Just out of curiousity, if you are truly diving halves with the stages do you breathe them dry or do you subtract some amount first and then breathe halves? Or am I totally confused?
Chad
Chad
The Rule of Thirds.
- Thread starter Uncle Pug
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OP
Here are the basic answers
These answers do not give all the salient details that many of you have included in your posts details count as extra credit unless your details are wrong in that case off with your head!
BTW . A couple of you did a stellar job as I would have expected from your previous post.
The Rule of Thirds
1) What is it? Answer: 1/3 of your gas is for going in, 1/3 of your gas is for coming back, and 1/3 or your gas is for your buddy in the event of an OOA at the furthest point.
2) What dives require the rule of thirds? Answer: ANY dive where egress can only safely be made at the point of entry.
3) When is the rule of thirds not enough? Answer: ANY dive where egress cannot be made at the point of entry or where more gas is require to exit than to enter.
4) When is the rule of thirds too much? Answer: ANY dive where access to the surface is available at every point in the dive.
5) What is the rule of halves? Answer: AFTER subtracting Rock Bottom, ½ the gas for going out, ½ the gas for coming back as long as the surface is available for egress at every point in the dive.
6) What is rock bottom? Answer: The gas needed for you and your OOA buddy to make it to the surface (or to the first gas switch) doing all stops and with a safe ascent rate.
7) What dives require only rock bottom? Answer: Dives that are not directionally constrained.
These answers present the minimum but adding in additional gas without knowing the baseline is just a shot in the dark. If you feel that you would like to pad your gas supply that is fine. In fact when we figure 1/3s or 1/2s or Rock Bottom we always round toward conservatism.
Now we can get down and argue about the details and those of you who got the wrong answers can try to explain why you think you were really right.
These answers do not give all the salient details that many of you have included in your posts details count as extra credit unless your details are wrong in that case off with your head!
BTW . A couple of you did a stellar job as I would have expected from your previous post.
The Rule of Thirds
1) What is it? Answer: 1/3 of your gas is for going in, 1/3 of your gas is for coming back, and 1/3 or your gas is for your buddy in the event of an OOA at the furthest point.
2) What dives require the rule of thirds? Answer: ANY dive where egress can only safely be made at the point of entry.
3) When is the rule of thirds not enough? Answer: ANY dive where egress cannot be made at the point of entry or where more gas is require to exit than to enter.
4) When is the rule of thirds too much? Answer: ANY dive where access to the surface is available at every point in the dive.
5) What is the rule of halves? Answer: AFTER subtracting Rock Bottom, ½ the gas for going out, ½ the gas for coming back as long as the surface is available for egress at every point in the dive.
6) What is rock bottom? Answer: The gas needed for you and your OOA buddy to make it to the surface (or to the first gas switch) doing all stops and with a safe ascent rate.
7) What dives require only rock bottom? Answer: Dives that are not directionally constrained.
These answers present the minimum but adding in additional gas without knowing the baseline is just a shot in the dark. If you feel that you would like to pad your gas supply that is fine. In fact when we figure 1/3s or 1/2s or Rock Bottom we always round toward conservatism.
Now we can get down and argue about the details and those of you who got the wrong answers can try to explain why you think you were really right.
That answered my question.
Spectre
Contributor
So... was this a freak thread borne of boredom, or do you plan on making this a regular feature?
Hi Chad,
I don't dive stages. this was info I read,
there was a half + (so there was a little more for the way back and/or margin of error on guages.
Since my answer was wrong according to Uncle Pugs answers I'm going to try to find were I read it. If I can find it again I'll post it here.
Hey PUG
good benificial thread.
throw out another one, and next time quote the text where you got the answers from, just to verify it is not oppinions.
with that said i agree with all your answers, just think it would be good in future tests
good benificial thread.
throw out another one, and next time quote the text where you got the answers from, just to verify it is not oppinions.
with that said i agree with all your answers, just think it would be good in future tests
Thanks for the reply Tavi.
Chad
Chad
here is my take on it I copied it from one of my post in another thread.
when calculating your gas requirements use your back gas as the 1/3 for emergencies.
and use your satged bottles as 50% in and 50% out. less 300psi
so if you are carring dble 80's then you would have 160 cu/ft of reserve gas. this would then allow for four staged bottles of 80 cu/ft each. or 320 cu/ft of gas.
on the way in you breath 40 cu/ft from each tank [your are carring on your back enough gas to get out nwithout using any of the stage bottles.
you then breath the other half of each bottle on the way out, as you consume each bottle and get closer to the entrance your reserve % is acualy increased. because you have not used any of the gas on your back.
so your penetration would be based on how far you can travel in one direction on 160 cu/ft of gas.
keeping in mind we are not discussing any decompression here
this is just my aproach to your question. I am sure those who do this type of diving a lot have a procedure for this type of diving.
__________________
when calculating your gas requirements use your back gas as the 1/3 for emergencies.
and use your satged bottles as 50% in and 50% out. less 300psi
so if you are carring dble 80's then you would have 160 cu/ft of reserve gas. this would then allow for four staged bottles of 80 cu/ft each. or 320 cu/ft of gas.
on the way in you breath 40 cu/ft from each tank [your are carring on your back enough gas to get out nwithout using any of the stage bottles.
you then breath the other half of each bottle on the way out, as you consume each bottle and get closer to the entrance your reserve % is acualy increased. because you have not used any of the gas on your back.
so your penetration would be based on how far you can travel in one direction on 160 cu/ft of gas.
keeping in mind we are not discussing any decompression here
this is just my aproach to your question. I am sure those who do this type of diving a lot have a procedure for this type of diving.
__________________
OP
now lets get down to business.
First thing we gotta remind ourselves about is that definitions are neither universal nor precise... for instance... cavers might think of one thing when they hear rule of thirds and tech divers might think of something else. Same goes for DIR vs non-DIR, ect.
Here on Scuba Board we have a broad representation of dive disciplines as well as across the gamit in terms of experience.
That said we need to be flexible in our discussion allowing others their point of view.
Doug brought forth an interesting item.... quoting the text where I got the answers from. Well the fact is I didn't get the *correct* answers from a text! And you are to feel free to correct my answers too
I posted this in the basic scuba discussions forum because I feel that too much cave/technical stuff is being adopted without understanding the reasoning behind things.... and this can lead to absurdities such as using the rule of thirds for 30fsw live boat reef dives. I can also lead to potentially dangerous situations like using the rule of thirds on a dive from an anchored boat.
Let me explain that last one.
If we use the rule of thirds using one third of our gas out away from the anchor line and plan to use one third back but have an OOA right at the turn point we do have enough gas to get back to the anchor line if all goes well... but we are now out of gas at the anchor line and still need to make an ascent. Oooops.
This is where the concept of rock bottom comes into play... cavers do not use this because their point of egress is the same as their point of entrance. OW divers still need to make the ascent.
So we must figure our Rock Bottom... the amount of gas to get both of us safely up to the surface (or first gas switch in deco diving.) This Rock Bottom amount is subtracted from the gas supply before figuring thirds.
First thing we gotta remind ourselves about is that definitions are neither universal nor precise... for instance... cavers might think of one thing when they hear rule of thirds and tech divers might think of something else. Same goes for DIR vs non-DIR, ect.
Here on Scuba Board we have a broad representation of dive disciplines as well as across the gamit in terms of experience.
That said we need to be flexible in our discussion allowing others their point of view.
Doug brought forth an interesting item.... quoting the text where I got the answers from. Well the fact is I didn't get the *correct* answers from a text! And you are to feel free to correct my answers too
I posted this in the basic scuba discussions forum because I feel that too much cave/technical stuff is being adopted without understanding the reasoning behind things.... and this can lead to absurdities such as using the rule of thirds for 30fsw live boat reef dives. I can also lead to potentially dangerous situations like using the rule of thirds on a dive from an anchored boat.
Let me explain that last one.
If we use the rule of thirds using one third of our gas out away from the anchor line and plan to use one third back but have an OOA right at the turn point we do have enough gas to get back to the anchor line if all goes well... but we are now out of gas at the anchor line and still need to make an ascent. Oooops.
This is where the concept of rock bottom comes into play... cavers do not use this because their point of egress is the same as their point of entrance. OW divers still need to make the ascent.
So we must figure our Rock Bottom... the amount of gas to get both of us safely up to the surface (or first gas switch in deco diving.) This Rock Bottom amount is subtracted from the gas supply before figuring thirds.
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