Air strategy for 4-5 tank dive days?

[northernone]

This is strange thread to me. Kind of like debating whether to wear a helmet when you bike.

You may want to take another look. Coming up with the ideal gas mix for maximizing bottom time over multiple days and dives while maintaining safely margins isn't so simple.

With the wrong mix with the wrong dive profile oxygen and nitrogen both can really hurt us.

EAN21 has it's place as well as EAN32 in my dive planning. Unless you're suggesting trimix on all dives?

Cameron

 

[Blasto]

Still, this doesn't seem to be a very strategic task to solve.

Your 1st gas selection priority in rec diving is to always stay within safe O2 limits. Keep the planned depth below 1.4 PPO2 and the rock bottom over most of the site below 1.6.

Your 2nd priority is to minimize your tissue supersaturation, and to do that, minimize your N2 load. So use the highest O2 mix that keeps you within safe PPO2 limits.

If we were talking deco dives in deep blue water, it would be seriously worth considering whether the risks of getting caught in a downcurrent outweigh the risks of longer deco. But as it stands, the rule is not to take either risk: use a safe O2 mix and dive only as long as your PDC permits with a solid headroom.

This is assuming the cost of nitrox or extra O2 is not a factor.

 

[Johnoly]

.... But as it stands, the rule is not to take either risk: ....

Yep,, and if diving the Coz Walls it's the boat that makes the rules, not the diver. If the boat says walls=air only, then the nitrox percent strategy just becomes a day by day choice of where the boat takes you and how long you stay at depth.

 

[ggunn]

Yep,, and if diving the Coz Walls it's the boat that makes the rules, not the diver. If the boat says walls=air only, then the nitrox percent strategy just becomes a day by day choice of where the boat takes you and how long you stay at depth.

I have never experienced a Cozumel DM or boat captain dictating which gases the divers must breathe.

 

[boulderjohn]

I have never experienced a Cozumel DM or boat captain dictating which gases the divers must breathe.

I have.

It was many years ago, perhaps 15. I was diving with Dive Paradise. I wanted to dive with EANx 32 on the first dive and air on the second. I got tanks for that plan. Once on the boat, the DM would not allow it. Having never met me, he declared to everyone on the boat that I must be so totally incompetent that I could not be trusted to stay above MOD on the first dive. (Okay, his wording was a little different.) I would therefore not be allowed to use nitrox on the first dive, and I would only (obviously grudgingly) be allowed to use it on the second. You never know what incompetent divers like me are going to do.

 

[ggunn]

I have.

It was many years ago, perhaps 15. I was diving with Dive Paradise. I wanted to dive with EANx 32 on the first dive and air on the second. I got tanks for that plan. Once on the boat, the DM would not allow it. Having never met me, he declared to everyone on the boat that I must be so totally incompetent that I could not be trusted to stay above MOD on the first dive. (Okay, his wording was a little different.) I would therefore not be allowed to use nitrox on the first dive, and I would only (obviously grudgingly) be allowed to use it on the second. You never know what incompetent divers like me are going to do.

That sounds like the exception rather than the rule, especially if the DM held you up for ridicule in front of the other divers on the boat. I don't blame you for being upset, but none of the Cozumel ops that I have dived with have done anything like that to anyone while I was aboard.

 

[boulderjohn]

That sounds like the exception rather than the rule, especially if the DM held you up for ridicule in front of the other divers on the boat. I don't blame you for being upset, but none of the Cozumel ops that I have dived with have done anything like that to anyone while I was aboard.

He said I could not use the 32% tank on the first dive because it was "very dangerous" to go to the 80 foot depth they were planning--I might lose buoyancy control and plunge past the 111 foot MOD. Now, he didn't exactly say I was incompetent, and he made it clear that he was speaking generically, but....

 

[Scuba_88]

I'm going to be diving with Horn34 in November and I'm also interested in coming up with a good Nitrox strategy. 3P is recommending 36% Nitrox on two of the 4 dives each day. I believe someone said this in an earlier post but it seems like if the third dive was on 36% this would be enough to keep us in safe zone. I think 80% - 90% of our dives will probably be in 80 ft or less.

 

[CajunDiva]

Opinions are like...well, you know Everyone's got one...

I concur with the advice of 3P. Believe it or not, they dive those waters every single day and might have just a little experience with the subject matter. Not knowing you, they're trying to give you a bit of generic friendly advice.

You could always just do without it and see how your NDL turns out...

 

[boulderjohn]

Here are two dives, one to 80 feet and the second to 60 feet. I used the PADI tables for EANx 32 and Air to calculate the dives and compare the difference between using nitrox on the first dive and using air on the first dive. For those who do not know tables, PG = Pressure Group, and the farther down the alphabet, the more decompression stress.

Starting with Nitrox, with the first dive to the Air limit
Dive #1 on EANx 32
80 feet for 30 minutes (45 allowed); PG M
1 hour surface interval; PG D
Dive #2 on Air
60 feet for 39 minutes (max allowed); Pressure Group W

Starting with Air, with the first dive to the Air limit
Dive #1 on Air
80 feet for 30 minutes (max allowed); PG R
1 hour surface interval; PG F
Dive #2 on EANx 32
60 feet for 39 minutes (max allowed); Pressure Group X

Summary: total dive time and final pressure group almost identical; First dive on nitrox severely limited (45 allowed), but it allowed for a longer second dive on air. If the diver had gone to the full 45 minutes allowed on nitrox on the first dive, the second dive on air would have been limited to 34 minutes, only a 5 minute loss. The diver would have gained 15 minutes on the first dive and lost 5 minutes on the second dive, for a net gain of 10 minutes of bottom time.

Note: If the diver had used EANx 32 on both dives and dived each to the limit, the first dive would have been 45 minutes and second dive would have been 62 minutes