Question about nitrox/air on two-tank dives

[Jake]

Hi all-

Not sure if this is the right place, so mods: feel free to move this somewhere else if appropriate.

Or, if this is discussed elsewhere please point me in the right direction.

Suppose a diver will be doing several days of two-tank boat dives where only one of the tanks each day will be nitrox. I'm wondering what the better order is for using the tanks, assuming it matters at all.

Let's assume two scenarios:

1. Dive 1 is deeper than dive 2, in the ranges of say 50-70fsw and 40-60 fsw respectively.
2. Both dives are approximately the same depth, around the 40-60fsw range.

I'm going back and forth on why one configuration may be better in each scenario, but don't know enough about specific tissue loading and depth to really come to a conclusion.

Thanks in advance.

 

[claymore]

Generally 1st dive Nitrox, within MOD, second dive air. Deeper dives load more Nitrogen so Nitrox will lower your Nitrogen load and shorten you SI time.

 

[tbone1004]

historically you were supposed to do the deeper dive first *whether actually deeper or equivalent air depth deeper*, this was to accommodate the USN dive tables that everyone was using and NAUI and others still do which are optimized for one dive per day. Long SIT's, long bottom time on dive 1, short bottom times on dive 2. PADI came around and spent lord knows how much money on the RDP which allowed for better repetitive diving and is pretty depth independent so long as your SIT is reasonably long.
Since everyone is diving computers these days, the answer is going to come down to which computer you have and what it says.

45min is a respectable bottom time for a 50-70ft dive on an AL80 and 60 mins for a 40-60ft dive so we'll assume those with actual depths used as 60ft and 40ft for average depths. We'll also assume nitrox is EAN32 and a 1 hour SIT
If I use GUE's deco planner which is what I use for deco, and use gradient factors of 50/80 which I deem reasonable for me when diving in Caribbean type water I get the following information. Deco Planner is a deco planner, so to get NDL's you have to manually mess with the times to come out with the data so this will be plus or minus a few minutes depending on your planner.

If I use EAN32 on dive 1 for 45 minutes to 60ft, take a 1 hour SIT and use air on dive 2 to 50ft for 60 minutes I have 3 mins of deco at 10ft on dive 2. If the SIT is 90 minutes, it goes away. If I swap gas mixes, I have a 2 minute deco stop at 10ft on dive 1 and no deco on dive 2. I'll use EAN32 on dive #1, and take a 90 minute surface interval.

If i do a pair of 60 minute dives to 50ft, on air, with a 90 minute SIT I'll have a 2 minute deco stop at 10ft which goes away if my SIT is 95 minutes. There was a huge symposium a while ago on reverse profiles *shallower dive first* and the conclusions were there is no measurable difference in risk of DCS for reverse profile dives so long as that you are in recreational limits *less than 130ft*, and there is less than 40ft differential between the two dives. I.e. don't go to NDL at 40ft, then go to 130ft for the same. Unsure why that matters but it was a finding. There has also never been any evidence of any studies on reverse-profile diving that came to the conclusions that you had to go deeper first, it was just a hindrance when using the USN dive tables.

TLDR, it doesn't matter from a DCS perspective assuming you are diving up to your NDL's on both dives. You need to run your expected dive times though to see if you are going to be limited by gas or by NDL. If you are limited by gas, I wouldn't bother diving nitrox

 

[tomfcrist]

I’m curious as to why someone would bother with nitrox on one dive and air on the other? Why not both on air, or both on nitrox? What is the perceived need to only have one tank filled with nitrox?

 

[Jake]

TLDR, it doesn't matter from a DCS perspective assuming you are diving up to your NDL's on both dives. You need to run your expected dive times though to see if you are going to be limited by gas or by NDL. If you are limited by gas, I wouldn't bother diving nitrox

Thanks for the great info. This is more of a hypothetical, as my actual plan for the trip that spawned this question is 32/36 in that order, and since I don't have proper deco training it's all NDL diving.

 

[Jake]

I’m curious as to why someone would bother with nitrox on one dive and air on the other? Why not both on air, or both on nitrox? What is the perceived need to only have one tank filled with nitrox?

I don't know why anyone would aside from budget. I happened to receive an estimate that was all air, mixed air/nitrox, and all nitrox, with the mixed and full nitrox progressively more expensive than air.

The mixed air/nitrox setup got me thinking about this just from a hypothetical perspective, as I was scratching my head thinking that if I were in that situation, which order would I dive it in.

 

[Superlyte27]

Buy a rebreather and then do both dives at 1.2

 

[shurite7]

I’m curious as to why someone would bother with nitrox on one dive and air on the other? Why not both on air, or both on nitrox? What is the perceived need to only have one tank filled with nitrox?

From what I have observed, especially while working at a resort, cost quite often is the deciding factor. Others have commented they felt nitrox was fine for the deeper dives, but not as beneficial, financially or physiologically, for the shallower repetitive (<50') dives.

 

[tomfcrist]

Buy a rebreather and then do both dives at 1.2

Yup, just tell your wife it’s more cost effective...then lie to her about the price.

 

[Jeremy Williams]

NOAA Manual 4th Ed. allows either a Nitrox dive after an Air dive or vise verse. I’m voting for it doesn’t matter, but I would generally reach for the Nitrox on the first (typically deeper) dive.