Repetitive dives, 2 nitrox tanks, 2 different 02% - which first?

[RJP]

Possibly a dumb question...

Doing two dives on Saturday, my two 119cf tanks are filled with 36% and 26% EANX. Dives look like this:

Two dives on same wreck
Max depth = 70' to sand
Water temp = 45*-50* f

Does it matter practically or theoretically which tank I dive first?

I'm a pretty efficient breather, and tend to run out of NDL time before "air" time. RDP gives me 55min at 80' on 36% EANX

I'm figuring that the 36% for the first dive makes the most sense, for the following reasons:

- second dive likely to be shorter due to cold/fatigue, so max bottom time on first dive

- if equipment, weather, etc craps out and second dive is called, better to have maxed first dive

Mathematically, the total nitrogen load and 02 exposure of the series of dives is the same but what are the practical implications?

Thanks - Ray

-

 

[H2Andy]

well, you should technically do your deep dive first, so shouldn't you use your
lean mixture first (your EAD will be deeper)

then on the second one, you will have a shallower dive and due to higher mix
your EAD will be shallower anway...

or you can do the reverse and balance out the deep dive with the richer mix
(assuming MOD is not exceeded) and then have a shallower dive with a leaner
mix...

man ... i don't know!

 

[mike_s]

Possibly a dumb question...

-

Actually, this is one of the better "real situation" dive questions I've seen on here in a while that I haven't seen asked before.

 

[spectrum]

I'm not passing this off as a conclusion but:

The 36 on dive two would encourage more nitrogen off gassing from the total dive day.

Pete

 

[Don Wray]

We were taught to work it out both ways and see which would be better, then go with that. Personally, your rationale looks logical to me.

 

[Kim]

If I was fairly sure (as you seem to be) that the second dive will be shorter then I'd use the 36% for the longer first dive.

 

[Cacia]

yea, I would do the .36 first, assuming a longer BT. ( Why do you have two different mixes BTW?)

**don't forget to switch your computer! thats my reoccurring mistake. I will do Nitrox first dive, set my Suunto and then do a tank of air and forget to switch my computer to .21 and take a larger slice of bottom time than I deserve. :shakehead
but then...most people probably don't do that.

what is the usefulness of purpose of .26? deeper MOD..?

 

[joe rock]

yea, I would do the .36 first. Why do you have two different mixes BTW?

**don't forget to switch your computer! thats my reoccurring mistake. I will do Nitrox first dive, set my Suunto and then do a tank of air and forget to switch my computer to .21 and take a larger slice of bottom time than I deserve. :shakehead

what is the usefulness of .26, just curious? hope it isn't expensive....

Catherine, doesn't your suunto default back to 21%? Even though in Nitrox mode it would be 21% or am I mistaken?

JR

 

[RJP]

Why do you have two different mixes BTW?


what is the usefulness/purpose of .26? deeper MOD..?

My last dive I only used about a half tank of 32% - my friendly neighborhood LDS will top-off half-full or better tanks with clean air from the nitrox side of the compressor for free.

Basically, I got one "36% fill" and one "26% fill" for $12.

 

[wedivebc]

Some people believe deeper dive first is beneficial, others say it doesn't matter. This decision seems to fall under the same rules.