Multi-level repetitive dives

[TSandM]

Honestly, multiple multi-level dives is the perfect setting where the iterative capacities of a dive computer are most useful. There are other approaches, but they require a fair amount of situational awareness (and some facility with arithmetic) and are not validated in any formal way.

 

[DCBC]

I'd much rather trust the DCIEM tables myself...

 

[Interceptor121]

The whole idea of multilevel and computer diving is that you plan your dive based on 2 or 3 depths and bottom times and then execute the dive with the computer.
It is clear that during your dive you may encounter some fish or aquatic life or plant that you want to observe and unless you really dive for academic purposes you will most likely have small variations that the computer will calculate but that would be difficult to account manually (otherwise why do we have computers after all)

If your concern is that you do many dives and may get DCI just get enriched air, if available where you dive and you are certified.
At that point you will be most likely limited by air consumption and not by depths or surface intervals

 

[DCBC]

Nitrox of course has it's own problems (O2 toxicity). On nitrox, the risk of an acute O2 attack may be the same or higher than the risk from nitrogen narcosis at certain depths. The major problem with oxygen is that you may get little or no warning of an attack and your chances of surviving one at depth are remote.

More and more people use tables and/or computers (that are based on tables) that are less restrictive. These tables allow more BT at depth. Often divers feel that this is great because they can stay down longer at depth and "be legal," regardless of the fact that the chance of a DCS hit is increased. In the case of DCS, the greater the exposure the closer to the line you come. Many tables are based in such a way that if you follow them, it's just a matter of time before you get DCS.

The DCIEM tables are the most modest and have been adopted by many commercial operators. For a comparison see http://rubicon-foundation.org/dspace/bitstream/123456789/7760/1/SPUMS_V33N1_7.pdf.

Personally, I believe computers are great, but you should plan your dive with a built-in safety factor. I never solely trust one or more computers that are based upon a table that will bring me closer to the line...

 

[Interceptor121]

Of course he would plan a dive with margin and then execute it with the computer making sure is leaving enough no decompression time.
For what concerns oxygen toxicity with an EAN32 mix you reach 1.4 PPo2 at 33.8 meters and every computer would give you alarms when you reach your MOD, some will even allow you to set the PPo2 to lower values such as 1.2 or 1.3

So Nitrox remains the best option for repetitive dives as computers can easily monitor CNS and even a simple depth gauge to be honest would be sufficient if you calculate your MOD manually

 

[DA Aquamaster]

But the Wheel and eRDP-ML are only useful AFTER the dive. It is very unlikely you will have an exact profile before the dive. I suppose you could use the Wheel under water but it's out of the question for me unless I can bring my reading glasses.

I liked the square profile approach of writing ANDLs on a slate. It was simple and I understood it. None of the other schemes seem to do much before the dive and after the dive they can only tell you whether you need to head for a chamber.

I'm a strong supporter of tables and square profiles. For boat dives it works pretty well. But for multi-level diving and the increased down time it delivers, I don't see a way around using a computer.

Richard

The dives where multi level planning is most useful are dives like wall dives where you can indeed pre-plan the profile and then dive the actual planned profile very closely. In most sites, if you know the site or get a decent briefing, you can pre-plan the levels fairly accurately.

The important point is the increased planning ability that is allowed, even if you end up diving the computer anyway.

For example, on a no deco multi level dive, I can to dive at 130 ft for 10 minutes, then come up to 70 feet for 15 minutes, then come up to 40 feet for 30 minutes and still be within the NDL at 130, and 70 ft and be well within it at 40 ft.

But actually diving that profile would require 98 cu ft of gas assuming a SAC of .6 and 114 cu ft assuming a SAC of .7.

Consequently, if you pre-plan the multilevel dive with a Wheel, tables, etc, you know what is possible in terms of depths, dive times and NDLs. You can then do some preliminary gas planning to see if it is practical from a gas plan perspective, and if not, re-adjust the dive plan to better match the gas you have available as well as the gas reserves you require.

I contrast, if you just ride your computer, it will monitor your deco status and show you the increase in bottom time as you ascend to shallower depths, but you will just be riding the SPG as well and reacting rather than really knowing how the dive will progress.

For example, now that I know the dive plan in terms of depths, times and NDLs as well as the gas required, I know what to expect and I can anticipate how much gas I will have left at any point. If I am breathing more gas than planned, it will become obvious early in the dive, as will any SPG failure where I may get an innaccurate / misleading reading (stuck needle, etc). Similarly, if the computer fails, I don't need to abort, I can stay on the plan and finish the dive in a normal manner.

Actually planning the dive takes a bit more effort - about 5 minutes more in this case - but I can then dive with a much higher level fo knowledge and a much higher degree of confidence with no surprises and less potential for an "oh ****" moment, so it is 5 minutes well spent.

 

[DCBC]

Of course he would plan a dive with margin and then execute it with the computer making sure is leaving enough no decompression time.
For what concerns oxygen toxicity with an EAN32 mix you reach 1.4 PPo2 at 33.8 meters and every computer would give you alarms when you reach your MOD, some will even allow you to set the PPo2 to lower values such as 1.2 or 1.3

So Nitrox remains the best option for repetitive dives as computers can easily monitor CNS and even a simple depth gauge to be honest would be sufficient if you calculate your MOD manually

Personally Nitrox 32 and it's depth restrictions don't usually work for me. If however it meets your needs, great!

 

[Brett1971]

Thank you all for your great responses! This is exactly why I asked this question, I've learned a lot from your replies.

I'm a photographer primarily, not a depth junkie. I usually spend my time between 40 and 60 ft. That's mostly where the stuff I like to shoot lives. With the type of diving we do the risks are probably minimal anyway, as long as we don't push the limits. We just want to safely maximize the number of dives we can do in our week there. Thanks again for the insight, it has really helped!

Brett