Sequencing Nitrox 'v' Air

[finbob]

I was diving last month doing 4-dives/day. The problem occurred when I had inconsistent Nitrox fills. In fact, one Nitrox tank was so low in psi I had to switch to normal air.

#1 - 30% Nitrox
#2 - 36% Nitrox
#3 -Air
#4 - 32% Nitrox

What sequence should I have used the tanks in? The usual occurs deeper dive first, with 1-hour surface interval between dives. I chose #3; #1; #4 & #2:
Was this the best choice to make?

Thanks! Finbob

 

[ade_eccles]

Hi Finbob

Difficult to answer without depths but assuming no max depths reached for the blend I would have used the tanks the other way around i.e. highest o2 first.

I may be wrong but would think that you would be taking more nitrogen in on the early (deeper) dives and a higher o2 / less nitrogen in the blend would reduce this exposure. i.e. higher o2 would have more of a benefit on the deeper dives than those at a more shallower depth

Not sure how much difference it would actually make. Guess would depend on how close to your NDLs you were on the 4 dives

Any reason why you chose low o2 1st ?

 

[g1138]

If you're diving below the MOD of the highest %Nitrox tank that you have then ade_eccles's advice would not work out.

Edit:
If this is the case:
In doing the deeper dive first it would be better to start with the lowest %Nitrox and work your way up, so that you're shallower dives will be farther from the MOD of your Nitrox.
Additionally if using tables I would use the lowest %Nitrox table that fits your %Nitrox tanks.

So for that[-]your specific[/-] case I think I would use an Air Table to plan all my dives. I would then dive Air (21%), 30%, 32%, 36%. Getting shallower with each progressive dive.

If you're using your computer then listen to your computer. Just plug in the correct numbers.[-], but once again following the pattern of tanks above.[/-]
Also Check your tanks before you get on site

 

[ade_eccles]

Hi g118

Don't agree with this: "so that you're shallower dives will be farther from the MOD of your Nitrox". Personally I normally only use Nitrox on deeper dives to extend NDLs, and where I will be close to the 1.4 pp of O2 / MOD for that blend

Without knowing the OPs max depth for the 4 dives then this is all very hypothetical but say his last dive was to 10m - why would you save the 36% EAN for this dive ?

 

[g1138]

Hi g118

Don't agree with this: "so that you're shallower dives will be farther from the MOD of your Nitrox". Personally I normally only use Nitrox on deeper dives to extend NDLs, and where I will be close to the 1.4 pp of O2 / MOD for that blend

Without knowing the OPs max depth for the 4 dives then this is all very hypothetical but say his last dive was to 10m - why would you save the 36% EAN for this dive ?

There's nothing wrong with your original statement.
If the OP wants to maximize his dive time on his first dive, he should take the highest %O2 safe for the depth. However, if he wants to have shortened SI's and max his future dive times, then he should dive with increasing %O2 on ever following dive.
The advantages of nitrox could be negated if he were to jump from a deep depth to a really shallow depth though.

I think as long as you follow your computer, it doesn't matter what order you dive the tanks. If you're trying to maximize your dive time and shorten your SI in a certain way, then tank order would matter depending on your overall dive plan. If the OP were on tables though, then essentially he would only be diving on "air".

Edit
In hindsight I guess I should have clarified that my original explanation was pertaining to MOD limits and tables.

 

[Deefstes]

OK, I thought this was an interesting question but now you've really gone and complicated the matter. I think it's pretty safe to assume that the OP didn't ask for advice on whether or not he should dive below the MOD of his mix on any particular dive so let's assume he wouldn't do that. Now... what is the better sequence in which to use the four different mixes on four consecutive dives of decreasing depth? I'd be interested to know as well.

 

[vladimir]

Now... what is the better sequence in which to use the four different mixes on four consecutive dives of decreasing depth? I'd be interested to know as well.

Yes, that would be a better way to frame the question to get to the root of the issue. Better still: assuming I have 4 dives planned to the same depth, in what order would I use my nitrox? From rich to lean or vice versa? My guess is leaner first, making the dives progressively shallower from a nitrogen-loading perspective, as the OP seems to have done. For dives of varying depths, the principle would hold: make the equivalent air depth shallower with each subsequent dive.

 

[ajduplessis]

Based on the question posted, I would use the higher nitrox mix on the last dive, air first. The more important question is around dive depth and where off gassing started within these profiles. You have to utilise the breathing gasses correctly to help improve off-gassing results. You can dive all kinds of fancy gasses, but if you shape your profile badly they will have little benefit overall.

 

[ajduplessis]

Finger problems.....

 

[Deefstes]

Better still: assuming I have 4 dives planned to the same depth, in what order would I use my nitrox? From rich to lean or vice versa? My guess is leaner first, making the dives progressively shallower from a nitrogen-loading perspective, as the OP seems to have done. For dives of varying depths, the principle would hold: make the equivalent air depth shallower with each subsequent dive.

Seems like a logical answer, at least to me it makes sense.

Based on the question posted, I would use the higher nitrox mix on the last dive, air first. The more important question is around dive depth and where off gassing started within these profiles. You have to utilise the breathing gasses correctly to help improve off-gassing results. You can dive all kinds of fancy gasses, but if you shape your profile badly they will have little benefit overall.

Thanks for the answer aj but I'm not sure I understand. Would you care to expound a bit please?

"Where does off-gassing start within your profile?" Is there even an answer to that question? I would think that off-gassing starts in the fastest tissues the instant you turn the dive. Obviously you're referring to something more complex than what I'm giving it credit for but how does one determine at what point in a dive does off-gassing start?

"You have to utilise the breathing gasses correctly to help improve off-gassing results." I think this is the part I'm specifically interested to hear what you mean.

As vladimir points out, if we do repetitive dives, we always do the deeper ones first and the shallower ones last. It just seems logical to me that, if I were to do consecutive dives of exactly the same depth, I'd use a leaner mix first and the richer mix last - which effectively is exactly the same as doing the deeper dive first and the shallower dive last, not so? If the dives are progressively shallower to start with, doesn't it just improve the benefit from diver leaner first, richer last that much more?