Nitrox and repetitive dives, what mix first?

Cyber_Ghost · Feb 7, 2009

Here's the thing, I've done BSAC nitrox certification, and was taught to plan the dives and mixes so that the mix will get richer with each repetitive dive, with depth becoming shallower (as taught in most cases).
Yesterday, while getting my gear ready for a set of 2 dives, was told by an ANDI instructor that according to ANDY the richer mix should be used for the first, deeper dive, and the percentage should decrease with the dive order.
Today it was a dive to 30m, then a dive to 18m. I had 2 cylinders, one a 12L with air, the other 10L with EAN36. Did the first dive on nitrox, the second on air. (amount of air for the deeper dive played the decisive factor here).
So, how do you guys plan your dives and mixes?

What are the reasons for choosing a richer mix for the last dive?

Blackwood · Feb 7, 2009

Cyber_Ghost:
was told by an ANDI instructor that according to ANDY the richer mix should be used for the first, deeper dive, and the percentage should decrease with the dive order

Does not compute. "Richer" and "should be used deeper" don't belong in the same sentence.

The only thing I can speculate is that he's somehow trying to keep you low on the CNS clock by using a leaner mix on the longer dives, but for these types of dives that's hardly a concern (and it doesn't make complete sense anyway).

Cyber_Ghost:
What are the reasons for choosing a richer mix for the last dive?

The higher the oxygen content, the higher the inert gas gradient and (theoretically) the cleaner your decompression (for the repet dive AND the residual loading) will be.

While I personally don't, if you're going to dive "best mix," decreasing the FO2 as you dive shallower is back asswards.

h2odragon1 · Feb 7, 2009

Which dive mix gives you the best profile at which depth? Then the deeper dive first. :popcorn:

rstofer · Feb 7, 2009

You need to go back to the book... The primary reason for using Nitrox is to reduce your exposure to nitrogen. So, how do you know when you are making a choice that will result in the least exposure to nitrogen? By the ending pressure group. The lower the group letter, the less nitrogen you have absorbed.

The next consideration is MOD - how deep can you dive? By my tables, 30m on EAN36 is past the limit (it's close but beyond the MOD). It doesn't leave any room for error assuming you believe in the 1.4 ATA limit. And you should...

For your dives, with your tanks, I would have used air for the 30m dive (because EAN36 is inappropriate) and come up with a higher pressure group. I would have done the SIT until I was within range of the 18m dive with EAN36.

OTOH, if I had EAN32, I would have made the first dive with Nitrox to take on as little nitrogen as possible thus ending with a lower pressure group. Then I would have planned the 18m dive for air (or EAN36). Since this is the last dive, the ending pressure group is only important if you are traveling to altitude (driving OR flying).

Another consideration: a dive to 100' should be done with a tank containing 100 cf. I would have wanted to use the largest tank for the deepest dive. Next time, fill the 12L with EAN32 and the 10L with EAN36. Use the 12L for the deep dive and the 10L for the shallow dive.

Richard

Cyber_Ghost · Feb 7, 2009

rstofer:
You need to go back to the book... The primary reason for using Nitrox is to reduce your exposure to nitrogen. So, how do you know when you are making a choice that will result in the least exposure to nitrogen? By the ending pressure group. The lower the group letter, the less nitrogen you have absorbed.

The next consideration is MOD - how deep can you dive? By my tables, 30m on EAN36 is past the limit (it's close but beyond the MOD). It doesn't leave any room for error assuming you believe in the 1.4 ATA limit. And you should...

For your dives, with your tanks, I would have used air for the 30m dive (because EAN36 is inappropriate) and come up with a higher pressure group. I would have done the SIT until I was within range of the 18m dive with EAN36.

OTOH, if I had EAN32, I would have made the first dive with Nitrox to take on as little nitrogen as possible thus ending with a lower pressure group. Then I would have planned the 18m dive for air (or EAN36). Since this is the last dive, the ending pressure group is only important if you are traveling to altitude (driving OR flying).

Another consideration: a dive to 100' should be done with a tank containing 100 cf. I would have wanted to use the largest tank for the deepest dive. Next time, fill the 12L with EAN32 and the 10L with EAN36. Use the 12L for the deep dive and the 10L for the shallow dive.

Richard

I see the point. Unfortunately the 10L tank wasn't certified for nitrox. My original request was 32 for the first dive, 36 for the second. Length of the dive wasn't an issue here because the 30m one was limited by air supply (12L would give me a run time of about 30min for the dive), and the second dive wasn't deep enough to go into any kind of CNS problem with a 10L tank.
The more I think of it, the more it seems I was right in my original request.

Cyber_Ghost · Feb 7, 2009

Ok, went back to the book (Not my course materials, but BSAC's Nitrox Workshop Student Workbook):

Each mix has a defined “Maximum Operating Depth” (MOD), which exceeds the Ocean Diver depth limit of 20 metres:
The following statements are only applicable to no-stop decompression diving: planning decompression dives on nitrox is part of the Sports Diver syllabus.
○ Nitrox 32 - 35 metres for NO-STOP dives
○ Nitrox 36 - 30 metres for NO-STOP dives

So MOD was OK form me (tough not sure about ANDI standards).

Good practice to increase oxygen % for the second dive
It is good practice to increase the oxygen (or keep the same) in the mix for subsequent dives as this will help to minimise the risk of DCI.

OK, so I see that the argument is still in my favor. However that's what I hate about these workbooks, where's the math/research data to prove it?
Any pointers for resources that deal with this?

Rhone Man · Feb 7, 2009

Leaving aside issues of depth, if there is a choice, I always feel you want to get a richer mix as you go, as it offsets those pesky RNT minutes.

Web Monkey · Feb 7, 2009

Cyber_Ghost:
Here's the thing, I've done BSAC nitrox certification, and was taught to plan the dives and mixes so that the mix will get richer with each repetitive dive, with depth becoming shallower (as taught in most cases).

I use the right mix for each dive. The right mix minimizes nitrogen absorption, while not violating my maximum PO2 (1.35).

Terry

simcoediver · Feb 7, 2009

Terry is right, its irrelevant whether its the first dive or the second or for that matter the third dive, use the right mix for the the depth and time. However ,follow the rule of thumb ,deeper dives first followed by shallower.

John_B · Feb 8, 2009

Web Monkey:
I use the right mix for each dive. The right mix minimizes nitrogen absorption, while not violating my maximum PO2 (1.35).

I also use the right mix for each dive, which is always 32% at 111' or above.

Put another way, there basically isn't a recreational dive I can't do with EAN 32. Makes the logistics, math, etc. ridiculously easy. :cool2:

Nitrox and repetitive dives, what mix first?

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