H2Andy's (Excellent) Underwater Journey Through Reef and Cave v1.2.06

6/1/2006 - Nitrox: Lean or Rich Mix First?

i remember from my Nitrox class asking the instructor, if i have

two different mixes, which one should i use for the first dive?

i don't recall what the answer was. recently, this topic came up

again, and i decided to run the numbers for myself.

if you are doing two dives to the same depth, and have two

different Nitrox mixes, should you use the lean mix first or

the rich mix first?

let's assume two dives to 70 feet. i have 36% nitrox and 26% nitrox.

which one should i use first? think about it before you look below for

the answer.

ok, well... here's the math:

Using 36% first:

1st dive to 70 on 36% for 40 minutes: come out at group O
after one hour surface interval, you are group E
at 26% you have a maximum bottom time at 70 feet of 25 minutes
(EAD of 63, so used 70 feet)

Total dive time using richer mix first: 65 minutes

Using 26% first:

1st dive to 70 feet on 26% for 40 minutes: come out at group T
after one hour surface interval, you are gruop G
with 36% you have a maximum bottom time at 70 feet of 37 minutes

Total dive time using leaner mix first: 77 minutes

so, it looks like the leaner mix first is the way to go.

is that what you would have guessed?

6/1/2006 - <i>Untitled Comment</i>

Posted by

Well Andy I would dive the leanest first anyway.
The way I work it out is that if diving a 26% mix, I would be diving to around the 40-45msw mark.
Using the 36% mix I would be diving to around the 30msw mark.

I generally do the deeper dive first.

If I am using various different mixes then I am generally using a "best mix".

Other wise I simply dive 32% or 21%

Edited by Azza on 6/1/2006 at 10:45 PM

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6/2/2006 - Untitled Comment

Posted by limeyx

Interesting.

I would have chosen 32% for both dives (although I read the post on scubaboard and see that the 26% came from a top-off -- I'd dive that like air most likely and I'd dive the 36% like 32%)

I'd say EAD = 70 - 20% = 54 feet. at 50 feet, I have 60 mins, at 60 feet 50 mins, so I'd say I have 55 mins BT.

I would leave one hour Surface interval and probably do two dives with an average depth of 70 feet both of 45 to 50 mins BT.

I would do a slower ascent on dive 2 (2 min stops at 30 20 and 10 feet instead of 1, followed by a 2-3 min ascent from 10 feet) to try to minimize post-dive stress. But since both dives are NDL, the stops are not mandatory (and if i am in current or lost the line, I might cut them short)

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