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Roughwater
01-12-2010, 04:46
Hi,

I'm used to diving in overhead environments using the rule of thirds. Recently I have been introduced to the rule of modified thirds (or halves + 15 bar).

While I understand the new rule there was one statement made that I'm confused about and was hoping someone could clarify it for me.

The statement was that the rule of modified thirds (halves + 15 bar) can only be used by manifold divers, and not by side mount or independent divers.

I've been sitting here doing calculations but for the life of me can not reason why this rule is reliant on a manifold system. Just wondering if someone could please give me a clear explanation, or have I been given incorrect advice?

Cheers

Roughwater

UCFKnightDiver
01-12-2010, 09:53
I believe what you are refering to is stage handling and NOT backgas. It's half + 200psi(or 14bar) for stage use where you reserve the rest of the third in your backgas as that stays with you, so that in the event of a stage bottle failure or some other failure you could potentially shed the stage earlier.

For only backgas diving 1/3rds alot of times is considered a pretty liberal rule. For sidemount diving 1/3rds still applies

jj1987
01-12-2010, 09:54
Hi,

I'm used to diving in overhead environments using the rule of thirds. Recently I have been introduced to the rule of modified thirds (or halves + 15 bar).

While I understand the new rule there was one statement made that I'm confused about and was hoping someone could clarify it for me.

The statement was that the rule of modified thirds (halves + 15 bar) can only be used by manifold divers, and not by side mount or independent divers.

I've been sitting here doing calculations but for the life of me can not reason why this rule is reliant on a manifold system. Just wondering if someone could please give me a clear explanation, or have I been given incorrect advice?

Cheers

Roughwater

Whoever told you that is an idiot.

UCFKnightDiver
01-12-2010, 09:56
Hi,

I'm used to diving in overhead environments using the rule of thirds. Recently I have been introduced to the rule of modified thirds (or halves + 15 bar).

While I understand the new rule there was one statement made that I'm confused about and was hoping someone could clarify it for me.

The statement was that the rule of modified thirds (halves + 15 bar) can only be used by manifold divers, and not by side mount or independent divers.

I've been sitting here doing calculations but for the life of me can not reason why this rule is reliant on a manifold system. Just wondering if someone could please give me a clear explanation, or have I been given incorrect advice?

Cheers

Roughwater

Whoever told you that is an idiot.

agreed!

Roughwater
01-12-2010, 14:16
So, it's OK for backgas regardless of whether it's independent or manifold, but not OK for sidemounts?

IndyDiver
01-12-2010, 14:20
Hi,

I'm used to diving in overhead environments using the rule of thirds. Recently I have been introduced to the rule of modified thirds (or halves + 15 bar).

Roughwater

Maybe next week this person can intoduce you to the game of Russian Roulette. It will ultimately produce the same results with less time and effort.

[Serious mode on]
Thirds is not a very conservative gas plan to start with. If you are at max penetration when you hit 2/3, and your buddy has worst case loss of all gas , you now have 1/3 each to get out. Assuming air consumption under stress is higher - you better have flow helping on your way out, or a more direct way to the entrance than the way in, or else you are going to be in a lot of trouble. Halves + 15bar is much less than that, so it is totally unacceptable for overhead environments.
[Serious mode off]

Halves + 15bar (220psi) is an interesting plan. I assume the gas is used as follows:

One half on the way in.
15bar to knock out your buddy, so you don't have to share with him.
One half on the way out.

Of course, it would be best if your buddy still dove thirds + rock bottom, so that if you are the one with the failure, he would have plenty of air to get you out.

IndyDiver
01-12-2010, 14:27
So, it's OK for backgas regardless of whether it's independent or manifold, but not OK for sidemounts?

No, as Rox pointed out in his earlier post, halves +15bar applies to bottom stages and not your back gas/sidemount gas. There is a pretty good thread on TDS that goes into the halves + 15bar rule for stages. The concensus is that it is a pretty crappy gas plan for linear penetrations.

Roughwater
01-12-2010, 14:48
So, it's OK for backgas regardless of whether it's independent or manifold, but not OK for sidemounts?

No, as Rox pointed out in his earlier post, halves +15bar applies to bottom stages and not your back gas/sidemount gas. There is a pretty good thread on TDS that goes into the halves + 15bar rule for stages. The concensus is that it is a pretty crappy gas plan for linear penetrations.

Sorry - when I ment for backgases I mean:

The rule of halfves + 15 only applies to a diver who is:

a) Using a stage bottle and
b) Using backgas

Where the stage bottle is used to 1/2 + 15 and the back gas uses 'modified thirds' (to take on the extra gas that's supposed to still be in the stage bottle).

And that this can be applied to both independents and manifold backmount divers, but not to sidemount divers?

ssmdive
01-12-2010, 14:53
So, it's OK for backgas regardless of whether it's independent or manifold, but not OK for sidemounts?

No, it is not a good idea on your main supply at all. Some people do this with ADDITIONAL air supplies (stages), but some think it is a bad idea there as well.

UCFKnightDiver
01-12-2010, 15:36
So, it's OK for backgas regardless of whether it's independent or manifold, but not OK for sidemounts?

No, as Rox pointed out in his earlier post, halves +15bar applies to bottom stages and not your back gas/sidemount gas. There is a pretty good thread on TDS that goes into the halves + 15bar rule for stages. The concensus is that it is a pretty crappy gas plan for linear penetrations.

Sorry - when I ment for backgases I mean:

The rule of halfves + 15 only applies to a diver who is:

a) Using a stage bottle and
b) Using backgas

Where the stage bottle is used to 1/2 + 15 and the back gas uses 'modified thirds' (to take on the extra gas that's supposed to still be in the stage bottle).

And that this can be applied to both independents and manifold backmount divers, but not to sidemount divers?

No in this scenario side mount divers could follow this plan as well as long as they have a stage bottle too.

Roughwater
01-12-2010, 15:59
[quote=Roughwater;358991][quote=IndyDiver;3
No in this scenario side mount divers could follow this plan as well as long as they have a stage bottle too.

Aah - right. So I was correct in being confused trying to follow what I was told. (That the rule of 1/2's plus 15 only applies to divers with a manifold).

Rather, it's correct to say that this formula can be used on any diving configuration where:

a) There is a twin tank supply on the diver. (Either backgas or sidemounts) and

b) They are using additional stage bottles.

I wonder where these guys got the idea it was only for manifold diving.

IndyDiver
01-12-2010, 16:03
And that this can be applied to both independents and manifold backmount divers, but not to sidemount divers?



No in this scenario side mount divers could follow this plan as well as long as they have a stage bottle as well.


Exactly. When someone told you that it didn't apply to sidemount divers, they were probably trying to make the point that sidemount bottles are not the same as stages and the rule doesn't apply to them.

However, properly managed sidemounts are about the same as backmounts with an isolation manifold - the worst case scenario is loss of 50% of gas from a single failure. (Actually, the worst case is an isolation valve failure and loss of 100% of gas, but I choose not to think about that).

So as rox@ufc11 says, if they have a stage bottle in addition the sidemounts, there is no reason the rule doesn't apply to sidemounts the same as it does to backgas.

IndyDiver
01-12-2010, 16:30
Now, that we have cleared up what the rule applies to, I still don't think it is a good way to manage gas.

I don't see why people try to manage gas in different tanks as separate entities and have rule#1 for backgas and rule#2 for bottom stages. It seems to make life very difficult for no reason. At the end of the day, any gas you have counts - and when you run out, its game over. So any management scheme has to fit into making sure you have enough total gas available.

Two possible rules are:

1. At no point in a normal dive should available gas be less than 2/3(S-R) + R
2. A single failure should never leave you with less than 1/3(S-R) + 1/2R

Where S is the total starting gas in all main/stage bottles and R is the reserve to allow for problem solving time, higher SAC under stress, and other issues for you AND your buddy.

Caveat:
This assumes no deco or deco gas that is managed separately.

ianr33
01-12-2010, 17:03
I don't see why people try to manage gas in different tanks as separate entities and have rule#1 for backgas and rule#2 for bottom stages. It seems to make life very difficult for no reason.

There are lots of good reasons to breathe stages to 1/2 + 200 psi rather than to thirds in a cave situation:
1)Stages will be dropped further into the cave. Their gas can be accessed sooner in an emergency.
2)Having 2/3rds remaining in a stage is not ideal if 2 divers need to breathe from it. Better to use more gas from the stage and keep the reserve in backgas where it can be shared more easily.
3)Should the dropped stage be lost/stolen you lose less gas.
4)Stages will be breathed empty sooner.In an emergency they can be dropped,so speeding up exit

The important thing to remember is that AT MOST 1/3rd of the TOTAL gas is used going in. Where that gas is is less important.

jj1987
01-12-2010, 20:45
Now, that we have cleared up what the rule applies to, I still don't think it is a good way to manage gas.

I don't see why people try to manage gas in different tanks as separate entities and have rule#1 for backgas and rule#2 for bottom stages. It seems to make life very difficult for no reason. At the end of the day, any gas you have counts - and when you run out, its game over. So any management scheme has to fit into making sure you have enough total gas available.

Two possible rules are:

1. At no point in a normal dive should available gas be less than 2/3(S-R) + R
2. A single failure should never leave you with less than 1/3(S-R) + 1/2R

Where S is the total starting gas in all main/stage bottles and R is the reserve to allow for problem solving time, higher SAC under stress, and other issues for you AND your buddy.

Caveat:
This assumes no deco or deco gas that is managed separately.
IMO diving stages to 1/3 in any environment is a terrible idea unless you carry them very far beyond the drop point. You really need to think about volume that's available during realistic failures, and not volume that's somewhere in the water unavailable to you during certain portions of the dive.

Here's something I wrote elsewhere that explains why...


A single AL80 holds 77cu ft of gas @ 3000psi (rated pressure), and a cave filled set of double 104’s holds 282 cu ft of gas as shown earlier.
Total gas = 77+282 = 359 cu ftNow, we have one slight problem. Only one person can use a stage at a time. Let’s assume that you dove your back gas to 1/3, and your stage to 1/3. You now have 188.38 cu ft of gas to exit the cave on back gas, and 51cu ft in your stage bottle. You will use 94.19 cu ft of back gas to get to where you dropped the stage, leaving your buddy 94.19 cu ft of your back gas to get to where you dropped the stage. Hypothetically, your back gas will be empty right as you come across your stage bottle. At this point, you switch to your stage and begin to breathe it. It has 51cu ft of gas, TWICE what you used going in. The issue here is that there is only one tank neck oring, one valve to shut down, no way to isolate, and most importantly, NO WAY TO SHARE THE STAGE BOTTLE while breathing it! This means that one stolen stage bottle after a catastrophic loss of gas will end up in a death, either you or your buddy (at least mathematically).

Let’s do this a better way.
Total gas = 77+282 = 359 cu ft
We will use 1300psi of the stage, or 33.36 cu ft, dropping it at 1700psi (1/2+200)Now, let’s subtract 300psi from back gas before figuring our penetration gas.
(3600-300)/3 = 3300/3 = 1100psi for penetration gas.
104/2650*1100*2 = 86.34 cu ft for penetration gas.So, we’re using 86.34+33.36cu ft (119.69) for penetration gas, out of a total of 359cu ft.
359/3 = 119.667 cu ft. This means we’re within .03 cu ft of 1/3 of or total gas volume.By diving to 1/2+200 instead of 1/3 of the stage, you’ve increased the volume of gas that you can share exiting the cave, AND the gas is in the BACK of the cave with you, so you have it during the entire dive.


The important thing to remember is that AT MOST 1/3rd of the TOTAL gas is used going in. Where that gas is is less important.
Totally disagree. Where the gas is located is every bit as important as staying within 1/3rds. You don't count the AL80 in your vehicle, and you shouldn't count the gas in the stage bottle that you can't share...it sounds extreme, but mathematically they have the same result.

LiteHedded
01-13-2010, 06:59
I don't see why people try to manage gas in different tanks as separate entities and have rule#1 for backgas and rule#2 for bottom stages. It seems to make life very difficult for no reason.

There are lots of good reasons to breathe stages to 1/2 + 200 psi rather than to thirds in a cave situation:
1)Stages will be dropped further into the cave. Their gas can be accessed sooner in an emergency.
2)Having 2/3rds remaining in a stage is not ideal if 2 divers need to breathe from it. Better to use more gas from the stage and keep the reserve in backgas where it can be shared more easily.
3)Should the dropped stage be lost/stolen you lose less gas.
4)Stages will be breathed empty sooner.In an emergency they can be dropped,so speeding up exit

The important thing to remember is that AT MOST 1/3rd of the TOTAL gas is used going in. Where that gas is is less important.
I'm gonna have to agree with james on this one. where it is is hugely important.
which is why we keep the majority of this reserve on our backs and NOT in a stage bottle that may or may not be there, and if it is, it can't be shared.

in_cavediver
01-13-2010, 11:10
I agree with James/Ian and all for two man teams - 1/2+200 is better than 1/3rds for stages. In three man teams, assuming the SAC's are close for all members, I personally feel is less of a concern and some benefit is gained by the simplicity of thirds. Its not a big benefit necessarly but it does give 'in the head' options vs 1/2+200.

The reason the three man team is less of a concern with a SINGLE total gas failure is that each of the two 'buddies' now has 1/3 of gas to share to get out. In theory, in this situation, you could get to the stage bottles with 1/6th of your gas available in both sets of backmounts.

In real life - I use the gas plan my team is most comfortable with. In some cases, such as peacock, there are multiple options for responding to an emergency. For instance - at the peanut restriction, a total gas failure has me getting out at olsen, even if I went in at Peacock 1. We have these options since we have been all over the system and personally verified closest openings. Diving thirds on everything doesn't bother me. In a place like Ginnie - I am far more likely to dive 1/2+200 and carry the stage farther simply to get it farther back into the cave and make it more likely to be there on the way out.

LiteHedded
01-13-2010, 12:20
why change the way you gas plan based on the number of team members? reserves are still more useful on your back than in a stage regardless of the # of team members

jj1987
01-13-2010, 14:14
For instance - at the peanut restriction, a total gas failure has me getting out at olsen, even if I went in at Peacock 1. We have these options since we have been all over the system and personally verified closest openings. Diving thirds on everything doesn't bother me. In a place like Ginnie - I am far more likely to dive 1/2+200 and carry the stage farther simply to get it farther back into the cave and make it more likely to be there on the way out.
So in an emergency you would do a visual jump down an unverified line to get out at Olsen? I hear people say this all the time and I can't for the life of me comprehend why people think it's OK to do that?

Scuba Pete
01-13-2010, 15:02
I dont think he said that he would do that. He said they have personally verified closest openings. He also said nothing about visual jumps.

jj1987
01-13-2010, 15:06
I dont think he said that he would do that. He said they have personally verified closest openings. He also said nothing about visual jumps.
Possibly, I'm just assuming the most logical route to get to the peanut restriction.

in_cavediver
01-13-2010, 16:46
why change the way you gas plan based on the number of team members? reserves are still more useful on your back than in a stage regardless of the # of team members

I said the gas plan the team was most comfortable with. I also stated that my comfort varied based on the system. I personally don't see that big of a risk in peacock with thirds for most dives - especially with multiple gas sources. In two man team, you have (2) '1/3rds' in reserve - lose one, then you are on the edge. In a (3) man team, you have (3) '1/3rds' in reserve - lose one and you still have plenty of gas to get out or to your stage bottles. This is one reason why teams of 3 are better than teams of 2.

If you have been around - there are some things people tend to take to religious extremes. I am flexible under many cases and frankly - for a lot of stage diving I do, I don't see that large of a safety benefit for 1/2+200 vs 1/3rds. Then again, I don't do a lot of long straight line penetrations either - mostly just a means to get more bottom time to poke around areas.

I dive with (and have dove with) different people who had differeing opinions on 1/3rds vs 1/2+200. In peacock - that won't stop my from diving with you - I'm good with either. Your risk tolerance may vary

in_cavediver
01-13-2010, 16:55
For instance - at the peanut restriction, a total gas failure has me getting out at olsen, even if I went in at Peacock 1. We have these options since we have been all over the system and personally verified closest openings. Diving thirds on everything doesn't bother me. In a place like Ginnie - I am far more likely to dive 1/2+200 and carry the stage farther simply to get it farther back into the cave and make it more likely to be there on the way out.
So in an emergency you would do a visual jump down an unverified line to get out at Olsen? I hear people say this all the time and I can't for the life of me comprehend why people think it's OK to do that?

First - its risk reward situation. We have been along all the major lines to the various sinks. We know the distances and air consumption and frankly, the choice for me, at the peanut restriction is obvious. Its 'T's into the upstream olsen line around 500-600 feet from olsen and 800-900 feet to challenge. Both of those options are a LOT closer than the 1700+ feet back to P1.

I guess you could say that I didn't follow that line earlier so it could have moved, been cut or taken away. In which case, I'd have to rely on the whole referencing thing to know where to go. Good thing I have that covered as well.

Now for visual jumps - where did I say that? where did I say which line I was on? (not that it matters, I'd go to the upstream olsen line toward olsen). To many assumptions on your part.

I personally see this as a better risk to go to the closer sink rather than retrace my route - since I have been to that sink MANY times previously. (its actaully one of our favorite 'warm up' dives when get down to cave country). You don't have to agree. You may be more comfortable swimming the peanut line back in an air share. I just would rather not if I could help it. I see that as a bigger risk due to time/distance and air consumption rates.

LiteHedded
01-13-2010, 17:34
i don't think thirds at peacock is unsafe either in most cases.
but 1/3's on stages doesn't make sense at all to me regardless of the team. there's plenty of safety benefit in getting the reserves out of that stage bottle and onto your back/sides

in_cavediver
01-13-2010, 20:27
i don't think thirds at peacock is unsafe either in most cases.
but 1/3's on stages doesn't make sense at all to me regardless of the team. there's plenty of safety benefit in getting the reserves out of that stage bottle and onto your back/sides

I can do both and am flexible. Funny enough - the most expierenced diver I have dove with preferred the 1/3rds for stages and consider the added complication of 1/2+200 more dangerous and more likely to cause errors in planning and under water. Her name appears on many of the popular cave maps as part of the survey team and she actively does exploration all over the world.

To each thier own. (so long as they accept the risks)

ssmdive
01-14-2010, 12:36
I wonder where these guys got the idea it was only for manifold diving.

Im gonna bet an internet forum :smilie39::smilie39::smilie39::smilie39: