Thirds

[AndyNZ]

Thanks for the various comments. Was mostly interested by the fact that JJ's book didn't really make much of a distinction about minimum/usable gas, whereas Fundies teaching material does. Admitedly, the book is intended to complement the Tech 1 materials, which I guess go in to more detail of gas planning to build on the Fundies teaching material.

It's always interesting to hear what people for for gas planning.

 

[b1gcountry]

there's a story in caverns measureless to man where it nearly killed him. his buddy had a catastrophic failure on backgas at max penetration and they barely made it back to their stage bottles. sheck ran out of backgas before he made it the last ~50 feet to his stage bottle.

And then he ran out of gas in his stage bottle before he reached his next one if I recall the story correctly.

I'm curious what other folks do in terms of deep dives where there is no penetration. Like a 150' dive with no penetration, and you are carrying a 70' bottle. Lets say there is little to no current, and it's a fairly small wreck. If you calculate rock bottom, do you go to your first gas switch? What if your backgas deco requirement is greater than your rock bottom?

Tom

 

[MonkSeal]

I'm curious what other folks do in terms of deep dives where there is no penetration. Like a 150' dive with no penetration, and you are carrying a 70' bottle. Lets say there is little to no current, and it's a fairly small wreck.

Calculate MGR, decide if the risk of not comming back to upline is acceptable, define TP if necesarry or decide that "blue ascend" is ok, calculate available gas based on chosen option.

If you calculate rock bottom, do you go to your first gas switch?

Yes, it's always up to next available gas source. And you can always add something to feel more comfortable and to avoid SPG error.

What if your backgas deco requirement is greater than your rock bottom?

Then you have wrong tanks or wrong plan or wrong calculation

 

[DA Aquamaster]

What if your backgas deco requirement is greater than your rock bottom?

From a lost gas planning perspective on a single deco gas dive, you need to ensure you have enough backgas to do the deco in the event you lose the 70' bottle.

So if the reserve back gas is not sufficient to do the deco, you either redo the dive plan to ensure you have adequate back gas for a lost gas contingency or you add more back gas (bigger tanks or a stage with bottom mix that you use first) or add another deco gas with sufficient back gas to to the deco if either deco gas goes missing.

Missing significant deco on a long 150' dive can kill or cripple you the same as on a shorter 300' dive, so don't take it lightly.

 

[MonkSeal]

I'll briefly add a simplified view. If we're talking about 150' (45m) dive and single deco gas (70' - 21m) then we can consider this as T1 level dive. Double AL80 or double 12l (or similar) are tanks of choice for this application, since they match with AL80 (or 7l) for recomended max deco (up to some 35 min). If you start planning your dive form "max deco you want" perspective, considering contingency, your gas management plan will give you these tanks. Going for bigger tanks and extending BT means more deco and you'll enter the area of T2 dives.

 

[Blackwood]

since they match with AL80 (or 7l) for recomended max deco (up to some 35 min).

Other than the 31st President of the United States, who only gets 35 minutes out of an Al80 on a T1 dive?

 

[MonkSeal]

Other than the 31st President of the United States, who only gets 35 minutes out of an Al80 on a T1 dive?

Of course you can get more from AL80 then from 7l since it's almost 60% bigger but if it's fully pumped with nitrox 50, it's quite negative and doesn't ride good. 35 min with 7l is calculated with some contingency i.e. sharing with team member.

 

[Blackwood]

Yah, I figured you were either carrying some sharing reserve or trying for two dives on the one bottle.

 

[b1gcountry]

What if your backgas deco requirement is greater than your rock bottom?

Then you have wrong tanks or wrong plan or wrong calculation

Won't your gas requirement for backgas deco almost always be greater than your rock bottom to your first gas switch?

 

[SparticleBrane]

Just going to throw this out there, and I'm sure it won't be popular but here it goes anyways...

How many people who have responded so far have taken the requisite classes to talk with certainty as to the differences between GUE recommending "traditional thirds" and "usable gas thirds" ?? I can think of 2, just looking through the list of responses. I have only taken GUE-F where we were taught "usable gas thirds" but I haven't taken any other GUE courses, so I'm not really the best person to ask.

Seems to me that the OP might have gotten a better answer had he posted the question to Quest, or perhaps a GUE cave or technical instructor. Asking people who have never taken a GUE course why or why not GUE changed a certain aspect of their training...you probably won't get the best answers.

Just a thought.