The Rule of Thirds.

[Spectre]

Uncle Pug once bubbled...
I need to answer your question better... the problem is... I am not a caver... but as I understand it the rule of thirds works since the path back is not always vertical and it takes the same amount of time going in as it does coming back out.

Ok... I'll buy that [while we wait for a caver to correct us].

So... in summation...

Rule of thirds: When you must exit at the same point as entry, and the gas needs for exit == the gas needs for entry.

If gas needs for exit is greater than gas needs for entry, then you need a rock bottom reserve. total gas - extra gas needed for exit = gas supply to be used in calculating 1/3s.

So for the example on hand:

Given: 3500 psi. 1000 psi rock bottom.

When arriving at anchor line on decent, you've burned 400 psi. 3500 - 1000 = 2500 - 400 = 2100.

So 700 out, 700 back to anchor, 700 reserve.

Do you concur, or do I need more lessons?

 

[Uncle Pug]

Spectre once bubbled...
Given: 3500 psi. 1000 psi rock bottom.

When arriving at anchor line on decent, you've burned 400 psi. 3500 - 1000 = 2500 - 400 = 2100.

So 700 out, 700 back to anchor, 700 reserve.

I would figure this 3500-1000=2500 round down 2400 / 3 = 800
3500-800=2700 turn pressure.

Whatever is used in the descent is part of the first 1/3...


400 for descent ?!?! How deep are you doing?

 

[AquaTec]

Uncle Pug once bubbled...
I need to answer your question better... the problem is... I am not a caver... but as I understand it the rule of thirds works since the path back is not always vertical and it takes the same amount of time going in as it does coming back out.

Cavers please correct me here.

PUG I will thow in my 2 cents. I do have a full technical cave certification, however my caving experience is not as in depth as others, or that i would like it to be.

in general terms you figure your 1/3 from the time you leave the surface. not a lot of caves have deep descents prior to going horizantal. gas consumtion is more a fact of flow of the cave. most dives are conducted on the spring side or against the flow [current] and then coming back with the flow. you use 1/3 going in and probably less coming out. [to the surface]
I was recently at Gennie Springs and after a 45 minute swim in the swim out took about five minutes. so obviously the gas consumption was less coming out.

however you don't plan for that, we turned the dive upon consumption of the first 1/3, needless to say on the way out we only used a few 100 psi

not withsatnding any deco requirements

 

[Spectre]

Uncle Pug once bubbled...
400 for descent ?!?! How deep are you doing?

Just an example to make the math easy

 

[Uncle Pug]

AquaTec once bubbled...
in general terms you figure your 1/3 from the time you leave the surface. not a lot of caves have deep descents prior to going horizantal.

That is what I had surmised.

 

[AquaTec]

to your supprise i was simply agreeing with you

 

[Uncle Pug]

AquaTec once bubbled...
to your supprise i was simply agreeing with you

...that I got it right. Never been in a cave... Shane wants to go to Florida and take cave training... I'm game... but... well I guess I'm not sure I see the allure...

 

[MikeFerrara]

In cave, or for that matter in any overhead thirds is the most liberal gas plan we use. When diving a siphon (in flow) or a cave with no flow or a dive that is tricky for any reason we save more gas. As of yet I don't scooter but that requires a modification of the gas plan. The idea is to always have at LEAST twice the gas needed to exit.

At times we do what some call a recalc. There are lots of possible applications but lets say I'm on a cave dive and know my turn presure. I'm on my way out but spot a side tunnel I want to look at. IF, I know how much gas I need to exit from that point, I can reserve twice that to exit (from that point) and calc thirds again on the remaining gas as a plan for checking out the new passage.

Or maybe we get in to a certain point and instead of traveling further we want to poke around the area were in do that alot since distance means nothing to me I want to see the cave. The guys who just eat up line go far and get to brag but they miss alot of great stuff. I reserve at LEAST twice what I need to exit and have the rest to play.

The rule of thirds or any other rule is a guid. The idea is to always have more than what you need to get back.

Of course you need to take into account that you can't empty a tank. You need IP plus ambient in the tank to be able to breath. You also need to account for back gas that will be used for deco.

When I calc thirds I take (start presure - (something to power the reg) - deco gas)/ 3

Rarely do we use a full third going in. The rule of thirds assums that you can get out on the same gas you got in with. If you move slower of breath harder or anything that causes you to use more gas you won't make it.

Nobody ever died for having too much gas.

We add gas for things like

unfamiliar cave
new team members
no flow
siphon
bad vis
complex caves (tricky line layout)
if we don't feel lucky
If we see the dive as stressful (phisicly or mentally)

 

[Divesherpa]

Hey Pug,
A couple of points
1) regarding your answer to question #1, 1/3's is not JUST for your OOA buddy. How many buddies run out of air? 1/3's is more important for catastrophic failures and general safety. No one should ever be out of air in an environment that requires 1/3's unless they have a catastrophic failure, severe entanglement, or lost line in overhead. (maybe something else that is left off)
2) What type of diving "MUST" the diver return to the anchor line for? This may be another thread

Cheers

 

[Uncle Pug]

Divesherpa once bubbled...
2) What type of diving "MUST" the diver return to the anchor line for?/B]

MUST is a relative word.... we would not expect someone to run completely OOA and drown when the surface was available...

However.... there are many dives where it is very important for one to exit at the point of entry and in OW boat diving or even certain shore dives this will require more than thirds to dive responsibly.

I will post a few examples later but want to give others a chance to answer this question:

What recreational diving scenarios might require more than the rule of thirds?

Please folks... remember the location of this thread... general scuba discussions.

The reason I posted this and posted it here is because too many recreational divers are trying to borrow Tech/cave procedures without understanding how and when to apply them.

The rule of thirds is uncalled for in shallow reef diving... and because of that there are those who would disregard the whole idea for any and all recreational diving.

However there are recreational dives where MORE than the rule of thirds is called for!

So for all you tech/cavers responding to this thread try to keep that educational goal in view and let's help the recreational divers apply the rules appropriately to various recreational dives.