NOAA Nitrox 32 No-Decompression Table Questions

[Dan]

Last Sunday when I went diving in Flower Garden Banks, Gulf Mexico, Texas with Fling Charter Liveaboard, one of the diver got bent after the first dive of the day & doing 5 dives on the day before, as posted here: A diver air lifted off the coast of Galveston, Texas by US Coast Guard on July 16, 2017 So, that prompted me to review my own dive profiles and see how far I was from the No-Decompression Limit (NDL). I used Nitrox 32 for those NDL dives & figured that I could use NOAA Nitrox 32 No-Decompression Table to see how far I was to the NDL. However, I'm a bit rusty in using the table since I've been relying on my dive computer in the last 12 years. Here are my dive profiles (AD = Average Depth in feet, DT = Dive Time in minutes, SI = Surface Interval in hours:minutes)
#. AD, DT, SI
1. 68, 54, 02:17
2. 71, 46, 02:33
3. 57, 56, 02:32
4. 60, 58, 12:55
5. 72, 44, finish

I pretty much follow my dive computer, SUUNTO D4, for NDL dives. From Chart 1 of the Table, below, Dive 1 is in the pressure group J. After SI = 02:17, Chart 2 shows the pressure group drops down to group H. Going across to Chart 3, for Dive 2 around 70 ft, the chart only allows the dive time of 14 minutes with 46 minutes of residual nitrogen. I pretty much already went over the allowed dive time by 32 minutes. Am I reading the table correctly?

 

[elgoog]

Almost every table I've seen is meant to be used with max depth, not average depth and assumes a perfectly square profile. Unless you plan them all as multi-level dives using tables, this will not be a good method for you.

 

[Dan]

Almost every table I've seen is meant to be used with max depth, not average depth and assumes a perfectly square profile. Unless you plan them all as multi-level dives using tables, this will not be a good method for you.

That explains it. I'll retract the flow charts again based on the max depth & time at that depth. Thanks!

 

[2airishuman]

I guess tables are a lost art.

1st dive.

Upper right portion of table, read row with 70' depth (round up from 68'), across, max no-stop time in red is 60 minutes, actual dive time falls between columns I&J, round up to J, read down to SI table in bottom right, SI falls in row H, follow left to row H for 2nd dive.

2nd dive

Row H for an 80' dive (you're supposed to round up) gives a no-stop time of 9 minutes.

But as pointed out upthread, this assumes a square profile, and is based on bottom time rather than run time. With a 30 fpm ascent and a 3 minute safety stop that can make a substantial difference.

The NOAA tables are more conservative for repetitive dives than the PADI tables.

 

[2airishuman]

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

 

[2airishuman]

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.

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

 

[tursiops]

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

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

 

[tursiops]

if you believe the PADI tables.

Why would you not?

 

[Bob DBF]

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 he top of your game don't push the limits.


Bob

 

[boulderjohn]

Several people have said something along these lines already, but I want to spell it out as clearly as possible.

• Average depth does not work when using tables.
• If you did a multi-level dive using a computer and then put your maximum depth and time onto a table, it will not work either. The computer constantly adjusts to your changing depths, as if you were doing a series of mini dives of different depths. The tables assume you were at the maximum depth the entire time. If you do a full length multi-level dive (assuming a standard tank size), you will probably exceed the table limits. That does not mean your dive was unsafe.