Understanding Decompression Sickness

[Angelo Farina]

The PADI tables were created when the standard ascent rate was 60 FPM, and that is what was used as the standard when they were doing their research. If you were to go back a dozen years of so on ScubaBoard, you will find posts by me in which I incorrectly said that ascending slower than 60 FPM was not an accurate use of the table. The table instructions said to go "no faster than 60 FPM." When I did my research for an article I wrote on ascent rates several years ago, I learned from PADI headquarters that the research found no harm with slower ascent rates. The tables were still valid.

But at some point, if you go too slowly, you violate NDLs and go into deco. How can you tell when? That is the key problem with using tables. There is no way of knowing how slow is too slow. If you are using a computer, the computer will tell you if you are hanging out too long.

It is actually very simple to account for very slow ascent when using tables.
If you are ascending from 30m at 10m/minute, it would take 3 minutes. If you ascend slower, let say taking 10 minutes instead of 3, you have an excess dive time of 7 minutes. These extra 7 minutes should be considered as having been spent at the bottom.
Example using old US Navy tables: 30 min at 30 m gives a mandatory deco stop of 3 min at 3m.
But if you take 7 extra minutes due to a slow ascent, you need to switch to the next entry on the tables, 40 min at 30m, which gives a deco stop of 15 min at 3m.

 

[boulderjohn]

It is actually very simple to account for very slow ascent when using tables.
If you are ascending from 30m at 10m/minute, it would take 3 minutes. If you ascend slower, let say taking 10 minutes instead of 3, you have an excess dive time of 7 minutes. These extra 7 minutes should be considered as having been spent at the bottom.
Example using old US Navy tables: 30 min at 30 m gives a mandatory deco stop of 3 min at 3m.
But if you take 7 extra minutes due to a slow ascent, you need to switch to the next entry on the tables, 40 min at 30m, which gives a deco stop of 15 min at 3m.

Your example is a dive with required deco. My statement referred to NDL dives with no required deco.

Can you point to standard instructions for dive tables telling how to add time slower than the designated ascent speed on tables for NDL dives? I am obviously untrained in most of the diving tables in the world. I know it is not an accepted technique for PADI tables. Most importantly, it cannot predict when you would go from an NDL dive to a required decompression dive by making a slow ascent.

 

[boulderjohn]

It is actually very simple to account for very slow ascent when using tables.
If you are ascending from 30m at 10m/minute, it would take 3 minutes. If you ascend slower, let say taking 10 minutes instead of 3, you have an excess dive time of 7 minutes. These extra 7 minutes should be considered as having been spent at the bottom.
Example using old US Navy tables: 30 min at 30 m gives a mandatory deco stop of 3 min at 3m.
But if you take 7 extra minutes due to a slow ascent, you need to switch to the next entry on the tables, 40 min at 30m, which gives a deco stop of 15 min at 3m.

Here is a real example for you to demonstrate your methodology, using any tables you wish.

On 6/23/22 I did a dive using EANx 32 to a maximum depth of 126 feet/38 meters. I began the ascent immediately after hitting the maximum depth, so my total bottom time for table purposes, counting descent time, will round off to 5 minutes. I then took 78 more minutes to reach the surface, so my total dive time was 83 minutes. It was an NDL dive--I had loads of NDL time left on my computer when I surfaced.

Using your technique with the PADI tables is a challenge, because PADI does not really define an ascent rate other than saying "no faster then 60 FPM." Let's use 60 FPM for this. That would mean it should take me just over two minutes to reach the surface. That means I had 76 minutes of excess time. I did do a safety stop, so we'll take that off and just say I had 73 minutes of excess time. As I understand it, I should add 73 minutes excess ascent time to the bottom time, for a total of 78 minutes bottom time. The PADI EANx 32 tables give 18 minutes at that maximum depth, so I would have 60 minutes of decompression to take care of--quite a difference from a NDL dive.

Or did I misunderstand?

 

[John C. Ratliff]

From the NOAA Dive Manual, 1991, 3rd Edition, page 14-31:

"To shorten the necessary surface interval before flying,​

oxygen may be breathed instead of air. Table 14-13 lists for the various Repetitive Dive Group classifications,the length of oxygen breathing time necessary before flying is allowed."​

View attachment 797355​

For comparison, if air is breathed, Table 9-6 from the US Navy Dive Manual Rev 7A, the required surface interval before ascent to altitude gives Groups A-C as requieding 0:00 before flying (8000 ft); Group D is 1:45; Group E is 4:39, Group F is 7:06; Group G is 9:13; Groupo H is 11:04,; Group is 12:44; Group J is 14:13; Group K is 15:35; Group L is 16:50; Group M is 18:00; Group N is 19:04; Group O is 20:05; and Group Z is 21:01. This suggests a massive decrease in wait-time-before-flying if one breaths pure oxygen during the wait time.

Here is why you should NOT use these numbers:
1) NOAA has taken table 14-3 out of its dive manual, which is now in its 6th edition; the 3rd edition was the last appearance of Table 14-3, shown above.
2) Not all talbe use the same repetitive groups, so (for example) you cannot say your are in group G from a PADI RDP, and then use the Navy or NOAA tables; Group G means very different things.
3) If you are using a computer, you don't get any repetitive group labels anyway, so you can't trasnfer you computer into to ANY tables.

​

Table 14-3 in the original edition (1975) of the NOAA Diving Manual was titled "Classification of Underwater Habitats," and was not about decompression. Also, there was no Table 14-13. So apparently this came a bit later than January 1975.

All the decompression tables in the first edition (1975) of the NOAA Diving Manual were in the Appendix D, “Standard Air Decompression Tables.” Table 3 of that appendix is titled “Table 3–Recompression Treatment of Decompession Sickness and Gas Embolism using Air.” They also have an interesting whole section title “Surface Decompression Tables,” and go into surface decompression using oxygen and air.

SeaRat

 

[JMarc]

Can you point to standard instructions for dive tables telling how to add time slower than the designated ascent speed on tables for NDL dives? I am obviously untrained in most of the diving tables in the world. I know it is not an accepted technique for PADI tables. Most importantly, it cannot predict when you would go from an NDL dive to a required decompression dive by making a slow ascent.

From the usage instruction of the tables I've learned to dive (instructions from the FEBRAS -- belgian member of the CMAS -- manual, the tables themselves were the US NAVY 1970 one, I don't know if the instructions were modified by the FEBRAS) in the 80's (on the fly translation/summary from french by me):

Time to use as entry in the tables:
- time between immersion and the moment the ascent at 18 m/min starts
- if the ascent speed is slower than 18 m/min (IT IS ALWAYS FORBIDDEN TO ASCENT MORE RAPIDLY), time between immersion and the moment the diver reaches the deeper of 9 m or the first stop depth. In theory, one can subtract the time that one would have used for an ascent at the 18 m/min if the ascent has been regular, in practice it is easier and safer not to do so as the procedure can be unsafe depending on the precise profile if the ascent has not been regular.

(I don't remember having ever done the subtraction, but I've not dived with tables for close to 30 years)

 

[JMarc]

As I understand it, I should add 73 minutes excess ascent time to the bottom time, for a total of 78 minutes bottom time. The PADI EANx 32 tables give 18 minutes at that maximum depth, so I would have 60 minutes of decompression to take care of--quite a difference from a NDL dive.

What time would you use to enter in the table? I'd be surprised if it was not higher than with @Angelo Farina method. (Theoretically you can take a time weighted averaged depth for multilevel ascending time, but that seems unwise to plan using that outside professional setting with surface support to monitor the depth and time and adapt the decompression procedure on the fly; I'm certain I never tried that when I was still using tables. And that would be an adjustment of depth, not of time). The possibility of doing slow ascent without getting into deco has been a driving factor in the popularization of dive computers around me.

 

[boulderjohn]

What time would you use to enter in the table? I'd be surprised if it was not higher than with @Angelo Farina method.

What time would I enter into the tables in the example I provided? I wouldn't enter any time. I would not and could not use the tables for that dive. That was my point.

The possibility of doing slow ascent without getting into deco has been a driving factor in the popularization of dive computers around me.

Exactly. You can do that dive as an NDL dive with a computer. You can't with tables.

 

[boulderjohn]

From the usage instruction of the tables I've learned to dive (instructions from the FEBRAS -- belgian member of the CMAS -- manual, the tables themselves were the US NAVY 1970 one, I don't know if the instructions were modified by the FEBRAS) in the 80's (on the fly translation/summary from french by me):

Time to use as entry in the tables:
- time between immersion and the moment the ascent at 18 m/min starts
- if the ascent speed is slower than 18 m/min (IT IS ALWAYS FORBIDDEN TO ASCENT MORE RAPIDLY), time between immersion and the moment the diver reaches the deeper of 9 m or the first stop depth. In theory, one can subtract the time that one would have used for an ascent at the 18 m/min if the ascent has been regular, in practice it is easier and safer not to do so as the procedure can be unsafe depending on the precise profile if the ascent has not been regular.

(I don't remember having ever done the subtraction, but I've not dived with tables for close to 30 years)

I read this several times and am not sure I understand it. Please apply it to the example I gave.

 

[Angelo Farina]

Your example is a dive with required deco. My statement referred to NDL dives with no required deco.

Can you point to standard instructions for dive tables telling how to add time slower than the designated ascent speed on tables for NDL dives? I am obviously untrained in most of the diving tables in the world. I know it is not an accepted technique for PADI tables. Most importantly, it cannot predict when you would go from an NDL dive to a required decompression dive by making a slow ascent.

When using the old US Navy tables, all dives are deco dives (or, at least, this is what was taught me back in 1975).
Let's make the same example with a shorter bottom time, 25 min at 30m, which was within NDL with those tables.
If during ascent you take an excess time up to 5 min, you will follow the table for 30 min, 30m, doing a deco stop of just 3min at 3m.
II your ascent is even slower, so the extra ascent time is, say, 7 minutes, then you fall back to a bottom time of 40 min, which gives you a deco time of 15 min at 3m.
So the fact that the planned dive was within NDL, or outside it, does not make any difference to the logic.
Furthermore, as originally the US Navy tables were coinceived for an ascent rate of 18 m/min, instead of 10, they were already considered "not safe enough" at the time for sport divers.
So the method recommended was very simple: instead of computing the dive time from the beginning of descent to the beginning of ascent, we were taught to play it safe, counting the dive time from the beginning of the dive to the moment of reaching the deepest deco stop (9m for recreational diving).
This automatically acvounts fot any excess time during ascent.
At that point you look at your timer, and that is the total dive time. You look at the depth meter, and the red arrow shows you the max depth reached.
With these two numbers you look at dive tables, choosing the first depth equal or just larger than your max depth and then the dive time equal or slightly larger than the total dive time.
In most cases this would create enough excess deco stops for being safe, despite the US Navy tables, if followed strictly, where not 100% DCS safe.
Of course computers made this approach entirely obsolete...

 

[Angelo Farina]

I read this several times and am not sure I understand it. Please apply it to the example I gave.

@JMarc described the very same approach used here by Fipsas (Italian branch of Cmas).
The dive time starts when leaving the surface and ends when you reach the deepest deco stop at 9m.
So, in the example given, the dive time was probably around 70 min.
And yes, some deco stops are required for that diving profile, as when using tables you always assume that the dive time was entirely spent at max depth.