2 tanks: Nitrox and Air - Which one do you use first?

[paolov]

if you are going to do just that two dives at that shallow depths.

you may use nitrox first so you would be less tired (if it applies to you)

 

[Diver0001]

...snip....

The dives in question are shallow (~25 fsw), with long (90-120 minute) bottom times.

....snip....

You know.... At those depths I don't think it matters one little bit. The chances of getting bent on those profiles are so astronomically slim that I would be more worried about getting abducted by horny aliens.

As a general rule I would suggest using the Nitrox on the dive that puts you closer to your NDL.

R..

 

[Curt Bowen]

Simple,

first dive is 200 feet, I would use the air and then use the nitrox as a decompression gas. Skip the 2nd dive

 

[Scubaguy62]

I really haven't researched it as I'd like (mainly because I breathe nitrox 100% of the time), but from practical experience, my breathing rate tends to slow down when breathing nitrox...go figure... On those two dives I was breathing 36% on the first dive, then went to 21% on the second, still breathing at the same rate. It wasn't a question of breathing shallow, it was a question of breathing so slow on 21% that too much CO2 built in my system.

I discussed the situation with a tech instructor friend of mine and right away he mentioned that it could have been due to my breathing rate for 21% being slower because I had been breathing 36% the previous dive.

But as I said before, I haven't researched it as I'd like. Grier, you being a PharmD, maybe you can shed more light into this...

 

[AADiveRex]

In terms of tissue loading, always using progressively lower PN2s during decompression eliminates the possibility of causing a reverse gradient, which could result in ongassing when you would expect to be offgassing. Extending this concept to a no deco model over multiple dives, it would seem to make sense to use the lower PN2 gas for the second dive, as it induces a more favorable gradient towards offgassing of residual nitrogen.

Dive Safe
Adam

 

[glbirch]

I really haven't researched it as I'd like (mainly because I breathe nitrox 100% of the time), but from practical experience, my breathing rate tends to slow down when breathing nitrox...go figure... On those two dives I was breathing 36% on the first dive, then went to 21% on the second, still breathing at the same rate. It wasn't a question of breathing shallow, it was a question of breathing so slow on 21% that too much CO2 built in my system.

I discussed the situation with a tech instructor friend of mine and right away he mentioned that it could have been due to my breathing rate for 21% being slower because I had been breathing 36% the previous dive.

But as I said before, I haven't researched it as I'd like. Grier, you being a PharmD, maybe you can shed more light into this...

You might find this some interesting reading then: http://scuba-doc.com/CO2acclim.html

 

[Scubaguy62]

Thanks for the link glbirch. You're right, very interesting reading!!

 

[Bigcape]

The general rule of diving is to do your deepest dive first and use the mix with the lowest O2 first.

200fsw dive first, 80fsw dive 2nd, 60fsw etc.......

21% O2 first, 32% mix 2nd, 36% etc.......

 

[in_cavediver]

The general rule of diving is to do your deepest dive first and use the mix with the lowest O2 first.

200fsw dive first, 80fsw dive 2nd, 60fsw etc.......

21% O2 first, 32% mix 2nd, 36% etc.......

You realize this thread is 2 years old and the OP's tanks have surely been used by now?

 

[ZzzKing]

You realize this thread is 2 years old and the OP's tanks have surely been used by now?

Maybe he got a reeeeeeaaaaaaaalllllllllyyyyy long bottom time out of that second tank.