Question about nitrox/air on two-tank dives

[Centrals]

Unless it is a square profile around 30m otherwise I won't consider using Nitrox at all.

 

[Jake]

Unless it is a square profile around 30m otherwise I won't consider using Nitrox at all.

I didn't commonly dive nitrox on vacation, but I had a friend get skin bends a few months back on a dive that looked perfect on paper. 5 of us did the same dive and she took the hit, even though based on things like fitness she was the last one you would have expected.

After observing what she went through in the chamber and recognizing that I'm not in the shape I once was, I've decided to add some extra conservatism to my life.

 

[tbone1004]

I didn't commonly dive nitrox on vacation, but I had a friend get skin bends a few months back on a dive that looked perfect on paper. 5 of us did the same dive and she took the hit, even though based on things like fitness she was the last one you would have expected.

After observing what she went through in the chamber and recognizing that I'm not in the shape I once was, I've decided to add some extra conservatism to my life.

only more conservative if you shorten the NDL's. Nitrox does nothing if you dive up to the NDL's so the conservatism can be built into air diving just as easily. it's a setting in a computer *obvious exception being if you dive nitrox and tell your computer you're diving air

 

[tomfcrist]

Echo of tbones comment.

Breathe a rich mix and dive air NDLs. Even the there are no sure things. Sometimes, $h!t happens.

 

[Centrals]

Nitrox is not a magic bullet to avoid DCS. The only solution is not breathing compressed air, nitrox included, under water.

 

[A. Boyette]

I’m curious as to why someone would bother with nitrox on one dive and air on the other? Why not both on air, or both on nitrox? What is the perceived need to only have one tank filled with nitrox?

The way we usually do it is, on a 2-tank dive, the boat goes to one location (usually the deeper dive) for the first dive and then to another location for the shallower 2nd dive on the way back in. So I could see where EAN would be beneficial on a first dive to say 100' but not make much difference on a shallower second dive to 50'.

The reason I usually do it (when I seldom do) is because it's just what I have in the tank. I "normally" dive 36% in the ocean, but on the local reef at 30 feet, I run out of gas before I get anywhere near a NDL on either gas. So $12 worth of 36% is a waste of money, so I dive air on the reef. EAN doesn't do anything for me at 30 feet. If my buddy says, "Hey let's go diving, see you at the boat ramp in the morning." And I have a cylinder with 36% and a cylinder with air, and my third is empty, I'm not going to dump $6 worth of air and go get a nitrox fill at 7pm. I'll dive EAN and air on a 2-tank trip.

 

[MargaritaMike]

While this is not a "scientific" answer, and probably will be the subject of "M² you are stupid" remarks, I will make this comment anyway.

I do MOST of my diving in the Caribbean and I am limited by gas MOST of the time with dives consistently over an hour. Because of that I never bothered to Nitrox certify. Last year I decided to add Nitrox to list of certs. Looking back over my dive logs my first dives are typically 85'-105' at the beginning of the dive working my way up to about 40'-60' toward the end of the dive until I run out of gas. I typically don't hit NDL. Notice I said MOST. Since I started diving Nitrox, hitting an NDL is really hard to do, but I have while Lionfish hunting because the more shallow depths have been somewhat fished out. On this last trip, with both dives to near or over 100', I saw MOD depths and NDL of 0 but no deco. I was diving Nitrox 32/36 on all of my dives for 12 days straight.
All of the above was to state that when I was diving these profiles on air, I felt really draggy after 2 or 3 days of diving. When I switched to Nitrox I never felt tired after diving, even after doing 6 dives in 24 hours. Some of the locals (ex-pats) used air/Nitrox mix and they used air on the first dive probably because of MOD, which they never came close to, and EANx36 on the second dive. Because of the depths and profiles we were diving, I don't see the benefits of doing that other than cost. The only advantage that I saw was lack of fatigue and THAT was worth the extra cost.

This probably didn't make sense. The point was the ex-pats dived on air first and then Nx36 on the second in Cozumel on rec diving profiles of Max 90' on the first dive and Max 50'-70' on the second dive.

Cheers -
(I now have my armor on. Let the bombardment begin)

 

[CWK]

only more conservative if you shorten the NDL's. Nitrox does nothing if you dive up to the NDL's so the conservatism can be built into air diving just as easily.

I dive nitrox to NDLs and it does do something. Nitrox allows me to offgas more efficiently after a dive so I stay on Nitrox until I'm taking my rig off on the boat - this is 5 to 10 mins on Nitrox at the surface. There are two consequences. Firstly, the direct consequence is that I'm offgassing nitrogen more efficiently thereby reducing the risk of DCS. Secondly, the indirect consequence is that since I'm offgassing more efficiently than my computer expects, my computer is always calculating NDLs based on more nitrogen in my system than there actually is. This creates another layer of conservatism.

 

[Scuba Scott]

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

 

[tbone1004]

I dive nitrox to NDLs and it does do something. Nitrox allows me to offgas more efficiently after a dive so I stay on Nitrox until I'm taking my rig off on the boat - this is 5 to 10 mins on Nitrox at the surface. There are two consequences. Firstly, the direct consequence is that I'm offgassing nitrogen more efficiently thereby reducing the risk of DCS. Secondly, the indirect consequence is that since I'm offgassing more efficiently than my computer expects, my computer is always calculating NDLs based on more nitrogen in my system than there actually is. This creates another layer of conservatism.

5-10 mins on EAN32 instead of air is irrelevant....
80ft for 35 mins on EAN32 is a no deco dive running 50/80
% M-value does go down in the super fast tissues, but they're so fast that they're going to clear anyway

If plan a second dive with a 90 minute SIT, I can go for 35 minutes with or without that 10 minutes of nitrox. The simple fact is the gradient for EAN32 vs. air is just not big enough for it to make any appreciable difference. It doesn't hurt, but it really doesn't do anything measurable for you.