The Rule of Thirds.

[Spectre]

Uncle Pug once bubbled...
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.

Ok teach... ya lost me. Now I don't cave dive yet, so maybe I'm misunderstanding.

What I read above is. "Cave divers start their 1/3 when they enter the water, and end their 1/3 when they exit the water". "OW divers start their 1/3 when they leave the anchor, and end their 1/3 when they return to the anchor line".

If I was to dive thirds in open water... I would have always assumed it would be from the point of entry in the water to the point of exit in the water... Interpreting rule of thirds from leaving anchor line to arrival at anchor line, IMHO, is an error in the adaptation of 1/3s to OW diving.

[disclaimer... my method....]
Since many of my current buddies don't have an accurate SAC rate at this point in time, I can't dive true rock bottom, so I currently dive OW with my ascent going at my rock bottom [with plenty of leeway] + 500 psi. My pony also contains a personal rock bottom amount as well... so in a pinch, I've got my rock bottom in the pony, and my buddy gets my rock bottom + 500 that's on my back.

Doah! I just realized I gave away my secret. I kept telling my buddies I have the pony 'cause I don't trust them!

 

[AquaTec]

PUG said
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.

good points again PUG, I am worried that you are getting wise in your old age and a little soft as well.

It appiers that a lot of people on this board get their diving education here and on the net or even in books, without getting any true instruction.

tests like this are valuable and fun, but it is important that everybody knows that nothing on this board can replace training, no matter what agency they get it from

 

[Uncle Pug]

Spectre once bubbled...
If I was to dive thirds in open water... I would have always assumed it would be from the point of entry in the water to the point of exit in the water...

Keep in mind the scenario of an anchored boat this means that you must arrive back at the anchor line.

How much gas does it take you to descend and go out to the turn point? Will that same amount suffice to return to the anchor line and ascend? No... and that is why the rule of thirds does not work in this case. Go back to the answer of #3. It takes more gas because the ascent takes longer than the descent.

Rock Bottom is the amount set aside for the ascent.

 

[Aquamaniac]

Uncle Pug once bubbled...


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.

I have a question pertaining to this VERY good thread.

We always dive up current from the anchor line....ALWAYS.
I always know what my "Ascent gas" will be.
Therefore if we are on our way back to the Anchor, on our second third, and we have an OOA emergency, our proceedure is to keep on traveling toward the anchor until we hit our last third PLUS Ascent gas, then we stop and ascend AT THAT POINT, Still leaving our last third as the safety
From here we drift back to the boat on the surface,and take the mickey out of the man who went OOA.

Any thoughts on that model?

Dave

 

[Spectre]

Uncle Pug once bubbled...
How much gas does it take you to descend and go out to the turn point? Will that same amount suffice to return to the anchor line and ascend?

Ok... so then how does the rule of thirds work with cave diving. Don't they have to deal with the same issue? decent being faster than ascent?

 

[Aquamaniac]

Aquamaniac once bubbled...


I have a question pertaining to this VERY good thread.

We always dive up current from the anchor line....ALWAYS.
I always know what my "Ascent gas" will be.
Therefore if we are on our way back to the Anchor, on our second third, and we have an OOA emergency, our proceedure is to keep on traveling toward the anchor until we hit our last third PLUS Ascent gas, then we stop and ascend AT THAT POINT, Still leaving our last third as the safety
From here we drift back to the boat on the surface,and take the mickey out of the man who went OOA.

Any thoughts on that model?

Dave

I posted that before I read your response UP, I see where your heading now......

 

[AquaTec]

Uncle Pug once bubbled...
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.
.

PUG
one problem with your theory.

In your senario why would you not emediatly ascend to the surface. [of course still using proper ascent rates etc.

I do agree with what you are trying to say. on an open water recreational dive why do you need to use the rule of thirds. if there is an out of air situation just surface!!

 

[Uncle Pug]

Dave... we do the same thing if we have current... we use it... but the plan does count that because up here currents are unpredictable... they can be going different ways at different depths.

Jeff... remembering we are primarily talking about NDL diving here in this forum, what do cavers do when the ascent takes longer than the descent, say in a cave that has an initial vertical drop of 80 feet?

 

[Uncle Pug]

AquaTec once bubbled...
In your senario why would you not emediatly ascend to the surface. [of course still using proper ascent rates etc.

For this scenario that is not possible... if that were a possibility then we would be talking about the rule of halves.

In this situation the divers must make it back to the up line.... we could come up with other specific examples of this... I just chose the anchored boat scenario.

 

[Uncle Pug]

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.