Sequencing Nitrox 'v' Air

[knotical]

Any sequence is "safe" as long as you follow an accepted DCS model.
But some sequences will require you to wait on the surface for longer periods of time between dives.

So the rules of thumb have become:
Deeper dive first, and
Air before nitrox.

I'd have chosen the same sequence as did the OP.

See also:
http://www.scubaboard.com/forums/advanced-scuba-discussions/349157-nitrox-tank-used-first-last.html

 

[ajduplessis]

I should have known better to post unnecessary information in this tread. Please accept my apology.

I am going to try and answer your questions here without adding more "clutter". If you really need more detail I can recommend some reading material.

Off-gassing start at various depths depending on compartments, depth and time. You have to understand how the tissue compartments work and what their half times is. In short, fast tissues will start to off gas once you leave the bottom and will continue to do so as you lower your depth. The slow tissue compartments will continue to load until you leave the water, or once you switch to O2 (no more nitrogen) and then you have the “in-betweens” Any dive involves loading of nitrogen (does not matter how deep or long) and you need to get rid of it, but I am sure you are well aware of this. It is how you get out of the water, or how much the various compartments are loaded. The less they are loaded, the better. By controlling your ascent and time spend doing this, you have control over the rate of on/off-gassing within these compartments (profile shape, gradient). Getting off the bottom to slow or spending longer time than needed during deep stop will not optimise the rate at which you off-gas (on-gassing happening in slow compartments). This will increase your SI as some compartments are still loaded. This is not really something to be concerned with during recreational diving, follow your table or your DC and all will be fine. It’s a balancing act. Please PM if you really want more info or read up on deco.

 

[ClayJar]

#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?

I had a similar set of dives on one trip last year, although it was only a three-tank day so scratch out the 30%. When I was considering the order in which to use the mixes, I had to consider a few things.

The first dive could not be 36% due to MOD limits, but that was the only fixed criterion. If I dove in order of increasing oxygen fraction (21-32-36), I would get the greatest total bottom time. On the other hand, the last dive would be by far the longest (possibly even pressure-limited instead of NDL-limited), and I've done that shallow site many times. I would much rather spend as much time as possible on the deeper wrecks that I'd rarely if ever done. So, since I couldn't do 36% on the first, it came down to 32-36-21, with the last dive being significantly shortened from what it would have been, but the first two (much more desirable) dives being significantly increased.

So, basically, the order in which you dive depends on what you want to get out of the series of dives. You can actually use a chart with MOD, EAD, depth, and oxygen fraction to plan dives using the richest mixes that do not violate MOD and fall in order of decreasing EAD if you want to front-load your diving. I don't have one made up, but I think I'm going to do one, just for data's sake. :biggrin:

 

[DivemasterDennis]

If you properly calculated your no decompression limit for each dive using appropriate tables, or if you stayed within your NDL according to you properly set nitrox programmable computer, it made no difference. I suppose because of the depth limits being different for each fill, if my depth was over 90 feet on a dive, I would have used air for deepest dive, so as to not flirt with an oxygen toxicity problem.
DivemasterDennis

 

[lowviz]

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

Not enough information to answer your question. -need the depths.

 

[Deefstes]

Alright, thanks for the explanation. I guess in order to really understand I should probably read up more but for now, I'm not sure it's necessary that I really understand deco in all its intricacies.

I'm still intrigued by this question though as I think many recreational divers would be confronted by it (I certainly have) and it might be useful to have a basic understanding of which mix to use first and why without going into a full blown study of technical decompression.

So I did a little exercise and this is by no means scientific:

Consider the mixes of the OP (21%, 30%, 32% and 36%) and the following four dives, all square profiles with a 30min surface interval between all dives.
Dive 1: 30m, 15min
Dive 2: 25m, 15min
Dive 3: 20m, 15min
Dive 4: 15m, 15min

These repetitive dives cannot be done on air without going into deco on the second dive. When using any combination of the Nitrox mixes above, the dives can be made while staying within NDL. So, assuming the dives are done in the order listed, which is better, doing richest mix first or leanest mix first? Either way round, you will still be doing "deepest" dive first and "shallowest" dive last. I'm putting "deepest" and "shallowest" in quotes because I'm referring to EAD and not physical depth.

In other words, if the dives were done using the gasses in the order (EAN21, EAN30, EAN32, EAN36), the EAD's would be (30m, 21m, 16m, 10m). If the gasses were used in the order (EAN36, EAN32, EAN30, EAN21), the EAD's would be (22m, 20m, 17m, 15m). This means that both approaches ("rich before lean" or "lean before rich") would still satisfy the rule of thumb of "deep before shallow".

I don't have V-Planner but I decided to run these four repetitive dives through Suunto Dive Planner and see what the Nitrogen load is after the entire series and compare that for the two gas sequences. Here's what I got:

The light green bars show the Nitrogen load when diving rich before lean while the dark green bars show the Nitrogen load when diving lean before rich. Remember, in both scenarios the dives are still done in the same order, to the same depths, for the same duration and with the same surface intervals. So it would seem that, all other things being equal it makes sense to dive lean before rich, at least from a Nitrogen load perspective, and specifically as regards the faster tissues.

But what about the oxygen clock? When diving the sequence of dives rich before lean, I've calculated the OTU at 128 while diving lean before rich gives an OTU of 120. So it seems that from an Oxygen exposure point of view it also makes sense to dive lean before rich.

Of course it will always be difficult to establish a catch all rule so this is still a rule of thumb. But I have played around with the values and I have yet to find a scenario where it appears better to dive rich before lean except for those where a specific profile would violate the MOD of a specific EAN mix.

 

[Codiak]

Why do we over complicate everything?

Nitrox vs Air = reduced nitrogen loading period... irrespective of mix... irrespective of depth.

While it's possible to create a situation where per the tables you are "reverse profiling" it's not a large enough delta to stress over.



Getting "Air" thrown into the mix isn't that uncommon.... for these gas blends the simple rule of thumb is to just pick the highest percentage mix for the planned mod and go blow bubbles.

 

[GrumpyOldGuy]

Why do we over complicate everything?...

Nitrox vs Air = reduced nitrogen loading period... irrespective of mix... irrespective of depth.

While it's possible to create a situation where per the tables you are "reverse profiling" it's not a large enough delta to stress over.



Getting "Air" thrown into the mix isn't that uncommon.... for these gas blends the simple rule of thumb is to just pick the highest percentage mix for the planned mod and go blow bubbles.

I agree, but I would dive the air and lowest percents first if possible for the simple reason that if I somehow space out and forget to change the computer to the next mix between dives I am still inside the safe DCS profile based on the previous mix.

 

[ClayJar]

A Tale of Three Mixes

Consider the mixes of the OP (21%, 30%, 32% and 36%) and the following four dives, all square profiles with a 30min surface interval between all dives.
Dive 1: 30m, 15min
Dive 2: 25m, 15min
Dive 3: 20m, 15min
Dive 4: 15m, 15min

While I certainly appreciate your analysis, the assumptions seem quite artificial, so I decided to complement your analysis with one of my own using a different set of assumptions which more closely approximate the diving practices of at least a certain subset of divers with which I am familiar.

As the number of possibilities becomes unmanageable very quickly as more dives are added, I'm going to go back to my three-dive trip (based on an actual trip last year). The depths to be visited and the cylinders available were:

Three dives: 110', 90', and 70'
Three mixes: 36%, 32%, and 21%​

Three dives and three different mixes give a total of six possibilities, but two would violate the MOD on the first dive:

1. 21-32-36
2. 32-21-36
3. 21-36-32
4. 32-36-21
5. [-]36-21-32[/-] (violates MOD)
6. [-]36-32-21[/-] (violates MOD)

Constraints and Assumptions:

1. Each dive begins with one full cylinder holding 77 cubic foot at 3000 psi (Luxfer AL80, rounded from 77.4)
2. The diver's SAC is 0.5 cubic feet per minute throughout the dive
3. The diver must surface with at least 450 psi ("500" after rounding) remaining
4. The diver must not enter deco at any point on the dive
5. The diver will remain at depth on each dive as long as possible within the gas-remaining and no-deco constraints
6. A one-hour surface interval is completed between dives
7. A 3-minute deep stop at half depth, plus a safety stop, are not included in the "Time on Wreck"

The following are the results of simulating the dives in Suunto Dive Manager:

Series 1: 21-32-36:

Time on Wreck:

• Dive 1: 13 min on 21%
• Dive 2: 26 min on 32%
• Dive 3: 38 min on 36% (PSI-limited)

Total: 77 min
​

Series 2: 32-21-36:

Time on Wreck:

• Dive 1: 22 min on 32%
• Dive 2: 17 min on 21%
• Dive 3: 38 min on 36% (PSI-limited)

Total: 77 min
​

Series 3: 21-36-32:

Time on Wreck:

• Dive 1: 13 min on 21%
• Dive 2: 30 min on 36%
• Dive 3: 37 min on 32%

Total: 80 min
​

Series 4: 32-36-21:

Time on Wreck:

• Dive 1: 22 min on 32%
• Dive 2: 29 min on 36%
• Dive 3: 25 min on 21%

Total: 76 min
​

The following is a combined final tissue saturation graph of all four series:

​

All four series avoided deco and surfaced with a "boat's rule" of air remaining. While I certainly do not begrudge anyone of placing additional constraints on their diving (I do in mine), the constraints listed are common and while they are not necessarily "conservative", they are not untenable.

I would suggest, then, that the particular tissue saturations in the graph above are not significant to the decision of which mix to dive on which dive in the given three-dive sequence. Any decision within the constraints (e.g. MOD) is *acceptable*, so which order is *optimal*? *THAT* should be the question, and it is not a question that pure data can answer. So, what of it?

Well, of considerable note to me is that diving 32% instead of 21% on the first dive increases that dive's length by just shy of 70%! Adding an additional 9 minutes to a 13-minute dive is quite significant to me. A 22-minute dive may give me time to peruse the entire wreck without feeling rushed, and that 9 minutes of theoretical time will only stretch more given the non-square bottom of the real dive.

If I decide I want that time, then I've got to choose from Series 2 or Series 4. If I do air for the second dive, I'd only get 17 minutes on it, whereas if I go with my 36%, I can get 70% longer, again -- 29 minutes. Of course, there's a cost to everything, and that cost is that I significantly shorten my third and final dive. Instead of getting 38 minutes, I can only stay a more modest 25 minutes. For me, however, a 25-minute dive is not unacceptably short. (As a third dive, if I'm feeling chilled or seasick, it may be longer than I need, regardless.)

In the case of my trip, the third dive was going to be on a near-shore site I'd been to plenty of times before, so I had absolutely no reason to hesitate to trade time on it for time on the other less-traveled sites. Each of my dives in Series 4 is in the 25 +/- 5 minutes range, which seems like a decent way to split the time. If I went with Series 1, I'd have a short, medium, and long dive, which may be more to my liking if there's chilly water below the thermocline on the deep sites. If I wanted more time at both of the shallower sites (for photography, perhaps), I could dive Series 3 and have two extended dives and one shorter.

So, basically, what is the "correct" order to dive the mixes? There isn't one. Sure, you can end up with a bit lower nitrogen loading at the end of the series of you dive one way versus another, but if you stay within your constraints *any* of the available options are acceptable. It all comes down to what you want to get out of the dives.

(Don't fall for the "lower nitrogen loading is *better*" concept. If you take that too far, you end up giving up diving completely and moving to Denver or something like that. Your nitrogen loading should be kept at acceptable, non-DCS-producing levels, certainly, but it need not be your focus of optimization.)

 

[Deefstes]

Good point and well argued. I like your analysis. It specifically shows just how little of a difference the choice makes to your N2 load but how significant a difference it makes to your bottom time. I have to admit that one of the prime reasons why I took the Nitrox course is so that I could have more time on these deeper dives which are so frustratingly short and seeing as we do the deeper dives first, it does seem counterproductive in a way to dive the richer mix last.

Interesting discussion.