Which Tank First?

[stever2002]

We all know that you dive to the deeper depth first, and then repetitive dives go more shallow. My question revolves around diving and the various PO2 rates in your nitrox tanks: Are there any guideline/safeguard for which tank you should dive with first?

Maybe an example is best: I was recently in Florida where my two tanks of nitrox were verified to have PO2 mixes of 35.2% and 34.5%. With the first dive planned to 60' and the second dive planned to 50', Does it matter which take I dive first?

Obviously, the richer the blend (higher PO2 %), the higher the partial pressure (for a given depth). Further, I realize that the more rich (higher PO2) the more shallow my max depth can be. So, should I conclude that I should use the less rich tank for the deeper dive?

I know some dive computers can reset to a default PO2 rate between dives, or keep the same PO2 (unless changed). If I were to forget to set my PO2 rate, and my computer was set to keep the same rate, I guess it would be best to use richer tank first - as some sort of margin of error.

Before I get the obligatory "It depends on what kind of diving you are doing" - Lets assume that I'm a sightseer - no tech or wreck - Just cruising the reef looking at the creatures. :scubadive

...steve...

 

[jeckyll]

I would dive the 'appropriate' mix first. Your tanks were within 1% so it doesn't really matter, but if you had one 32% and one 40% and your first dive was going to be to 105' then you probably don't have that many options. In that case you dive the higher % first.

I had one situation where we were doing 3 boat dives, everyone brought 2 tanks (32 & 36) and the boat only had air.

So I dove 32% to 80', 36% to 60' and something like 25% to 50'. The circumstances dictated what we could do.

HTH

Bjorn

 

[CompuDude]

Figure best mix and go from there, but I don't know that it matters that much unless you're really riding those NDLs.

To me it kinda makes sense to use the richest mix on the last dive, where it will help with the offgassing the most, plus the second dive is the one where you are more likely to be limited by NDLs than the first dive.

 

[Thalassamania]

Within the limits of the media's MOD of course, and in terms of EAD I'd try and make the deepest dive first and then make each dive successively shallower. But I can see where it might make more sense to dive a richer mix on an earlier dive in order to get closer to best mix at greater depth.

 

[in_cavediver]

We all know that you dive to the deeper depth first, and then repetitive dives go more shallow. My question revolves around diving and the various PO2 rates in your nitrox tanks: Are there any guideline/safeguard for which tank you should dive with first?

Maybe an example is best: I was recently in Florida where my two tanks of nitrox were verified to have PO2 mixes of 35.2% and 34.5%. With the first dive planned to 60' and the second dive planned to 50', Does it matter which take I dive first?

Obviously, the richer the blend (higher PO2 %), the higher the partial pressure (for a given depth). Further, I realize that the more rich (higher PO2) the more shallow my max depth can be. So, should I conclude that I should use the less rich tank for the deeper dive?

I know some dive computers can reset to a default PO2 rate between dives, or keep the same PO2 (unless changed). If I were to forget to set my PO2 rate, and my computer was set to keep the same rate, I guess it would be best to use richer tank first - as some sort of margin of error.

Before I get the obligatory "It depends on what kind of diving you are doing" - Lets assume that I'm a sightseer - no tech or wreck - Just cruising the reef looking at the creatures. :scubadive

...steve...

Assuming both mixes are good for the dives you are going to do, you have two options to maximize (you may be able to do both).

1) Dive the deeper EAD first (forward profile)

2) Dive the richer mix last (richer deco mix)

Personally, I'd go for the deeper EAD for the first dive following the idea of forward profiles. Fortuneately, it the recreational context, I doubt it really matters.

 

[dsteding]

I would dive the 'appropriate' mix first. Your tanks were within 1% so it doesn't really matter, but if you had one 32% and one 40% and your first dive was going to be to 105' then you probably don't have that many options. In that case you dive the higher % first.

Maybe this is me not getting this, but when you say "dive the higher % first" (40% in your hypothetical) on the first dive (105') you'd be ending up with a PPO2 of ~1.8 . . . :11:

Of the top of my head, the MOD for 40% is, what, 3.5 ATA, or 75 feet?

 

[cummings66]

Brain misfire I think.