2 tank dive air or nitrox first

[rustybatey]

I just got my Nitrox cert and I only have 1 new steel 120 for Nitrox so far. My other tanks are aluminum and I have not had them 02 cleaned yet so I will be doing air and nitrox dives on the same day for a couple of weeks. Does it really matter which tank I use first and has anyone had a better experience using one or the other first?

I will be spearfishing at about 65ft

 

[Teamcasa]

I just got my Nitrox cert and I only have 1 new steel 120 for Nitrox so far. My other tanks are aluminum and I have not had them 02 cleaned yet so I will be doing air and nitrox dives on the same day for a couple of weeks. Does it really matter which tank I use first and has anyone had a better experience using one or the other first?

I will be spearfishing at about 65ft

OK, it really does not matter very much.

To address the tank issue. (Making the assumption that your instructor did not clearly explain the different type of filling procedures for Nitrox and stopped after the O2 clean requirement, many sadly do.)

Pre-blended 32 or 36 Nitrox (typically stored in banks) does not need O2 cleaning before use. PPB or Partial Pressure Bending (where pure O2 is added and air is added later) requires O2 clean tanks.

Don't worry, you are not going to die either way.

 

[Teamcasa]

BTW, Welcome to ScubaBoard!

 

[LeadTurn_SD]

rustybatey: Welcome to scubaboard!

You may want to reword your question.

Maybe something like:

1.) I plan on diving to 65' maximum depth. I'll be making two dives.
2.) I would like to use a steel 120 with __% Nitrox, and a AL80 with air.
3.) Should I plan both dives using air tables only for safety (as long as I stay within the MOD for the nitrox), or can the dive be safely executed using Nitrox tables for the 120 and switching to air tables for the 80?
4.) Does it make any diffence which (air or Nitrox) I dive first in this case?

That is what you were trying to find out, right?

I don't dive with that "voodoo gas" stuff, so I'll let someone who does provide the best answer (though I know what I'd do in your case It rhymes with air tables....).

Best wishes.

 

[Reg Braithwaite]

I just got my Nitrox cert and I only have 1 new steel 120 for Nitrox so far. My other tanks are aluminum and I have not had them 02 cleaned yet so I will be doing air and nitrox dives on the same day for a couple of weeks. Does it really matter which tank I use first and has anyone had a better experience using one or the other first?

I will be spearfishing at about 65ft

Here's how a n00b would answer your question (I ought to know!)

Diving Nitrox to 65' gives you an equivalent air depth of less than 65'. This is mix-dependent, but for the sake of argument let's say your EAD on Nitrox is going to be 50'. Now we can re-phrase the question: I will be doing 65' and 50' spearfishing dives on the same day for a couple of weeks. Does it really matter which dive I do first and has anyone had a better experience doing one or the other first?

Many agencies teach that you should do the deeper of two dives first. Do you believe this? If so, do you agree with the n00b's conjecture that you should dive air first and Nitrox second?

 

[The Chairman]

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[BuoyantC]

For me the quick answer without doing a lot of analysis paralysis, would be as the above poster alluded to. Do the deeper EAD dive first. Assuming both dives to be at the same actual depth, that would be the air dive first.

 

[knotical]

From a risk / safety perspective I don't think it matters, as long as you follow the rules of whatever computer or table model you are using.

One practical advantage of diving air first can be seen from the following example:

Assume PADI tables for both air and EANx32.
Assume two dives to 65 feet for 35 minutes.

If you do the air dive first, you exit the water as a Q-diver.
To do the nitrox dive second, you must wait until you are an H-diver, or 43 minutes.

If you do the nitrox dive first you exit the water as an M-diver.
BUT to do the air dive second, you must wait until you are an A-diver, or 135 minutes – more than three times as long !

 

[psychocabbage]

OMG.. Leave it to the others to overcomplicate things here.

If I were you, I would dive nitrox first. Then do the air. I would set my computer to nitrox and the % I am running. Do my dive. Afterwards I would change my computer to air and do my 2nd dive.

Typically, your first dive will be the longest, so, better to have less nitrogen than more. Right?

You are diving with a nitrox capable computer right?!??!

 

[Reg Braithwaite]

Assume PADI tables for both air and EANx32...

This question has come up on Scubaboard before. I'm thinking it ought to be part of the training curriculum.