NOAA Nitrox 32 No-Decompression Table Questions

[elmo]

Tables are a good lo-tech reference but are limited because they cannot account for your actual dive profile and rely on pressure groups for repetitive diving, which cannot consider all tissues so make a conservative guess as to which might be controlling. If using tables for planning, it's safest to use maximum depth. But if you're looking back for a comparison with your dive computer after you've finished diving, average depth would be a closer (but less conservative) approximation.
If you're interested, why don't you download your dives into Subsurface? You can then look at the NDLs (or deco requirement) calculated by Subsurface at any point in any dive. You can also see the effect of changing gradient factors or compare Buhlmann gf with VPM-B models. What's reported by your dive computer may also be shown (depending on what your dive computer can do and what data gets downloaded).

 

[2airishuman]

Why would you not?

Some people use tables (or algorithms) that are more conservative.

 

[tursiops]

If using tables for planning, it's safest to use maximum depth. But if you're looking back for a comparison with your dive computer after you've finished diving, average depth would be a closer (but less conservative) approximation.

The tables are valid when used with max depth. There are any number of profiles for which use of average depth would be way off the mark. This is poor advice in the Basic Scuba forum.

 

[tursiops]

Some people use tables (or algorithms) that are more conservative.

What is your measure of conservatism, so that you can rank-order various tables? For example, it would seem that FOR REPETITIVE DIVES the Navy/NOAA tables would be more aggressive....given their design and validation.

 

[Dan]

I pulled out a PADI EAN32 table.

1. 68, 54 vs. no-stop time of 60, pressure group (T), 02:17, (B)

2. 71, 46, 02:33
This exceeds the adjusted no-stop time for 80 feet of 35 minutes. Many divers would interpolate which would give a no-stop time of 47 minutes and an RNT of 11 (U) 02:233 (B)

3. 57, 56 vs. adjusted no-stop time of 76, RNT of 14, pressure group (T), 02:32 (B)

4. 60, 58 vs adjusted no-stop time of 76, RNT of 14, pressure group (U), 12:55, table shows no residual nitrogen

5. 72, 44, vs no-stop time of 45 minutes (at 80 feet), finish

Thanks! Applying the average depth data into the PADI tables seem to be closer to what my dive computer shows. I can see the 2.5hrs SI helps to bring the next dive plan to the lower pressure group (lower RNT, higher NDL). I can also see that I was pushing the limit, although my body felt fine by the 4th dive.

 

[boulderjohn]

With surface intervals over two hours though the residual nitrogen time is small, if you believe the PADI tables.

hy would you not?

Some people use tables (or algorithms) that are more conservative.

I assume the following explanation is what you mean:

With old Navy tables people were diving decades ago, the surface interval was based on the 120 minute compartment, which had never existed in previous decompression models and was created when those tables were designed. There was no real research for this decision, as I understand it. This leads to a 12 hour washout (6 iterations of that compartment length), and very, very long surface intervals that affected recreational divers wanting to do 2-tank dives. It did not affect Navy divers all that much, because they usually only did one dive per day.

PADI's subsidiary DSAT then did extensive testing using Doppler bubble imaging on people doing typical recreational, no-decompression stop diving to design a new set of tables. They wanted especially to see if the 120 minute compartment was the right one to use. Their research indicated that no, the 40 minute compartment was probably best. They went with the 60 minute compartment instead to be more conservative, leading to a 6 hour washout and much shorter surface intervals. They further shortened the surface intervals by roughly doubling the number of pressure groups, thus eliminating a lot of the rounding that was necessary with the Navy tables.

Some dive tables today still follow the old Navy system, and divers following those tables will have much, much longer surface intervals than anyone following PADI tables. I do not know if any current computer algorithms are consistent with the old Navy tables. The computers using DSAT are following the PADI research, and most other algorithms that I know are reasonably close to DSAT as well.

So it is possible that there are people who do not believe the PADI tables and still follow the old Navy tables. I recently read a thread that said the current NAUI tables still use the old Navy system, but I can't say that for sure. I would suggest, though, that if you do not believe the PADI tables, you should be able to point to the flaws you find in that research that led you to your disbelief.

 

[2airishuman]

You did catch that he provided AVERAGE not MAXIMUM depth, right? The tables are not usable....

We're in vehement agreement, c.f. my earlier post, to wit:

I don't think it's possible to reach any real conclusions without considering the actual profiles. If your profiles were reasonably square then using maximum depths and actual bottom time (not total run time) would be insightful.

What is your measure of conservatism, so that you can rank-order various tables? For example, it would seem that FOR REPETITIVE DIVES the Navy/NOAA tables would be more aggressive....given their design and validation.

My metric is which tables allow a dive that others do not. The example dives nearly fit within the PADI EAN RDP limits but do not fit within the Navy/NOAA table limits. Of the widely used tables, the PADI tables are usually the least conservative for repetitive dives.

 

[2airishuman]

My previous reply was crossposted with bouderjohn.

I would suggest, though, that if you do not believe the PADI tables, you should be able to point to the flaws you find in that research that led you to your disbelief.

I dive a computer that uses an algorthm that closely matches the PADI tables. I use the PADI tables. I believe in the PADI tables, with some caveats that you probably share. Since the premise of the thread is to understand how close a series of repetitive dives was to the published NDLs, and since the OP posted copies of tables derived from the Navy data, I just wanted to point out there are differences between them, and that the OP is, as far as I'm concerned, entitled to believe in either of them. People choose more conservative algorithms or tables for all kinds of reasons.

The caveats are based on things the tables themselves mention in the instructions: that they are based on research that does not adequately address repeated dives over multiple days, that cold water or strenuous dives may contribute to DCS before the no-stop limits are reached, that there are variations in human physiology and the tables are designed to be safe aroudn 99% of the time, and so on.

 

[Dan]

Rather than all the math, since you are using a computer that gives you a readout in minutes of NDL, just stay several minutes further away from NDL. If you feel overly fatigued, some may say is a symptom of sub-clinical DCS, back off even more.

Regardless of what the calculations say, individually one can suffer from DCS within NDL, especially when doing aggressive multi-dive days. If you are not feeling at the top of your game don't push the limits.


Bob

I kept an eye on the remaining bottom time & ascent whenever it got down to 1-2 minutes left and didn't feel fatigued after the dives. I just felt a bit shakened up when a diver in front of me who, I "ass-u-me", had similar dive profiles, got bent & had to be dragged out of the water. That made me wonder whether I had also pushed my limit without realizing it.

 

[tursiops]

the tables are designed to be safe aroudn 99% of the time

LOL. 99.99% is more like it, according to the studies done..