Need Help with Nitrox

[Narced Out]

So i did my nitrox class a few weeks ago...but need a little clarifying. I want to do my first nitrox dive within the next 2 weeks. I want to dive between 60 and 90 feet. So if im reading the charts right, it looks like i can spend 50 minutes at 80 feet with a 32% mix. I want to have some sort of margin in case i go deeper on accident than planned. If im wrong can someone please help me? Thanks,

Jacob

 

[TSandM]

I don't know which tables you are using, but 50 minutes at 80 sounds about right. What do you mean by having "margin in case I go deeper"?

 

[Tool Belt]

The way I read mine 50 mins at 80 feet exceeds the NDL.

 

[Narced Out]

Im not the best at reading charts, Sandman my buoyancy is still a work in progress i tend to sink a little deeper than planned on some dives... but for the most part i can hold it pretty stead...and for the chart i was looking at i just googled nitrox 32% table.

Nitrox, Wreck, Trimix, Technical Diver

scroll to the bottom it tells you the times. So if i use a 36% mix i can last 60 mins..?

 

[nshuff]

I've got a ndl time of 39 mins at 80 ft with my computer

 

[rongoodman]

The chart you're quoting is more generous with its NDLs than some others, but as far as leaving a margin for going deeper, the MOD for 32% is 110'. For recreational diving, that's the only mix I use.

 

[TSandM]

Jacob, why are you using a printed table (the origin of which is unclear to me) instead of the tables you should have gotten in your Nitrox class?

The author of that page says those are the NOAA times, and I went to the NOAA site and found their table, and they do indeed list 50 minutes as the NDL for 32% at 80 feet. Different models WILL give different no-decompression times, which is why other folks are giving you different numbers.

 

[Tortuga68]

So if i use a 36% mix i can last 60 mins..?

You're probably going to run low on gas before you clock up 60 minutes at the depths you mentioned

Those tables you linked are based on a max PP02 of 1.6, which is generally only used for decompression dives... most agencies recommend 1.4 or less for non-deco dives

I'd suggest you re-read your nitrox course materials

Did you do it online?

 

[pjohnson]

"Equivalent Air Depth" EAD= (Depth + 33fsw) × Fraction of N2 / 0.79 − 33fsw

P x FO2 = pO2
P = Pressure absolute(ATA) FO2=Fraction of Oxygen(mix) pO2=Partial Pressure Oxygen

Understanding these two formulas is essential when contemplating any nitrox mix for use at a specific depth. It is unfortunate that most Nitrox courses sometimes miss the boat in making sure students get the basis for depth limits of a particular mix.

We need to remember that basic courses are just that "basic". Expanding our knowledge on our own and practicing with the calculations will help to build an understanding of the information dumped on us in these "sometimes too condensed" classroom sessions. Don't guess, rely on dive computers or take someone elses advise. Know and understand the material, you will be a more a more confident and safer diver.

 

[DaleC]

Jacob, the others are right when they tell you to stick to one set of tables and to understand the parameters they are based on. When I use EAN I consider the following variables:

What is my working depth
What is my contingency depth
How long do I want to be down for
Which mix has the appropriate MOD
What are the other members of my dive party using

The accepted PO2 (by most but not all agencies) for the working portion of the dive is 1.4, with some diving conservatively at 1.2 and some aggressively at 1.6. I choose 1.4 myself.

If you are diving to 80' for example, the mix with a 1.4 equivalent MOD is 40%EAN but if you think you might slip down to 90-100' then a mix of 36% or 32% is a better choice.

I would caution you not to immediately try to extend your bottom time too much with EAN as that puts you at the same risk as pushing the NDL's on air. Before pushing the limits that way you should have your ascent rate/bouyancy skills dialed in and a good understanding of all the ways you can create/mitigate DCS risk.

Lastly, think about what your teammates are going to do. You should all be on the same page or at least willing to dive to limits of the member with the least capacity. For example: If you have EAN 32% and your buddy has air, you will be limited by his air NDL's. Conversely, he will be limited by your MOD.