reduces G to F. F gives a Residual Nitrogen time of 98 minutes for a repetitive 25 ft dive. (Dive 2) Another 100 minute dive to 25 mins -- use 100+98 = 198 minutes -- gives Group J, which reduces to I for a 10-52 min SI. I gives an RNT of 167 mins at 25 ft for a TBT of 167+100=257 minutes. (Dive 3) Going back to 25 ft for 257 minutes yields Group L, which reduces to K, and give RNT=286 mins at 25 ft. (Dive 4) 386 mins at 25 fit gives Group N, which reduces to M, with an RNT at 25 ft of 354 mins. (Dive 5) 454 mins at 25 ft give Group N again. This means 5 dives to 25 fit with 10-52 min SIs pushes your N2 load up to nearly deco, but not quite, and then it stays at that high level.
Conclusion: the Navy tables allow unlimited 100-minutes dives to 25 ft, using SI of 10-52 minutes.

 

[ginti]

The USN tables have a special table for "shallow water." They begin at 30 ft, and go deeper in 1 ft increments.
At 30 ft, the NDL is 371 minutes; at 35 ft the NDL is 232 minutes, at 40 ft 163 minutes.
The normal tables (Table 9-7) increment in 5 ft intervals, beginning at 10 ft. For 10, 15, 20 ft the NDL is unlimited, but you still accumulate N2 until your reach about 400 minutes, attaining a (Navy) repetitive group of E to K for 10 to 20 ft, respectively.
At 25 ft the NDL is 1102 minutes.
The Navy answer to the previous discussion is that NDL is NOT unlimited above 35 ft, but it is above 20 ft.

Repetitive 25 ft dives? (Dive 1) 25 ft for 100 mins give Group G. An SI of 10 to 52 minutes reduces G to F. F gives a Residual Nitrogen time of 98 minutes for a repetitive 25 ft dive. (Dive 2) Another 100 minute dive to 25 mins -- use 100+98 = 198 minutes -- gives Group J, which reduces to I for a 10-52 min SI. I gives an RNT of 167 mins at 25 ft for a TBT of 167+100=257 minutes. (Dive 3) Going back to 25 ft for 257 minutes yields Group L, which reduces to K, and give RNT=286 mins at 25 ft. (Dive 4) 386 mins at 25 fit gives Group N, which reduces to M, with an RNT at 25 ft of 354 mins. (Dive 5) 454 mins at 25 ft give Group N again. This means 5 dives to 25 fit with 10-52 min SIs pushes your N2 load up to nearly deco, but not quite, and then it stays at that high level.
Conclusion: the Navy tables allow unlimited 100-minutes dives to 25 ft, using SI of 10-52 minutes.

Could you post the tables?

 

[tursiops]

Could you post the tables?

The tables only are at https://www.divetable.info/workshop/USN_Rev7_Tables.pdf. You want Tables 9-7/9.8.
The full USN Dive manual is at https://www.navsea.navy.mil/Portals...ING MANUAL_REV7.pdf?ver=2017-01-11-102354-393
See Appendix 2A for the shallow-water tables.

 

[mc42]

(Disclaimer: this is based upon software I wrote that does DSAT calcs and other math. Use at your own risk!)

Based on DSAT M-values and breathing air, 7.4m (24' and a bit) is the depth where you cannot saturate any compartment beyond its M-value, no matter how long you stay under. This would be based upon the longest compartment (480 minutes) which has the lowest M-value, 13.3msw. The calculation of the maximum no-limit depth will depend on the compartment M-values of the model you're using (DSAT, Bühlmann, USN) so each should be different. This calculation has no conservatism whatsoever.

Researchers have also empirically found the point where a diver can saturate and still make a direct ascent to be “significantly shallower than 25.5fsw, probably in the range of 22fsw.” (DSAT RDP paper). This aligns with my calculation above. The USN table also seems to agree by having 1102 minutes as the 25’ limit, 371 as the 30’ limit, and no limit at 20’ and below.

Given this, it seems tenuous that 25’ is that magical point that you can saturate and not get DCS. It will depend on your physiology. You’re at the edge of the table here. A mere 5’ increase in depth can affect your NDL by 741 minutes! [I see tursiops posted pretty much this just now as I was typing this out]

It’s up to you to find some sort of useful planning limit to fit your conservatism level: you can (1) use the lowest (35’) RDP values, (2) use a different table, like the USN table, or (3) use your DC or planner and see what it says.

Should you wish to fly after your 25’ saturation dive, you'd need to wait about 6-1/2 hours according to my program and with zero conservatism. Since you shouldn't be saturated, this is the worst case, so standard no-fly rules (12/18/24hrs) seem reasonable here, certainly 24 hours.

We know that breathing 32% nitrox at the same depth would reduce the compartment pressure by the difference in N2 fraction, thus the maximum attainable compartment pressure for the same dive on nitrox would be .68/.79 (86%) of the maximum, or 11.45msw. This certainly buys additional safety room. It also lets you theoretically get on the plane after waiting only 2-1/2 hours to get your N2 levels low enough.

That reduced no-fly interval is actually interesting, and I hadn't considered that the time isn't 86% of the original no-fly interval, but because of exponential offgassing, nitrox seems to make a big difference here too.

 

[Compressor]

The tables only are at https://www.divetable.info/workshop/USN_Rev7_Tables.pdf. You want Tables 9-7/9.8.
The full USN Dive manual is at https://www.navsea.navy.mil/Portals/103/Documents/SUPSALV/Diving/US DIVING MANUAL_REV7.pdf?ver=2017-01-11-102354-393
See Appendix 2A for the shallow-water tables.

Thanks for the links @tursiops.
Love the NAVY syntax.