DIR Pre-dive Planning?

[limeyx]

Sitting down and doing the calculations, the incremental advantages of 21/35, 25/25, and 30/30 are just not great enough to get exercised about.

Right, once you start to dip below 100 feet, using a High PPO2 to get more "MDL" time gets increasingly more ugly.

Sometimes it's just best to choose an appropriate gas and plan to do the deco, or do a shallower/shorter dive as appropriate.

Some people like to use 26/17 down to 150 and then take 21/35 down to 190, which I see as pushing things a bit too far for me, I'd rather have less O2 and more He anytime.

 

[rainman_02]

I was referring to the "I'll dive 30/30 but disregard the agency set depth limits for the gas." What's the point of using a standard gas if you're going to violate the standards for it?

 

[Soggy]

Some people like to use 26/17 down to 150 and then take 21/35 down to 190, which I see as pushing things a bit too far for me, I'd rather have less O2 and more He anytime.

That's the NAUI Helitrox gas (26/17).

My team uses 28/15-20 or so down to 120'. Sometimes we just use 28% if we don't have a spent set of 21/35 to top off. Below that we switch to 21/35. I just don't have a lot of use for 30/30.

 

[Soggy]

I was referring to the "I'll dive 30/30 but disregard the agency set depth limits for the gas." What's the point of using a standard gas if you're going to violate the standards for it?

People have different beliefs on what is an acceptable PO2. It's not like diving 30/30 to 120' is an incredibly ridiculous thing to do. A 1.4 PO2 on a recreational dive is hardly suicidal. You need to step back and remember how these standard gases came to be.

 

[Steve R]

I was referring to the "I'll dive 30/30 but disregard the agency set depth limits for the gas." What's the point of using a standard gas if you're going to violate the standards for it?

To fend off the 'lemming' discussions.

But then it brings out the other crowd of complainers, so it's just a matter of not winning regardless.

Now I have to admit I dived 28.7% while my buddy was on 31.4% a month ago to 110ft.

Somehow we managed to keep the wheels on 'er and survive.

I love standard mixes, but have no issue dealing with the realities of 'field-mixing' when it becomes an issue. I usually like to plan for it though, so it doesn't.

 

[rainman_02]

People have different beliefs on what is an acceptable PO2. It's not like diving 30/30 to 120' is an incredibly ridiculous thing to do. A 1.4 PO2 on a recreational dive is hardly suicidal. You need to step back and remember how these standard gases came to be.

I agree completely. I do not see a problem with 30/30 down to 120'. I see a problem with the standard that says "you can use all these other gasses down to 1.4 but aim to keep the average at 1.2, but this one (30/30), you can only use down to 1.2."

I do wonder about how 30/30 came to be a part of the standard gasses when not a single thing about it was similar to all the other standard gasses, or what its origins are. The best answers I've seen so far are, "GI3 refused to dive any trimix with less than 30% He" and "It was thought up in an airport terminal."

 

[Soggy]

I agree completely. I do not see a problem with 30/30 down to 120'. I see a problem with the standard that says "you can use all these other gasses down to 1.4 but aim to keep the average at 1.2, but this one (30/30), you can only use down to 1.2."

Do the standards say that? I was under the impression that the MODs on the other gases were based on a 1.2, also
21/35 - 150'
18/45 - 190'
15/55 - 230'
12/70 - 300' (not GUE, but as I understand it is the Wakulla gas)


10/70 is a weird one, too and I'd bet good money that I will never do a dive that requires it, so I'm not going to even bring it up

The best answers I've seen so far are, "GI3 refused to dive any trimix with less than 30% He" and "It was thought up in an airport terminal."

Yeah, the minimum 30% He thing seems completely devoid of logic or support from a physiological perspective.

 

[Dan Gibson]

George's statement has likely been taken out of context.

Yeah, the minimum 30% He thing seems completely devoid of logic or support from a physiological perspective.

 

[limeyx]

I was referring to the "I'll dive 30/30 but disregard the agency set depth limits for the gas." What's the point of using a standard gas if you're going to violate the standards for it?

Some habits die hard, I guess. It gets irritating when something changes under your feet (the MOD for 30/30 used to be 120, but then got changed)

Same as 21/35 is now "clarified" to be 150 MOD

Stubborn-ness ?

I have the choice of 21/35 and a deco gas
abusing 30/30
using a non-GUE gas (25/25)
not diving between say 100 and 130

 

[limeyx]

George's statement has likely been taken out of context.

Quite possibly -- my post was supposed to be a joke (Although I did read that quote somewhere). I am guessing he was meaning very different kind of diving than I am used to

From what I understand, GUE has "clarified" the use of bottom gases to make MOD be a PPO2 of 1.2 (so 21/35 is now 150 and not 190 -- if it ever was 190)
GUE (from my understanding) is saying that this is not a change, but a "clarification" of existing procedures.

Generally, I would try to average the PPo2 to about 1.2 and keep END max 100 feet.
So that means I would use 30/30 down to 110-120 for 1/2 the dive, and do the other 1/2 shallower. Same for 21/35 -- I would go 1/2 the dive at 160, the other half at 140 to average 1.2 @ 150 etc.

My understanding of 30/30 was that it was introduced for day 4 of tech1 (where you were about 100 feet anyway) to show the benefits of Helium without having to go into mandatory deco. It then snuck its way into Triox etc. etc.