Nitrox vs Air

[ianr33]

Okee dokee - I kinda' mistyped what I was trying to say. Thanks for not getting bent over it.

So, when you have 10 min of deco left, and the Oceanic goes into the yellow, how long does it take to go into the green...?

Well I am sure it depends on the profile,but probably around 10 minutes,for modest deco dives anyway.

Put another way,running V Planner on +2 conservatism more or less equals the computer JUST going into the green. Just going into the yellow from red(deco) is probably more aggressive than V Planner on nominal conservatism. Really not a good idea to surface at this time!

I have only ever "bent" my Aeris computer on one dive. That was doing 150 feet for 30 minutes with 2 deco gases (32% and 80% if I recall corectly) I have done lots of dives with one deco gas and had to do at most an extra 2 or 3 minutes deco beyond the tables to get it back into the yellow. (The computer knows nothing of the deco gases)

Bottom line is that surfacing when this type of computer has JUST come out of deco is a real BAD idea

 

[Charlie99]

May I change a little bit the focus of this discussion to ask other thing about Nitrox...

.... but when you dive, you do 15min at the bottom with air and ascend till 6m when the DM gives you a pure O2 from his 15l cilinder.

What do you think about this?

I think it's a bad way to dive, for several reasons.

You are blindly trusting someone else rather than understanding your own deco plan.

What do you do if for some reason you are separated from the DM and the O2? Do you have enough backgas to deco on your own.

From what little you described, there are probably many other problems with the dive plan. For example, if the current keeps you from returning to the line and you do your ascent and decompression stops while drifting, you should have a large marker buoy that you can send to the surface so that the boat can keep track of you and come pick you up.

 

[TheRedHead]

But what I want to know from you guys, is what do you thing doing this 53m dive with air and to finish with 100% O2 at 6m. Off course is a deco dive, but should we use Trimix? Should we use EAN 32 from 20m? This is the kind of info I would like to see.

At that depth (approx. 174 feet), I would be narced out of my mind on air. If I were to dive to that depth, I would use Trimix.

 

[DA Aquamaster]

But what I want to know from you guys, is what do you thing doing this 53m dive with air and to finish with 100% O2 at 6m. Off course is a deco dive, but should we use Trimix? Should we use EAN 32 from 20m? This is the kind of info I would like to see.

170 ft on air is very deep, but if the viz is good, no penetration is done and you have deep air experience, I'd assert that trimix is still a good idea but not absolutely required, although trimix divers will almost certainly adamantly disagree with this (while deep air divers will mostly stay silent and just go dive since they can afford to dive a lot.)

I would however never conduct a moderate deco dive like this (or even a minor deco dive) while relying on someone else for deco gas. A diver should always have it with them on an open water dive like this in the event they are unable to get back to the ascent line.

I also want to be doubly sure that I have enough gas plus a reserve (1.5 for back gas and at least 1.2 for deco gas) as well as have enough gas in reserve (either a second deco gas/deco bottle or enough back gas) to complete contingency deco plans in the event access to one of my deco gasses is lost. In short, you want to be fully independent, fully redundant and well equipped with contingecy deco plans to compensate for the loss of one of your planned deco gasses. Having a buddy is nice, but having to actually depend on a buddy is a recipe for eventual disaster in deco diving.

On a profile like this it is also worth using 2 deco gases. For a deep air dive, I would probably choose 50% for the stops from 70 ft on up and then switch to either 80% at 30 ft or 100% O2 at 20 ft. This gives you faster off gassing but more importantly, gives you more flexibility in dealing with the loss of one or more of your gasses as you can have the primary plan for both deco gasses, a contingency plan for each of the gasses alone as well as a plan for decompression on your back gas alone (if you have enough).

If you were diving trimix, it may make more sense to use Nitrox 36 for the first deco gas given that the initial stops are often deeper and then finish with 80% or 100% at the shallow stops.

 

[Charlie99]

Put another way,running V Planner on +2 conservatism more or less equals the computer JUST going into the green. Just going into the yellow from red(deco) is probably more aggressive than V Planner on nominal conservatism. Really not a good idea to surface at this time!

I have only ever "bent" my Aeris computer on one dive. That was doing 150 feet for 30 minutes with 2 deco gases (32% and 80% if I recall corectly) I have done lots of dives with one deco gas and had to do at most an extra 2 or 3 minutes deco beyond the tables to get it back into the yellow. (The computer knows nothing of the deco gases)

Bottom line is that surfacing when this type of computer has JUST come out of deco is a real BAD idea

Pelagic computers are sold under many brand names, including Aeris. Assuming that your particular Aeris computer is the standard Pelagic algorith where the NDL model is a straight unpadded DSAT (PADI) model, then surfacing with your computer right at the yellow/red transition is equivalent to doing a square profile dive to the limits of the PADI RDP. More agressive than I choose to do, but not what I would call "a real BAD idea".

The DSAT and Buhlman models are similar enough that you can get pretty much the same results as your Aeris computer by running any Buhlmann program (Decoplanner, the old GAP, etc) with either no gradient factors, or high gradient factors like 99/99. More agressive than my preference, but not crazy.


It is interesting that you note that +2 on Vplanner roughly corresponds to the green/yellow transition. I did a few checks and found that on my Oceanic Data Plus 2 it corresponds to a gradient factor about about 80%-85% on the ZHL16B model (which isn't all that different from 80%-85% on the DSAT model).

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As for how long it takes to clear the yellow zone back into the green, DA Aquamaster is right on target. After a short deep dive, the controlling compartment is a fast one, and the bargraph clears quickly. After long 60-80' dives, after several repetitive dives, or after "riding the NDL curve" up from a deeper dive, the controlling compartment will be slower and take much longer to clear back to green.

 

[DandyDon]

You may not have the gas to get back in the green, so you are not going to want to go there in the first place if you want to exit the water in the green.

Tell me! Been there some. :11:

 

[LavaSurfer]

There's something here that doesn't stack up.
What SAC did you calculate on?

I ran my own calculation for the profile you gave at a SAC of 13liter/min (given that my average is 12 and would probably be less on this dive because it's a dive which requires almost no physical effort) and came up with only 79 cu ft?

I used 0.7CF. @ 0.5CF it comes out to
89.7 cu ft Air

How that converts to Liters is beyond me:05:

I only play with vPlanner for fun and experience right now. Someday I will use it in practical situations but not today.

Even at 79 cu ft, unless you use a larger tank than an AL80, its still a dual setup.
I doubt the rental supplies any 120's but I might be wrong

 

[miketsp]

I used 0.7CF. @ 0.5CF it comes out to
89.7 cu ft Air

How that converts to Liters is beyond me:05:

I only play with vPlanner for fun and experience right now. Someday I will use it in practical situations but not today.

Even at 79 cu ft, unless you use a larger tank than an AL80, its still a dual setup.
I doubt the rental supplies any 120's but I might be wrong

OK Firstly I doubt that you'll find many air-hogs on a dive like this so a reasonable SAC would be 0.4 to 0.45 in imperial units.

Secondly I ran this calculation on the profile you supplied which is very conservative.
The profile that emelotto quoted with a 1st deep deco stop is not the one that was in use last time I was in Noronha in 2002.
At least then they were using the CMAS table based on a full air dive but with the last stop replaced with O2 maintaining the duration in order to create an additional safety factor. If for some reason the diver was unable to access the O2 bottles he would still be within the table plan.
The CMAS profile would be descend at 18m/min, 15mins from start of descent to start of ascent, ascent at 9m/min, 3mins at 9m, 4 mins at 6m and finally 7mins at 3m.
If you run the calculation with a reasonable SAC you will find it is in fact a single AL80 dive.
The only criteria you don't meet is rock-bottom to get both members of the buddy pair to the surface on a single remaining cylinder after catastrophic failure but this situation is covered by the accompanying DM with large doubles and extra stage.
This dive is normally done with a low ratio of visitors to DMs.

 

[LavaSurfer]

OK Firstly I doubt that you'll find many air-hogs on a dive like this so a reasonable SAC would be 0.4 to 0.45 in imperial units.

Secondly I ran this calculation on the profile you supplied which is very conservative.
The profile that emelotto quoted with a 1st deep deco stop is not the one that was in use last time I was in Noronha in 2002.
At least then they were using the CMAS table based on a full air dive but with the last stop replaced with O2 maintaining the duration in order to create an additional safety factor. If for some reason the diver was unable to access the O2 bottles he would still be within the table plan.
The CMAS profile would be descend at 18m/min, 15mins from start of descent to start of ascent, ascent at 9m/min, 3mins at 9m, 4 mins at 6m and finally 7mins at 3m.
If you run the calculation with a reasonable SAC you will find it is in fact a single AL80 dive.
The only criteria you don't meet is rock-bottom to get both members of the buddy pair to the surface on a single remaining cylinder after catastrophic failure but this situation is covered by the accompanying DM with large doubles and extra stage.
This dive is normally done with a low ratio of visitors to DMs.

0.4 SAC you would be pushing it. Personally most people I know get SAC rates a little higher than that. The tank would be at about 250 lbs on surface so I guess in a perfect world its doable. I would want more buffer. Personally!

 

[miketsp]

..snip..
I would want more buffer. Personally!

The point I wanted to make is that it is possible on an AL80/12 liter cylinder.

In fact we just looked at the logs and the operator did in fact supply 15liter cylinders. I suppose that's about 100cuft.
So there is a bit more safety margin than I had remembered.