Question about nitrox/air on two-tank dives

[Hatul]

Intuitively I would have thought it better to use EAN on the first dive and air on the second, because there is more nitrogen uptake on the deeper first dive, but that assumes the same length for both dives and air consumption may change this. And John Adsit's table calculation gives the opposite result.

To challenge the premise of the OP, it's not logical to try to save by using air on the second dive. After paying thousands of dollars on the dive trip, trying to save the cost of an extra nitrox tank does not make sense. Just go with nitrox on both dives.

In my Cozumel diving the more common pattern was a deep dive sometimes 100+ or on a wall with no bottom for the first dive, and a shallow <60 ft dive for the second dive. In this pattern I choose air for the first dive and EAN36 for the second. On such a deep first dive with possibly no bottom, air is safer for keeping away from MOD in the event of loss of buoyancy or in a downcurrent.

 

[CWK]

In a square profile, say diving 100/100 to 30m, upon reaching NDL one can surface at 10m/min and surface at M-value. If one were to ascend at 30m/min, I believe that one would surface in excess of M-value. If, however, one were to ascend at 2m/min, one should surface below M-value.

I dive with 2 computers. On one EAN32 dive the settings were as follows:
- Perdix GF 65/80
- Suunto Zoop Novo - Personal setting of 0.

The dive profile above is from my Shearwater Cloud. It is a semi-square profile because the ascent is slower than 10m/min. The run times to note are as follows:
- 19 min - Perdix at GF65/80 begins to show a ceiling.
- 24 min - I unhook from the reef and begin ascending because the NDL on my Suunto probably showed 1 min. I estimate this to be about GFXX/95.
- 35 min - ceiling on Perdix clears.
- 43 mins - I surface.

The reason I brought up this profile is that I reached NDL on my Suunto at 24 min but surfaced 19 mins later at 43 mins. During that 19 minutes of ascent, I was offgassing. I was diving on EAN32 and had 19 minutes of more efficient offgassing than I would have had doing the same ascent profile on air. The point that I am trying to make is that it is not just 10 mins at the surface after the dive that I was offgssing more efficiently, but also for 19 minutes of ascent, therefore I was offgassing more efficiently for 29 mins. So we are not just looking at a 3% difference that results from 10 mins of EAN32 at the surface.

Are you able to calculate the difference between air and EAN32 taking both the 19mins ascent and 10 mins at surface? Would it be 5%? 10%?

 

[tbone1004]

@CWK yes but if your ascent is to look at pretty stuff, it's part of your dive and factors into your average depth, so what profile do you want me to run? This is 24 minutes at 100ft on EAN32, then a 19 minute stop at 20ft to roughly replicate whenever you did on ascent.

If you made a direct ascent
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 63.8 79.1 86.3 87.4 83.5 80.6 76.1 71.5 67.4 65.4 64.2 63.4 63.4 62.6 62.5 62.7
%M-V Grad 45.1 65.3 75.3 75.2 65.0 54.4 38.6 21.0 3.8 -9.9 -21.4 -32.7 -45.4 -55.1 -66.4 -77.9

After 19 minute stop at 20ft on EAN32
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 38.1 50.5 62.8 71.2 74.1 75.7 74.1 71.1 67.8 66.0 64.9 64.1 64.0 63.2 63.0 63.1
%M-V Grad 6.3 17.9 32.7 43.1 45.0 42.7 33.4 20.0 4.9 -7.9 -19.0 -30.2 -42.8 -52.8 -64.3 -76.0

19 min stop at 20ft on air

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 43.4 55.9 67.8 75.6 77.7 78.6 76.3 72.8 69.1 67.1 65.7 64.7 64.6 63.6 63.4 63.4
%M-V Grad 14.3 26.9 41.7 51.7 52.5 49.6 39.3 24.8 8.8 -4.6 -16.2 -27.7 -40.6 -51.0 -62.7 -74.6

The stop obviously does quite a bit vs a direct ascent, though you have chosen an acceptable gf of 80, so the delta between 45% on EAN32 and 52.5% on air is pretty irrelevant IMO.

 

[boulderjohn]

The reason I brought up this profile is that I reached NDL on my Suunto at 24 min but surfaced 19 mins later at 43 mins. During that 19 minutes of ascent, I was offgassing. I was diving on EAN32 and had 19 minutes of more efficient offgassing than I would have had doing the same ascent profile on air. The point that I am trying to make is that it is not just 10 mins at the surface after the dive that I was offgssing more efficiently, but also for 19 minutes of ascent, therefore I was offgassing more efficiently for 29 mins. So we are not just looking at a 3% difference that results from 10 mins of EAN32 at the surface.

We are getting a little complicated here, but in general there is some truth in this. My earlier comparisons assumed a straight, normal ascent to the surface followed by a standard safety stop. I see the same thing you are describing in technical diving when finishing the dive on oxygen. During the late stages of the last deco stop, if I go to the tissue bar graph on a Shearwater, it will show that my fastest tissues have dropped so far while I am not breathing any nitrogen that when I finally ascend to the surface and am walking around after the dive, those tissues will actually be ongassing. If you look at your tissue bar graph at the end of a safety stop on EAN, you might see something similar.

 

[CWK]

@tbone1004
Many thanks for generating the data. Most interesting.

What program are you using? I might buy it to model some stuff.

 

[tbone1004]

@tbone1004
Many thanks for generating the data. Most interesting.

What program are you using? I might buy it to model some stuff.

that is Deco Planner, but many other programs out there will spit out the same information. The algorithms are not complicated

 

[Landlocked123]

I dont do deco dives. I get right up to the NDL's and head up but as a habit after my 3 mintues at 15 feet for safety stop I do 1 minute at 10, 1 at 5, and I take another to get from 5 to the surface. It's good buoyancy practice and the way I see it further reduces any risk as you pretty much crawl from that 15 to the surface. My ears also seem to like this slow ascent from 15.

 

[KenGordon]

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.

A lung over expansion injury isn’t likely to be mild though is it?

I know you don’t believe in people getting bent like this. You should visit some chambers and listen to their war stories. Our local chamber gets 70 to 80 diving cases a year.

 

[KenGordon]

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.

This is the ‘no bright black line’ between bent and not bent point. It might even have been Eric Baker (Mr GF) who made the point that way.

Deco is about a complicated biological system rather than a hard mechanical one. Stuff is happening all over the body and influenced my many different factors. Deco algorithms basically ignore all that and focus on one or two easy to model aspects, calibrated by a small set of measured results. So the model is drawing a line that does not exist. Your precipice is more like a steep, slippery hillside on a windy day. How far you can go before falling depends on whether it is raining, how big the gusts are, how grippy your shoes are, there you are on all fours and how late you were drinking.

 

[boulderjohn]

A lung over expansion injury isn’t likely to be mild though is it?

I know you don’t believe in people getting bent like this. You should visit some chambers and listen to their war stories. Our local chamber gets 70 to 80 diving cases a year.

There are a variety of lung overexpansion injuries, and some can be quite imld and even unnoticed. The arterial gas embolism is the one we all fear, but there are different types of injuries, including mediastinal emphysema, pneumothorax, and subcutaneous emphysema, which in a mild case would present as a minor rash. A gas embolism's severity depends upon where the bubble goes.

A few years ago we had a thread that dealt with the various chamber's relationship with DAN and their insurance policy. Some documents were released, including one from DAN that talked about the very large number of cases of insurance claims for DCS in which DAN felt the diver did not have DCS. We had another thread a few years ago that mentioned the large number of people who have carpal tunnel syndrome fllareups after diving. These are frequently misdiagnosed as DCS. Left on their own, these flareups will improve in about the same time table as a DCS case getting treated in a chamber. (I used to have that precise problem after a weekend of diving, and the cure was carpal tunnel surgery.)