Question about nitrox/air on two-tank dives

[rsingler]

Tom,
What's the half time of that leading compartment? 97 after 10 minutes on the surface seems odd, after just a single 80' NDL dive...

 

[iamrushman]

Air first then nitrox. If I had tables with me i would demo some profiles for you.

i do air first then N2O2 to shorten SI...

 

[tbone1004]

Tom,
What's the half time of that leading compartment? 97 after 10 minutes on the surface seems odd, after just a single 80' NDL dive...

Sorry, quoted the m-value not percentage of m-value. Either way, if you're doing a 60 minute SIT, the first 6 compartments are going to be mostly cleared. 90 minute SIT and the first 8 are going to be mostly clear.
1-4min
2-8min
3-12.5min
4-18.5min
5-27
6-38
6-54
8-77
9-109
10-146
11-187
12-239
13-305
14-390
15-498
16-635

These are the values for compartments 1-16 without the 10 mins on the surface

M-Value 97.3 83.2 73.8 66.8 62.3 57.4 54.1 51.7 49.9 48.2 46.8 45.6 44.1 43.5 42.6 41.8
%M-Value 64.3 78.2 86.2 88.6 85.7 83.3 78.7 73.8 69.4 67.1 65.6 64.5 64.4 63.4 63.2 63.2
%M-V Grad 45.9 63.9 75.0 77.4 69.5 60.6 45.4 27.6 9.7 -4.6 -16.6 -28.4 -41.4 -51.8 -63.5 -75.4


The 10 mins on EAN32 is technically done at 1ft since you can't put a SIT in this software, but this is the final segment of the dive

M-Value 97.3 83.2 73.8 66.8 62.3 57.4 54.1 51.7 49.9 48.2 46.8 45.6 44.1 43.5 42.6 41.8
%M-Value 33.2 48.5 62.5 71.5 74.4 75.9 74.2 71.2 67.8 66.1 64.9 64.1 64.1 63.2 63.0 63.1
%M-V Grad -1.1 14.7 32.2 43.6 45.7 43.3 33.9 20.3 5.1 -7.8 -18.9 -30.1 -42.8 -52.7 -64.2 -75.9


After 10 mins on air, i.e. just surfaced and chilled out

M-Value 97.3 83.2 73.8 66.8 62.3 57.4 54.1 51.7 49.9 48.2 46.8 45.6 44.1 43.5 42.6 41.8
%M-Value 35.9 50.9 64.5 73.1 75.7 76.9 75.0 71.7 68.3 66.4 65.2 64.3 64.2 63.3 63.2 63.2
%M-V Grad 3.0 18.7 35.8 46.8 48.3 45.7 35.8 21.9 6.4 -6.7 -18.0 -29.3 -42.1 -52.1 -63.7 -75.5


If you did 10 mins on O2

M-Value 97.3 83.2 73.8 66.8 62.3 57.4 54.1 51.7 49.9 48.2 46.8 45.6 44.1 43.5 42.6 41.8
%M-Value 16.7 33.6 50.2 61.5 66.7 69.8 69.5 67.6 65.2 64.1 63.3 62.7 63.0 62.3 62.4 62.5
%M-V Grad -26.2 -10.0 9.9 23.9 29.2 28.9 21.8 10.5 -2.6 -14.3 -24.5 -34.9 -47.1 -56.3 -67.3 -78.6

So as you can see, there really isn't any real appreciable difference in the slow compartments unless you're on O2. The difference is really negligible in the really slow tissues, and in the ones where there is a difference, i.e. compartments 1-6, those are going to clear out after an hour SIT anyway.

sorry for formatting, it really didn't want to format anything at all

 

[rsingler]

Thanks, Tom

 

[boulderjohn]

Air first then nitrox. If I had tables with me i would demo some profiles for you.

I have the tables with me. Here are two dives proposed in the opening post, and I will do them on air and EAN 36, using the PADI tables.

1. Dive 1 is deeper than dive 2, in the ranges of say 50-70fsw and 40-60 fsw respectively.
​

I will propose that the diver has a SAC rate of about 0.5 and will be using an AL 80. I will make the dives to 70 and 60 feet, and I will give the diver a maximum gas usage of 60 cubic feet at depth for each dive. The depths are close enough to make a even estimate of 40 minutes per dive. (Just trying to make the math easier.) I will give an hour surface interval.

Scenario One--Air First, followed by EAN 36
First Dive: The diver hits the maximum allowed bottom time of 40 minutes. He is in Pressure Group T. After 1 hour, he is in Pressure Group G.
Second Dive: Switching to EAN 36 for the second dive to 60 feet, he has 83 minutes of maximum time allowed, of which he can only use 40 minutes because of gas usage. He finishes the second dive in Pressure Group R.

Scenario One--EAN 36 First, followed by Air
First Dive: The diver is done at 40 minutes, nowhere near the limit of 75. He finishes the dive in Pressure Group M. After One hour, he is in Pressure Group D.
Second Dive: Switching to Air for the second dive to 60 feet, he has a maximum of 39 minutes, so he will lose 1 minute of bottom time. He will finish in Pressure Group S.

So, in this scenario, diving air first followed by EAN 36 gives you one more minute of bottom time, and you finish one pressure group lower, thus lowering your decompression stress by a small amount.

 

[CWK]

Sorry, quoted the m-value not percentage of m-value. Either way, if you're doing a 60 minute SIT, the first 6 compartments are going to clear. 90 minute SIT and the first 8 are going to clear.
1-4min
2-8min
3-12.5min
4-18.5min
5-27
6-38
6-54
8-77
9-109
10-146
11-187
12-239
13-305
14-390
15-498
16-635

These are the values for compartments 1-16 without the 10 mins on the surface

M-Value 97.3 83.2 73.8 66.8 62.3 57.4 54.1 51.7 49.9 48.2 46.8 45.6 44.1 43.5 42.6 41.8
%M-Value 64.3 78.2 86.2 88.6 85.7 83.3 78.7 73.8 69.4 67.1 65.6 64.5 64.4 63.4 63.2 63.2

The 10 mins on EAN32 is technically done at 1ft since you can't put a SIT in this software, but this is the final segment of the dive

M-Value 97.3 83.2 73.8 66.8 62.3 57.4 54.1 51.7 49.9 48.2 46.8 45.6 44.1 43.5 42.6 41.8
%M-Value 33.2 48.5 62.5 71.5 74.4 75.9 74.2 71.2 67.8 66.1 64.9 64.1 64.1 63.2 63.0 63.1

After 10 mins on air, i.e. just surfaced and chilled out

M-Value 97.3 83.2 73.8 66.8 62.3 57.4 54.1 51.7 49.9 48.2 46.8 45.6 44.1 43.5 42.6 41.8
%M-Value 35.9 50.9 64.5 73.1 75.7 76.9 75.0 71.7 68.3 66.4 65.2 64.3 64.2 63.3 63.2 63.2


If you did 10 mins on O2

M-Value 97.3 83.2 73.8 66.8 62.3 57.4 54.1 51.7 49.9 48.2 46.8 45.6 44.1 43.5 42.6 41.8
%M-Value 16.7 33.6 50.2 61.5 66.7 69.8 69.5 67.6 65.2 64.1 63.3 62.7 63.0 62.3 62.4 62.5


So as you can see, there really isn't any real appreciable difference in the slow compartments unless you're on O2. The difference is really negligible in the really slow tissues, and in the ones where there is a difference, i.e. compartments 1-6, those are going to clear out after an hour SIT anyway.

Thanks for generating the % of M-values. Quite interesting.

Taking the first listed compartment as an example, the difference between EAN32 (33.2%) and air (35.9%) is 2.7% (35.9% – 33.2%). This 2.7% may be considered to be "negligible" but a negligible 2.7% can take you over the edge from 99.0% to 101.7% into DCS. It is marginal (negligible) increases that takes one over the edge (or treshold) into DCS.

Where is the edge of super saturation that takes one into DCS? The theoretical answer is M-value. But consider the quote below from a study of 320 divers suffering from DCS. Study link: Dive Risk Factors, Gas Bubble Formation, and Decompression Illness in Recreational SCUBA Diving: Analysis of DAN Europe DSL Data Base

• ✓ Only eight cases (2.5%) showed a GF > 1
• ✓ 14 cases had a GF > 0.9 (4.4%)
• ✓ The majority of cases (236–73.7%) showed GF-values between 0.70 and 0.90
  • * 37.5% between 0.8 and 0.9
  • * 36.2% between 0.7 and 0.9
• ✓ 46 cases (14.4%) had a GF lower than 0.70
• ✓ 10 cases (3.4%) lower than 0.60
• ✓ Only 3 cases had a GF lower than 0.50

Divers have suffered DCS when diving within NDL with a GF lower than 0.50!

From my perspective, a marginal (negligible) increase takes one over the edge into DCS and the edge can be quite far from M-value. Therefore, irrespective of whether I'm surfacing on 02, EAN50, or EAN32, I'm staying on enriched air for as long as possible.

 

[CWK]

BTW, the % of M-value listed appears odd to me.

Assuming that the 4 minute compartment is at 100% upon surfacing. After 4 minutes, it should be at 50%. This halves again after another 4 minutes to 25%. It should be lower than 25% after 10 minutes, but the listed % for air after 10 minutes is 35.9%.

Is my understanding of half times faulty?

 

[tbone1004]

@CWK the compartments aren't fully saturated, and there may be some errors in here due to me having to force it to say 1ft instead of surface. There are also ascent times factored in etc. so there will be a couple minutes in there that aren't called out specifically. I.e. the 35 minute dive is 35 minutes at the bottom and the total dive time is 39 minutes and you start offgasing from depth. When I put the 10 minute stop in there, the total run time is 48 minutes so there will be some fudge factors in there. It may also be using compartment 1b instead of 1. I'm not sure.

I edited the original post and included the @ m-v grad which I should have in the first place. This shows which tissues are ongasing and offgasing. Closer to 100 they get, the higher the risk you are of getting bent because 100 is the theoretical tissue pressure.

When you hit the surface, your leading compartment is #4 with a 77.4% m-v grad which is pretty close to your GF-hi of 80. Perfectly reasonable.

If you did 10 mins on EAN32, your leading tissue is actually fully offgased and is ongasing nitrogen when you come off the regulator, and compartment 5 has the most pressure in it at 45.7%.
If you did it on air, the leading compartment is offgasing, but compartment 5 is still leading the charge with a pressure of 48.3%. That is a pretty negligible benefit over air.

Does that 3% make a difference? It obviously doesn't hurt, but I'm not entirely convinced it helps significantly since you have already reached the surface. It will potentially minimize risk of getting a joint bent or something that is work related maybe, but at 3% it just isn't significant. You quoted that people were getting bent with GF-hi's less than 50, but what you don't know is what their ascent profile looked like, how many tissues were close to 50, what their physical condition was, and what caused the bend. People regularly dive gf hi's of 90 or more with no ill-effect, so
We do surface deco on just about every deco dive we do, and if you look at the percentage after 10 minutes on O2, you can see that the compartment 5 is still the leading compartment, but it is only at 29.2% and that is significant enough for me to pay attention to before I climb my 270lb self with 150+lbs of gear onto a pitching boat deck up a ladder, or climbing stairs to get out of a cave, etc.

 

[CWK]

@tbone1004,
I think that we are roughly on the same page but am happy to discuss further since you have now brought ascent rates into play.

The most surprising case of DCS that I have heard about involved a mother and daughter on their very first open water training dive. Max depth of 12m, total run of 15 min. The mother was doing lost regulator retrieval, failed to locate the regulator, panicked and shot to the surface. Instructor saw the mother shooting for the surface, grabbed the daughter by the hand and took off after the mother. Mother and daughter diagnosed in hospital with DCS (mild). So yes, ascent rates do matter.

I have trained with 2 different tech instructor trainers in recent months and both taught me to use what I consider to be conservative profiles (one was 35/75 and the other was 40/70). But over and above the conservative GFs, both taught me to be even more conservative in different ways. The first taught me to clear deco at 5m, then add 3 mins before surfacing. The second taught me to clear deco at 5m, then stop for 1 min each at 4m, 3m, 2m, and 1m. I put the conservatism down to the understanding that people were getting DCS within M-values. They were teaching me to add small margins of conservatism. To me, staying on enrich air until I am back on the boat and taking off my gear is an extension of that conservatism. The theoretical model is not perfect and added conservatism does not hurt.

Does 3% make a difference? Let us say you are on flat land and x steps takes you to the edge of a precipice, and x+1 steps takes you over the edge. If you could retrace steps, in order to get back onto flat land you would not need to retrace x+1 steps. All you would need is to retrace 1 step, the marginal (and negligible) step that took you over the edge. To me, the theoretical model is not perfect. One can dive 35/75 and be certain to have a 25% margin to M-value, nevertheless that 25% margin does not guarantee that one is safe from DCS. Nobody can guarantee that 3% will not make a difference.

I will look at ascent rates and the difference between air and Nitrox in my next post.

 

[boulderjohn]

Mother and daughter diagnosed in hospital with DCS (mild). So yes, ascent rates do matter.

The first key word in that sentence is "diagnosed." Being diagnosed with DCS and having it are not necessarily the same thing. The symptoms of mild DCS could also be caused by a number of other causes, and such a diagnosis is uncertain at best.

The second key word is "DCS." Were they diagnosed with DCS or DCI (decompression illness)? Decompression Illness is a term used to cover both decompression sickness and lung overexpansion. The reason to use this more general term is that the symptoms are often so very similar that it can be hard to impossible to tell which is the case in a specific incident. The correct diagnosis, however, is unimportant, since the treatment for both is the same.

EDIT: In the incident described, a lung overexpansion injury is very likely.