Nitrox Tank used first or last?

[MRTdiver]

Say you have a 2 tank dive. One filled with EAN32 the other is air. Is it safer to dive the Nitrox first or last? Dive profile 100/25 with 1 hour SI. I'll be swimming at the bottom, +/- 5ft on both dives.
I'm hoping for some technical physiology mumbo-jumbo. Maybe a reference to an article/book that specifically addresses this.

 

[Red_5]

I'd dive it on the first dive myself as this should be your deepest dive and it would pose the greater risk.


ETA:

looking at the PADI RDP's-

100' at 25:00 exceeds NDL for air where you could do it safely, with a safety stop, on EAN32.

you don't state the 2nd dives depth, but the first dive's given data makes it a moot point as previously mentioned.

 

[Tortuga68]

He said that the two dives would have the same profile, so is that still your answer?

 

[Otter]

As Red 5 pointed out, the first dive can only safely be done with in recreational dive limits on EAN, so its a moot point. He can't dive AIR either dive with that profile.

 

[Red_5]

As Red 5 pointed out, the first dive can only safely be done with in recreational dive limits on EAN, so its a moot point. He can't dive AIR either dive with that profile.

thanks Otter,

I get max bottom time of 9 min on the 2nd dive with air at 100'. if you were to use EAN32 on the 2nd dive, that would only get you about 13 minutes.


ETA:

I think what the OP is looking for, is which to use first, Air or EAN32 and the example profile given just didn't work for what he was asking. A better question (in hindsite) is to ask, which one to use first, all things being the same. is there a benifit of using one or the other first?

 

[aquaregia]

If you subscribe to "deepest dive first", you should dive the air tank first. Both dives have the same actual depth, but the air dive has a deeper equivalent air depth.

US Navy lets you do 25min @ 100', but only once. It reckons you'll owe 25min of deco on the second dive.

 

[deepstops]

V-planner plan for dive #1 on air:
DIVE PLAN

Surface interval = 2 day 0 hr 0 min.
Altitude = 0ft
Conservatism = Normal, Critical Volume = ON

Dec to 100ft (2) on Air, 50ft/min decent.
Level 100ft 23:00 (25) on Air, 0.85 ppO2, 100ft END
Asc to 40ft (27) on Air, -30ft/min ascent.
Stop at 40ft 1:00 (28) on Air, 0.46 ppO2, 40ft END
Stop at 30ft 2:00 (30) on Air, 0.40 ppO2, 30ft END
Stop at 20ft 4:00 (34) on Air, 0.34 ppO2, 20ft END
Stop at 10ft 8:00 (42) on Air, 0.27 ppO2, 10ft END
Asc to sfc. (42) on Air, -30ft/min ascent.

Off gassing starts at 71.3 ft
OTU's this dive: 19
CNS Total: 6.2%

61.6 cu ft Air
61.6 cu ft TOTAL

 

[Karl_H]

The key factors are time and depth, if you are planning deco diving you should allow adequate stops to offgas - using EAN allows accelerated offgasing for decompression but if the accelerated times are used it is no safer than air.

Recreational Nitrox is intended to reduce the partial pressure of Nitrogen during the dive - taking EAN down to the MOD doesn't really do much for your bottom time as the partial pressure of Nitrogen is still relatively high, although less than it would be for air.... Remember that using an EAN mix up to the NDL is no safer than doing the same dive up to the NDL on air, using EAN within the MOD on an air profile is usually safer than using air but this doesn't give you any additional bottom time.

Form a practical point of view I'd say use the EAN32 first as this will give you less gas loading at the start of your SI hence the gas loading for the start of the second dive would be lower. The SI after the second dive should be much longer (?) so you'll have more time to off gas before a 3rd dive or a lot more time to offgas if a third dive isn't planed.

If you dive the same profile for both dives you will have decompression on at least one of the dives, although at the depths mentioned the deco should be pretty simple - just make sure have enough gas and know how to follow the deco profile correctly.

 

[knotical]

aquaregia is correct.

Either order is OK, as long as you follow an accepted model.
However, time limits are different.

An example, using PADI RDPs (other models yield similar results).
Two square profile dives, 90 feet for 20 minutes with a one hour SI.
Dive 1 Air, Dive 2 EANx32 – you end as a Q diver
Dive 1 EANx32, Dive 2 Air – you cannot do dive 2 unless you have a longer SI.

You encounter less time penalty if you do the dive with more nitrogen-loading first.
So, in general, air before nitrox, deeper before shallower (see other threads discussing "reverse profiles").

 

[iztok]

It is "simple" if you subscribe to first dive deeper than second philosophy. First dive is to 100 ft while the second dive is less (around 90 ft from top of my head but I did not do the calculation here).

You basically use EAD (Equivalent Air Depth). So when you figure out which dive is first you need to calculate EAD and do the deepest first. If dives are to the same depth then air first, EAN second. Only time where you would do the other way around is if second dive is shallower than EAD of EAN.

So let's say that first dive is to 100 ft (EAD is 90 - from memory) and second dive is to 70ft then you would do EAN first. (Just my opinion.)