Thirds

[AndyNZ]

More of a curiousity question than anything else.

I've just been skimming through JJ's book "Getting clear on the basics: The fundamentals of technical diving". In it, he talks about what I would consider to be "traditional" rule of thirds.... i.e. 1/3 in, 2/3 for exit & emergencies.

This is a bit different from the (newer) Fundies material, which essentially does thirds on usable gas. So if you're doing a dive to 30m, which has a 100 bar minimum gas, and have a 230 bar fill that gives you 130 bar usable gas, round down to 120 bar and divide by 3 to give 40 bar. Turn pressure is then 230 - 40 = 190 bar. This is obviously more conservative than "traditional" rule of thirds, where turn pressure would be 160 bar.

Has this difference been due to a change in training materials/attitude over time? So, introducing the concepts of all/half/third usable gas scenarios to give more flexibility based on circumstance. Or is it a difference between Fundies & Tech 1?

 

[Blackwood]

What's appropriate for one environment isn't necessarily appropriate for another.

For example, straight thirds may make sense in a shallow cave but it isn't necessarily sufficient for deeper cave dives or ocean penetration dives where exiting the overhead still leaves you with a decompression obligation.

 

[TSandM]

I was taught in Fundies, in 2005, that thirds was for total gas in overheads, and for usable gas in "must return to the upline" dives. The same information was taught in Rec Triox in 2007.

 

[Thalassamania]

More of a curiousity question than anything else.

I've just been skimming through JJ's book "Getting clear on the basics: The fundamentals of technical diving". In it, he talks about what I would consider to be "traditional" rule of thirds.... i.e. 1/3 in, 2/3 for exit & emergencies.

Actually it's 1/3 in, 1/3 out and 1/3 for my buddy.

This is a bit different from the (newer) Fundies material, which essentially does thirds on usable gas. So if you're doing a dive to 30m, which has a 100 bar minimum gas, and have a 230 bar fill that gives you 130 bar usable gas, round down to 120 bar and divide by 3 to give 40 bar. Turn pressure is then 230 - 40 = 190 bar. This is obviously more conservative than "traditional" rule of thirds, where turn pressure would be 160 bar.

Has this difference been due to a change in training materials/attitude over time? So, introducing the concepts of all/half/third usable gas scenarios to give more flexibility based on circumstance. Or is it a difference between Fundies & Tech 1?

That leaves you, in worst possible case, with no reserve. As I understand it, a change in philosophy concerning good gas management planning calls for defined "rule of thumb" reserves ... thus the 100 bar minimum gas, so its 1/3 of gas minus reserve in, 1/3 gas minus reserve out and 1/3 gas minus reserve for my buddy.

 

[MonkSeal]

Has this difference been due to a change in training materials/attitude over time?

No, it's just applying appropriate safety to each segment of dive separately - in your example: MGR has built in enough safety to bring up two divers to next availabe gas source (surface or switch); active portion of dive applies rule of thirds to ensure safe return to point where MGR can be applied. If whole dive can be taken as one sequence and there's no mandatory return to upline, no penetration, etc. then the rules are simplified.

 

[karstdvr]

What's appropriate for one environment isn't necessarily appropriate for another.

For example, straight thirds may make sense in a shallow cave but it isn't necessarily sufficient for deeper cave dives or ocean penetration dives where exiting the overhead still leaves you with a decompression obligation.

1/3rds is not conservative enough be it shallow or deep cave. Sheck Exley who is the "founder" of this rule admitted that a loss of gas at max penetration would more likely result in a fatality.

 

[b1gcountry]

I just did the math for some staged deco dives last week, and the rock bottom/mgr number was so low that straight rule of thirds would have been about the same.

In MDL/NDL limit diving, you typically use a lot more backgas ascending than you do descending. With a all the staged deco dives we did last week, I calculated I used more backgas on the descent than I did on the way back up to my deepest switch. So straight thirds would have been conservative enough; especially at the snails' pace I normally dive at.

Tom

 

[Blackwood]

1/3rds is not conservative enough be it shallow or deep cave. Sheck Exley who is the "founder" of this rule admitted that a loss of gas at max penetration would more likely result in a fatality.

Hence the "may"

It's all circumstantial. If you lazily took your time swimming in, you could possibly get you and your buddy (who's instantaneously lost all his gas at the point of max penetration) out on the remaining 2/3. But as was mentioned, that leaves nothing in the tank if it takes as long to exit as to enter, and results in a deficit (i.e. the likely fatality) if it takes longer to exit than to enter.

My point in saying "shallow" was to eliminate the situation in which it definitely takes longer to get out than it did to get in (thus requiring a substantial "rock bottom" or "MGR" which absolutely breaks straight thirds).

I just did the math for some staged deco dives last week, and the rock bottom/mgr number was so low that straight rule of thirds would have been about the same.

I personally prefer precision+conservative MGR calculations to the "1/3 and hope" for OW dives. I consider thirds to be for penetration.

If am going to do a penetration dive in the ocean, I need to compute MGR and then run thirds (plus whatever exiting contingency I feel is appropriate) on my remaining gas supply when I arrive at the overhead minus that MGR.

 

[Thalassamania]

An additional reserve makes sense, I can't imagine a team having a failure at maximum penetration and being able to exit as quickly, smoothly and calmly as they entered.

 

[lamont]

1/3rds is not conservative enough be it shallow or deep cave. Sheck Exley who is the "founder" of this rule admitted that a loss of gas at max penetration would more likely result in a fatality.

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.