Best Mix

Hello,

I plan to make a wreckdive tour in shallow water. The deep is round about 35 meters bottom (round about 105feet). I plan to use Nitrox for the dives but I'am not sure about what best for me.

I have different options. The simplest option is to use EAN32 and the NOAA table for that gas.

Another option is to dive at 1.6PPO2 with EAN35 and calculate the EAD (it's round about 27 meter) and to use a air decompression table like US-Navy.

This looks good to me, because for a 30 minute dive at 35 meters I figured out a no decompression dive. (So I remember...the data is still from my memory, I've calculated it day's before)

The next option is to use different O2 mixes. EAN32 as bottom mix, EAN50 for the ascend and EAN100 for decompression. This will result in a 30 minute dive at 35 meter, an ascend with EAN 50 with two deep stops of 2 Minutes and a complete ascend time of 12 minutes to 6 meter (18 feet) and there a decompression for 7 minutes with EAN100.

I've calculated this for a bottom time of 40 and 45 minutes (with Zplan) and this looks good to me.

But when I use Zplan I only make one dive a day and now there is my question:

Should I do one Zplan calculated dive with a bottom time of 45 minutes with EAN32,EAN50 and EAN100 or should I make two dives one of 30 minutes with EAN35 and two hours later a second 30 minute dive with EAN35 and calculated with the EAD method using the US-Navy tables?

Any suggestions whats best?

If you're asking for recommendations on mix for a dive to 35 meters (closer to 115 ft than 105 ft) then you probably aren't ready for switching gases on a single dive. I'd recommend staying with a PO2 of 1.4 which you'll achieve with a mix of EAN 31.

Don't ever push it right to the limit when your planning. If you plan a dive for 1.6 PpO2, what happens when you analyse your mix and it's, say, 35.8%...

If your floor is 35 meters, your probably ok with EAN32 [running a Pp02 of 1.44], but if that's just the planned depth and the sand is at 40 meters, you'd be better off with 31 or 30.

Now the benefits between EAN35 [EAD 27] and EAN32 [EAD 29] are non-existant if your planning with tables, nominal if planning with a computer [mine gives me 23 minutes at 110 fsw on EAN35, 20 minutes at 110 fsw on EAN32].

I'm definately with walter in that if your asking this question, you probably don't have the training to be deco diving, let alone deco diving with travel gas...

If I were you, and I knew the sand was in the area of 35 meters, I'd run EAN32 and do a couple dives.

The question is:

Do it complicated or do it easy?

Make one or few long dives or many short dives?

How to get the maximum bottom time without a computer, with or without decompression?

Why I ask? Many reasons to think about different configurations in equipment.

Using EAN31 ist more complicated as using EAN32 (because there are NOAA tables for EAN32) and it is equal complicated as using EAN35 because I have to use the EAD method too.

Using EAN31 is not really a good idea.

The risk in using EAN35 at maximum PPO2 1.6 is low because the bottom of the wreck is in 35 meters and nobody will spend much time at a wreckdive tour near the ground. The top of the wreck is in 27 meters so PPO2 is in my opinion safe. Calculating the dive this way is in my opinion safe.

I do not have a Nitrox decompression computer. So I have to use other ways to get the maximum from my dives and there are a few solutions to make such a dive.

Hello Jeff,

this is what I'am not sure about! 23 minutes vs. 30 minutes (EAN35) is in my opinion a lot of time. I wan't to take some good (!!!) photos of this wreck.

Making a long dive at 35 meter with gas changes is probably not a good idea because there is high water flow!

So having a long bottom time and long decompression at 6 meter is heavy in water having 2-3 knots and a huge equipment. And will result in several othere risks as the PPO2 risk!

So EAN 35 is probably the best solution. Gives a long bottom time, no decompression, I can surface whenever I have to and it is possible to swim with a small tank.

So I'am not in doubt of my experiences, knowledge and training. The location is the problem!

Sometime things are not so easy as they seem to be.

First of all, don't think of deco as a penalty of diving. It's a mandetory fact of diving so enjoy it. Don't risk high PPO2 just to get yourself out of deco (which you have to do anyway). For that profile and under those circumstances, I would simply use 32% (or 30/30 trimix if I had it) in a set of doubles and 50% starting at 70'. I would not take 02 at all since the time difference in deco isn't significant enough to worry about. If I were making a second dive that day, I would like to have 50% and O2 on the second dive to clear out nicely.

Mike

By the way, if you screw up doing this stuff you'll die and perhaps kill your buddy. So, make sure you know what you're doing.

you probably don't have the training to be deco diving

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Off topic, but...off NC this weekend a buddy team was doing deco with a shared 40 cf bottle with one regulator. Apparently, their "buddy breathing deco" was planned. We watched them pass the reg a few times before we went up...some people want to die, I guess.

Originally posted by gerald
this is what I'am not sure about! 23 minutes vs. 30 minutes (EAN35) is in my opinion a lot of time. I wan't to take some good (!!!) photos of this wreck.

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Not sure where you got those NDL numbers... @ 110 fsw on my computer it's 23 minutes for EAN35, 20 minutes for EAN32. Running the simulator at 116 fsw [~35 meters] I get 20 minutes for EAN35 and 18 minutes for EAN32.

Running my simulation of EAN32 for 20 minutes @ 116 gives you about 7 minutes of deco obligations. Ascending at 30 f/sec ascent puts you to just a 3 minute safety stop by the time you get up to 15.

As far as I'm concerned, running at a PpO2 of 1.6 to save 2 minutes isn't worth the risk...

You should run 1.4 or less for bottom gas. Lost Yooper is absolutly correct in that you should not push your ppo2 to avoid deco. In fact on deeper dives we take the oposit approach. We keep the ppo2 low for the working portion of the dive. You incur a little more deco time but you must do time anyway but now your exposure is low enough that you can use the right gasses for deco without pushing exposure limits. Also remember that for cold or strenuous dives you should cut back the ppo2. I like it under 1.3.

Have you traines for multi gas and/or staged decompression dives? The theory is simple enough but there is enough to do and think about that it is easy for the inexperienced to task overload and make serious mistakes. Start small. When you get to the bigger dives you will enjoy them alot more. If you jump to far to fast, at best it won't be any fun and at worst...

Originally posted by gerald
Hello,

I plan to make a wreckdive tour in shallow water. The deep is round about 35 meters bottom (round about 105feet). I plan to use Nitrox for the dives but I'am not sure about what best for me.

Click to expand...

Have you had Nitrox or Advanced Nitrox training? Maybe I am missing something in your question here, but I would think that if you had Nitrox training you would know how to calculate a best mix. And Advanced nitrox would have trained you on gas switching alternatives and figuring your CNS clock for the different gasses as well as for your subsequent dives of the day.

Using 105' @ 1.4 P02 the best mix is 33.4% Nitrox.
Fg = Pg/P(ATA)
Fg = 1.4 / (105/33) + 1
Fg = 1.4 / 3.18 + 1
Fg = 1.4 / 4.18
Fg = 33.4%

Rounding that down (so as not to increase the PO2) results in using the Nitrox 32 table. At 105' that yields a bottom time of 20 minutes on both my IANTD 32 tables and US Navy Air tables.

I personally would base my dive plan on the 20 minute bottom time at the deep end of the wreck, as staying longer at this depth could impose deco obligations. You could then work your way up to the shallower parts of the wreck which will allow you to offgas and extend your bottom time a little. But you also have to consider tank size and consumption rate at depth to determine whether you have sufficient gas supply to complete this dive, do a deco obligation and still surface with adequate reserve. Is an additional 3-10 minutes at depth really worth it?

Anyone got $.03 change for my nickel?