When thirds are not enough....

[boulderjohn]

The content of this post will be very obvious to a lot of people, perhaps even to most of the people who visit this forum, but based on some discussions I have heard and especially discussions related to a recent incident, it might be good to talk about it in for the benefit of some who have not really thought about it enough. If only one person benefits from this by changing a dive plan appropriately, then this thread will have served its purpose.

We know the main idea of the rule of thirds for overhead environments: go in for a third of your gas, come out with a third of your gas, and leave a third for emergencies. The problem is that thirds are not always enough, and one needs to think about the circumstances of the dive and look beyond the simplicity of that math.

One reason is that all thirds are not created equal.

Let's say two divers do a dive to thirds in a low flow system, starting with twin 108's and a nice cave fill to 3,600 PSI. They turn as planned and have a faster exit than expected, leaving the cave with half their gas (1,800 PSI). Having nothing to do for a while, they decide to give it another go with their remaining gas, this time turning the dive at the new third, or 1,200 PSI.

Let's think of this in terms of time. On the first dive, depending upon their depth, that third held in reserve will give them 20-30 minutes to solve a problem such as a silt out, lost line, lost buddy, etc. On the second dive they will have half that time, or 10-15 minutes, to solve that problem.

An untrained diver who decides that the rule of thirds will let him enter that same cave with an aluminum 80 will have maybe 5-8 minutes to solve the problem.

There are many other circumstances why one would not apply the rule of thirds to a cave dive, and I hope others will share their thoughts as well. The bottom line is that you have to think things through and always be sure you have plenty of gas before starting a dive.

 

[fire_diver]

Don't know what else to add. All sound advice.

 

[DiverLS]

Thirds will not apply with dissimilar tanks. There is a special computation when that happens and I just barely can do it. That's why I always take that formula with me.

I cave dive with twin steel 84's with a cave fill. My buddy is on a rebreather. I brought up that his 40 bail out tank would not be enough for both of us in case of catastrophic failure. So I carry a 40 with a 40 deco tank that I drop. He carries an 80. Then we compute my 1/3rds and use that as the turn time.

Another time where 1/3's would not be appropriate is with a sump dive. There you might want to dive 1/5ths. I have not done a sump dive but I can see it in the future.

 

[scadreau]

Let's say two divers do a dive to thirds in a low flow system, starting with twin 108's and a nice cave fill to 3,600 PSI. They turn as planned and have a faster exit than expected, leaving the cave with half their gas (1,800 PSI). Having nothing to do for a while, they decide to give it another go with their remaining gas, this time turning the dive at the new third, or 1,200 PSI.

I would hope that if they did a 2nd penetration, they would have planned the turn at 1500. You should still end all of your diving with 1200 no? If anyone is diving on the principle that they can do another penetration on the same gas and start a new rule of thirds is certainly not being prudent to themselves or their buddy(s). What if the above scenario they exited the first dive with 1500psi. Hey we can go back and turn at 1000psi exiting with 500.

 

[Cave Diver]

Other places thirds don't apply: When diving low flow or syphon systems.

In low flow, you don't have the benefit of riding the flow out and your exit is likely to be slower as you tire during the dive.

In a syphon, you're fighting flow coming out and you're going to use more gas exiting than going in. If you dive to thirds, you're gonna come up short on the exit.

Thirds should be thought of as a minimum starting point.

 

[ucfdiver]

I would hope that if they did a 2nd penetration, they would have planned the turn at 1500. You should still end all of your diving with 1200 no? If anyone is diving on the principle that they can do another penetration on the same gas and start a new rule of thirds is certainly not being prudent to themselves or their buddy(s). What if the above scenario they exited the first dive with 1500psi. Hey we can go back and turn at 1000psi exiting with 500.

Why would you end all of your diving with 1200psi?

 

[tstormdiver]

Thirds by far are not a conservative gas management plan. If, both divers breathing nearly the same rate, with the same sized cylinders, 1 has a catastrophic gas failure at the maximum penetration of a low/ no flow cave, there can be many things that can affect how fast they can get out & breathing rates- Silt out that forces the divers online can significantly slow the team's egress. Just the general stress of the situation can increase respiration rates. Any of these could make exiting the cave alive questionable, when using the rule of thirds. In theory it should work, in the real world (with all its other variables) it may or may not.

 

[Cave Diver]

I cave dive with twin steel 84's with a cave fill. My buddy is on a rebreather. I brought up that his 40 bail out tank would not be enough for both of us in case of catastrophic failure. So I carry a 40 with a 40 deco tank that I drop. He carries an 80. Then we compute my 1/3rds and use that as the turn time.

I'm glad you've revised your gas plan. I would contend that a 40 cft was not sufficient bailout for either of you, never mind both.

If anyone is diving on the principle that they can do another penetration on the same gas and start a new rule of thirds is certainly not being prudent to themselves or their buddy(s).

That really depends on how you apply it.

Do you do a second dive and penetrate again til you hit thirds, then turn and come back, or do you recalculate thirds and spend half an hour poking around just past the cavern zone as you breathe your tanks down. In either case, you're still in an overhead and you're still down to recalculated thirds. The difference is the distance from the exit and how much time it will take you to reach it if you have a problem.

 

[ucfdiver]

People dive 1/3rds of AL80's all the time, if you can do that, then there's no logical reason you can't dive 1/3rds of HP130's when they're half full.

Sometimes I wonder how many people are ubber conservative on the internet, who don't actually do dives where these types of logistics really matter anyways. I had to get the sh!t scared out of me one good time before I really implemented any thoughtful process of backing off penetration gas, as did most everyone else I know who adds reserves.

 

[SuPrBuGmAn]

Restrictions, navigational decisions, new cave, new buddies, low flow, no flow, direction of flow, number of dive buddies(or solo), alternative(verified) exits, DPVs and many many other reasons are going to affect turn pressure one way or another.

1/3rds is a starting point that really gets used extremely rarely for me...