DIR Pre-dive Planning?

[JeffG]

I think Jeff's being sarcastic . . .

No, I was just being strict in my definition.

120 rule is time + depth = 120 and is based on air being used. You then need to manipulate it further to use it for nitrox.

ie...the diver requires some understanding to use it for nitrox. Just like a diver would have to have to use it for triox.

 

[TSandM]

Oops, the extent of my indoctrination is showing . . . I forgot to mention that MDL triox is 30/30, which is treated for decompression purposes as being the same as 32% Nitrox. Although, now that they're only recommending 30/30 to 100 feet, I guess you'd be on 21/35 for the deepest MDL dives, which would put you back on air tables.

 

[ekewaka]

Oops, the extent of my indoctrination is showing . . . I forgot to mention that MDL triox is 30/30, which is treated for decompression purposes as being the same as 32% Nitrox. Although, now that they're only recommending 30/30 to 100 feet, I guess you'd be on 21/35 for the deepest MDL dives, which would put you back on air tables.

Why are they only recommending 30/30 to 100 feet? Did they change the maximum ppo2 again?

I wouldn't want to dive anything deeper than 100 feet on air tables.

 

[Rainer]

Max PPO2 (for cold water divers) is 1.2 (GUE).

 

[Rainer]

Thats an "addition" to the rule. (or how to take an air based rule and use it for nitrox)

Agreed, it's an addition. It also happens to be what we were taught in GUE-F.

 

[PfcAJ]

What do you use for triox? It can't be the rule of 120, since the decompression is different for helium.

The deco isn't really different with He in the mix, it just ends up being more effiecient.

 

[rainman_02]

21/35 for 100'-130' is a waste when there's a better gas to use that is more in line with all the other "standard" gasses than 30/30. 25/25. Perfect middle-ground gas.

 

[rongoodman]

I wonder why 25/25 isn't used instead of 30/30. It does seem more in line with the other standard gases, and looks like it can be easily made from He and banked 32%. (It turns out the 25/25 has about 25% longer deco times, according to a couple of profiles I just ran with Nautilus...)

 

[limeyx]

I wonder why 25/25 isn't used instead of 30/30. It does seem more in line with the other standard gases, and looks like it can be easily made from He and banked 32%. (It turns out the 25/25 has about 25% longer deco times, according to a couple of profiles I just ran with Nautilus...)

because a certain someone said "if thou are using He, thou shalt never use less than 30% He for it shall do thee no good"

25/25 has about a 10% EAD compared to 30/30 @ 20%EAD so effectively your no deco times will be a bit less, but with the He, doing some deco on it is no big deal as long as you plan for it.

 

[TSandM]

30/30 is kind of a b***ard child gas. Because of the depth range where it's used, I suspect the 30% was chosen to be close to the 32% we're used to planning MDLs with. As far as the He percentage goes, one would think it would have been chosen to keep END less than or equal to 100 fsw in the range where the gas is used (which was originally to 120 feet). Maybe it was, but the END of 30/30 at 120, using GUE's methods, is 84 feet. And you can't make it by adding 30% He and topping with 32%. I don't know where this one came from, except that it was an attempt to come up with a gas that would be useable by a recreational diver, keeping him out of deco on deeper dives and providing the benefit of a clearer mind with the helium.