A little help with Nitrox, please...

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DandyDon

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I thought I knew all the basics of recreational Nitrox diving - in spite of my instructor. Pretty sure I knew more than him by the time I got my card.

I know how to plan a dive using Padi 32% or 36% charts, how to calculate Soft Bottom & Hard Bottom depths for any recreational O2 percentage. When I can get a custom blend, I prefer 30% EANx for most of the North Carolina & Florida Keys wrecks I've done, 28% for the Bibb and a few of the NC's.

But let's say I'm doing 2 morning dives, to about 60-80 feet? My initial analysis is based on a 70 foot average:
  • a: 2 Air dives: First to 35 min = Q, 81 min SI = C, only 28 min left for second = T, after 63 min at 70 feet;
  • b: 2 Nitrox 32% dives: First to 36 min = M, 86 min SI = B, 35 min = Q, after 71 min at 70 feet - however - as much as they charge for Nitrox, do I really want to do both?
  • c: Or one of each - and which one should be the Nitrox...?

On a recent Cozumel trip, some divers said if doing 1 of 2 similar dives on Nitrox, do it on the second...?
  1. 1: First on Nitrox to 36 min = M, 86 min SI = B, then Air on the second to 31 min = T, after 67 min at 70 feet; or
  2. 2: First on Air to 35 min = Q, 81 min SI = C, then 32% Nitrox on the second to 37 min = S, after 72 min at 70 feet

Looks like they were right, in that I'll have 2 more minutes in plan #2 in this example, but finish in 1 dive group lower.

Did I approach this correctly, do it correctly, and end up with correct analysis...?

thanks, don
 
I didn't run through the tables but...

We know that with a Haldanian model (our tables) you get a penalty (less bottom time on the second dive) for doing the shallower dive first (reverse profile).

If you use EAD and plan the dives with an air table and the two dives are the same actual depth then it essentially becomes a reverse profile and you loose bottom time or end up in a higher rep group. So...if I understood the question I'd say that they were right.
 
Thanks, Mike - that's another way to look to look at it, that I hadn't thought of!
 
You're welcome.
 
DandyDon, interesting post. Looks like I learned something new today after all. :D
 
You gotta play with the EAD tables to figure it out ... but generally...

Doing two different dives:
Use NO2 on the second for (slightly) more bottom time. except you'll usually do better if you use Nitrox on the deeper one (assuming O2 limits allow).
 
DementialFaith:
Wait........ I'm confused........ I dont remember seeing any penalties related to reverse profile on my EAN chart..............

If you plan 2 dives doing the shallow one first, you'll have a shorter adjusted NDL and/or higher rep group on the second dive than you would if you had done the deep dive first.
 
DandyDon:
I thought I knew all the basics of recreational Nitrox diving - in spite of my instructor. Pretty sure I knew more than him by the time I got my card.

I know how to plan a dive using Padi 32% or 36% charts, how to calculate Soft Bottom & Hard Bottom depths for any recreational O2 percentage. When I can get a custom blend, I prefer 30% EANx for most of the North Carolina & Florida Keys wrecks I've done, 28% for the Bibb and a few of the NC's.

But let's say I'm doing 2 morning dives, to about 60-80 feet? My initial analysis is based on a 70 foot average:
  • a: 2 Air dives: First to 35 min = Q, 81 min SI = C, only 28 min left for second = T, after 63 min at 70 feet;
  • b: 2 Nitrox 32% dives: First to 36 min = M, 86 min SI = B, 35 min = Q, after 71 min at 70 feet - however - as much as they charge for Nitrox, do I really want to do both?
  • c: Or one of each - and which one should be the Nitrox...?

On a recent Cozumel trip, some divers said if doing 1 of 2 similar dives on Nitrox, do it on the second...?
  1. 1: First on Nitrox to 36 min = M, 86 min SI = B, then Air on the second to 31 min = T, after 67 min at 70 feet; or
  2. 2: First on Air to 35 min = Q, 81 min SI = C, then 32% Nitrox on the second to 37 min = S, after 72 min at 70 feet

Looks like they were right, in that I'll have 2 more minutes in plan #2 in this example, but finish in 1 dive group lower.

Did I approach this correctly, do it correctly, and end up with correct analysis...?

thanks, don


Okay, if I end up at S after 2 dives, then take a 1 hr 50 min SI for lunch, I'll be down to a B, with 31 min possible on another 70 foot dive. 2 hr 39 min SI is more likely, bringing me back to A, with 35 min left to 70 feet.

In actualality, if we start at 80 feet and drift up to safety stop level as often done in Roatan, the one tank of Nitrox a day sounds like it'll help, but be plenty...
 
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