"averaging" table NDLs and multi-level diving

[Makhno]

Greetings, and thanks in advance for your replies.

I recently went diving with an experienced scuba instructor (he's logged a lot more dives and certified a lot more students than I have) who said that he only uses dive computers in gauge mode because he doesn't trust computers to provide him with accurate no-decompression stop limits (NDLs). He's not yet a tech diver, but he has done the GUE Fundamentals course and is an avid DIR convert (as applied to recreational, no-deco stop diving); hence his opposition to replying upon dive computers for NDLs. I commented that I generally trust my computer as long as it's in good working order -- and I suggested that human error is more likely than computer error -- and I noted that using tables really limits one's NDLs on multi-level dives, compared to computers (or to PADI's wheel). He replied that he "averages" the table NDLs of the depths dived during a multi-level dive to extend his dive times beyond those allowed by his table's NDL for the deepest part of a multi-level dive.

We were doing fairly short, briefly deep but mostly shallow dives which turned out to be well within my computer's NDLs (and he was on EAN32 and I was on air so his margin of safety was even greater than mine), and I didn't want to get into a big and under the circumstances, moot, argument about it, but it seemed to me that his approach was quite wrong.

NDLs are determined by a specific M-value for a specific tissue compartment, and when using any table, a different tissue compartment and its M-value determine the NDL for a 30-meter dive than those which determine the NDL for a 15-meter dive, so "averaging" those NDL times (in whatever proportional way he does it -- he didn't elaborate) is like averaging apples and oranges. So in principle, it's wrong. Correct?

Please enlighten me if I'm mistaken, but: all tables assign a pressure group at the end of the dive and those pressure groups take into account not some abstract "total" nitrogen loading, but rather they take into account nitrogen loading vis-a-vis the M-values of each different tissue compartment used in that table's decompression model. So it seems to me that for a diver unwilling to rely upon a dive computer or PADI's wheel, then rather than "averaging" the NDLs of the dive's several depths a much better approach would be to determine the pressure group at the end of the deepest part of the dive and use that to work with the table to then determine the NDL for the next depth, and so on for any subsequently shallower depths. Does that sound correct?

In addition, instead of the standard safety stop at 5 meters for 3 minutes (remember, this is all recreational no-deco stop diving), he insisted upon doing something like "20 seconds at 15m, 30 seconds at 9 m, 50 seconds at 6 m, and 1 and a half minutes at 3m" (I forget the precise times, but you get the idea...). I've read Pyle's "The Importance of Deep Stops" and some related material, but for no-deco stop dives when one is ascending at no faster than 9m/minute, is there really any justification for a safety stop at 15m for a no-deco stop dive starting at 30m? And then additional (very short) stops at 9m and 6m? And then finishing at 3m rather than 5 m? Although the pressure gradient is steeper at 3m than at 5m so theoretically offgassing is a tad faster, many divers find it quite a bit harder to maintain a consistent 3m stop than to maintain a 5m stop due to buoyancy control difficulties or simply due to big waves (I'm at 3m -- here comes a big wave, and now I'm at 4.5m! wave quickly subsides and I'm at 1.5m! etc. etc. Much easier to maintain a consistent depth of about 5m). To me, simply ascending from a no-deco stop dive slowly (9m/minute) and then stopping for 3 minutes at about 5m makes the most sense (and is well-supported by empirical evidence from DAN studies). So, is there any empirical or theoretical support for anything other than a slow ascent rate and a 5m/3 minutes safety stop on no-deco stop dives?

Anyone care to comment? I'm not going to tell this guy what to do -- I'm just curious about the theory and about whatever empirical data might exist. I'd be grateful for your well-informed replies.

Cheers,
M

 

[Walter]

There are methods of using tables for multilevel diving. I do not recommend them, but I have used them before I bought my first computer back in '86. Computers are better, in my opinion.

 

[JeffG]

I recently went diving with an experienced scuba instructor (he's logged a lot more dives and certified a lot more students than I have) who said that he only uses dive computers in gauge mode because he doesn't trust computers to provide him with accurate no-decompression stop limits (NDLs). He's not yet a tech diver, but he has done the GUE Fundamentals course and is an avid DIR convert (as applied to recreational, no-deco stop diving); hence his opposition to replying upon dive computers for NDLs.

Inaccurate NDL's are only one reason computers are not used by DIR divers and the word inaccurate is also a bit misleading.

Some of the older algorithms may (or may not) behave the same for some dive profiles, but they almost always fault to the side of being conservative.

One of the other reasons they avoid the computer is the "riding of the NDL" countdown style of diving. You can either be proactive or reactive to shaping your dive. If all you do is watch your NDL countdown, you can only be reactive. If you have an understanding of decompression, you can determine the "deco" price for any dive action. (i.e. Do I go deeper by 10ft for X mins...how does that effect the rest of my dive.)

NDLs are determined by a specific M-value for a specific tissue compartment, and when using any table, a different tissue compartment and its M-value determine the NDL for a 30-meter dive than those which determine the NDL for a 15-meter dive, so "averaging" those NDL times (in whatever proportional way he does it -- he didn't elaborate) is like averaging apples and oranges. So in principle, it's wrong. Correct?

If you don't know what he is doing, how can you say it is wrong.

(Hint: He is not averaging NDL times)

I'm not going to tell this guy what to do

Why not. You're a DM. Isn't that part of your responsibility?

 

[atedeschi]

i been taking my average depth before final ascent while cave diving and using the tables with my average depth and it compares to my DR DUO. When diving in the ocean most of my profiles are pretty square so there is not much advanctage, but in a dive to little river max depth 105' average 88', therefore I would use 90' tables. if you have you computer download, and a deco software you can compare the two and see. i would not try this unless you understand it, and i always do at least a 5min safety stop on NDL or DECO dives.

 

[contender]

Greetings, and thanks in advance for your replies.

I recently went diving with an experienced scuba instructor (he's logged a lot more dives and certified a lot more students than I have) who said that he only uses dive computers in gauge mode because he doesn't trust computers to provide him with accurate no-decompression stop limits (NDLs). He's not yet a tech diver, but he has done the GUE Fundamentals course and is an avid DIR convert (as applied to recreational, no-deco stop diving); hence his opposition to replying upon dive computers for NDLs. I commented that I generally trust my computer as long as it's in good working order -- and I suggested that human error is more likely than computer error -- and I noted that using tables really limits one's NDLs on multi-level dives, compared to computers (or to PADI's wheel). He replied that he "averages" the table NDLs of the depths dived during a multi-level dive to extend his dive times beyond those allowed by his table's NDL for the deepest part of a multi-level dive.

We were doing fairly short, briefly deep but mostly shallow dives which turned out to be well within my computer's NDLs (and he was on EAN32 and I was on air so his margin of safety was even greater than mine), and I didn't want to get into a big and under the circumstances, moot, argument about it, but it seemed to me that his approach was quite wrong.

NDLs are determined by a specific M-value for a specific tissue compartment, and when using any table, a different tissue compartment and its M-value determine the NDL for a 30-meter dive than those which determine the NDL for a 15-meter dive, so "averaging" those NDL times (in whatever proportional way he does it -- he didn't elaborate) is like averaging apples and oranges. So in principle, it's wrong. Correct?

Please enlighten me if I'm mistaken, but: all tables assign a pressure group at the end of the dive and those pressure groups take into account not some abstract "total" nitrogen loading, but rather they take into account nitrogen loading vis-a-vis the M-values of each different tissue compartment used in that table's decompression model. So it seems to me that for a diver unwilling to rely upon a dive computer or PADI's wheel, then rather than "averaging" the NDLs of the dive's several depths a much better approach would be to determine the pressure group at the end of the deepest part of the dive and use that to work with the table to then determine the NDL for the next depth, and so on for any subsequently shallower depths. Does that sound correct?

In addition, instead of the standard safety stop at 5 meters for 3 minutes (remember, this is all recreational no-deco stop diving), he insisted upon doing something like "20 seconds at 15m, 30 seconds at 9 m, 50 seconds at 6 m, and 1 and a half minutes at 3m" (I forget the precise times, but you get the idea...). I've read Pyle's "The Importance of Deep Stops" and some related material, but for no-deco stop dives when one is ascending at no faster than 9m/minute, is there really any justification for a safety stop at 15m for a no-deco stop dive starting at 30m? And then additional (very short) stops at 9m and 6m? And then finishing at 3m rather than 5 m? Although the pressure gradient is steeper at 3m than at 5m so theoretically offgassing is a tad faster, many divers find it quite a bit harder to maintain a consistent 3m stop than to maintain a 5m stop due to buoyancy control difficulties or simply due to big waves (I'm at 3m -- here comes a big wave, and now I'm at 4.5m! wave quickly subsides and I'm at 1.5m! etc. etc. Much easier to maintain a consistent depth of about 5m). To me, simply ascending from a no-deco stop dive slowly (9m/minute) and then stopping for 3 minutes at about 5m makes the most sense (and is well-supported by empirical evidence from DAN studies). So, is there any empirical or theoretical support for anything other than a slow ascent rate and a 5m/3 minutes safety stop on no-deco stop dives?

Anyone care to comment? I'm not going to tell this guy what to do -- I'm just curious about the theory and about whatever empirical data might exist. I'd be grateful for your well-informed replies.

Cheers,
M

There are other important factors in doing Ratio Deco that you probably don't understand, once you understand them it will make sense to you. Remember, there is no such thing as a non deco dive.

 

[TSandM]

The concept of "minimum deco" comes from the idea that there really is no such thing as a "no decompression" dive. ALL dives involve absorption of nitrogen, and offgassing or decompression. "No deco" dives are the ones where the M-value line intersects the surface, and staged decompression dives are the ones where you hit the maximum M-value before you get there. A pure Haldane/Buhlmann dissolved gas approach will drive you shallow very quickly, and have you sit shallow to offgas. "Shallow", for a no-deco dive, is the surface.

Bubble models introduce deeper stops and slower ascents. I believe NAUI is now suggesting that even "no deco" dives should have a brief stop at half maximum depth.

The MDL tables we are given as DIR divers are derived from Decoplanner, for the times that give an ascent profile with one minute stops from half maximum depth. Therefore, our ascents are at 30 fpm to half maximal depth (or half average depth, depending on where the ascent begins) and 10 fpm thereafter. This is often accomplished by a "thirty second move, thirty second stop" strategy, which I suspect is what your buddy was doing. We often also do two minutes from ten feet to the surface, if conditions permit.

Because we have built decompression into our model, we are able to do some depth averaging to determine the total time we can spend at depth. The tables built and taught by other agencies are made with other assumptions, which is why you are taught to take the maximum depth of the dive and run your tables from there. But it has never made a great deal of sense to me (nor is it the way any of my computers has run) to consider the entire dive done to 130 feet, if we went down there for two minutes, and spent the majority of the rest of the dive above 60.

You are not required to accept this approach to diving, but there are a lot of people using it and diving actively around the world with an excellent safety record.

 

[Makhno]

If you don't know what he is doing, how can you say it is wrong.

(Hint: He is not averaging NDL times)

I wasn't saying it was wrong; I was asking whether or not it was wrong (" ... in principle, it's wrong. Correct?"), which is very different.

He told me he was -- and I quote -- "averaging" NDL times, but if he's not doing that then he misspoke.

Why not. You're a DM. Isn't that part of your responsibility?

No, not necessarily.

If I had been sure he was wrong AND was wrong in a dangerous way, then I would have spoken up, but I wasn't sure he was wrong which was why I asked here. And the plans he proposed for those dives were quite conservative and they fit well within my NDLs, so as concerned those safe dive plans any disagreement would have been a moot argument about theory which I don't purport to thoroughly understand. If he had proposed a dive plan which put me or someone else at risk, then it would have been my responsibility to speak up, but he didn't. He proposed quite conservative dive plans for those dives and those plans were fine with me (as I said, well within my NDLs), and I later posted my questions here simply to try to increase my knowledge of decompression theory.

 

[Makhno]

There are other important factors in doing Ratio Deco that you probably don't understand, once you understand them it will make sense to you.

Thanks, I'll look into learning more about Ratio Deco.

Remember, there is no such thing as a non deco dive.

Yeah, I didn't forget; that's why I referred to them as "no-deco stop dives" rather than "no-deco dives".

 

[Makhno]

The concept of "minimum deco" comes from the idea that there really is no such thing as a "no decompression" dive. ALL dives involve absorption of nitrogen, and offgassing or decompression. "No deco" dives are the ones where the M-value line intersects the surface, and staged decompression dives are the ones where you hit the maximum M-value before you get there. A pure Haldane/Buhlmann dissolved gas approach will drive you shallow very quickly, and have you sit shallow to offgas. "Shallow", for a no-deco dive, is the surface.

Bubble models introduce deeper stops and slower ascents. I believe NAUI is now suggesting that even "no deco" dives should have a brief stop at half maximum depth.

The MDL tables we are given as DIR divers are derived from Decoplanner, for the times that give an ascent profile with one minute stops from half maximum depth. Therefore, our ascents are at 30 fpm to half maximal depth (or half average depth, depending on where the ascent begins) and 10 fpm thereafter. This is often accomplished by a "thirty second move, thirty second stop" strategy, which I suspect is what your buddy was doing. We often also do two minutes from ten feet to the surface, if conditions permit.

Because we have built decompression into our model, we are able to do some depth averaging to determine the total time we can spend at depth. The tables built and taught by other agencies are made with other assumptions, which is why you are taught to take the maximum depth of the dive and run your tables from there. But it has never made a great deal of sense to me (nor is it the way any of my computers has run) to consider the entire dive done to 130 feet, if we went down there for two minutes, and spent the majority of the rest of the dive above 60.

You are not required to accept this approach to diving, but there are a lot of people using it and diving actively around the world with an excellent safety record.

Many thanks, TSandM -- that comes the closest yet to the reply I was looking for when I posted my questions. Very helpful!

 

[Dr Deco]

Many thanks to the responders. I am better at deco physiology and not deco techniques.