diving thirds vs rock bottom

[lowviz]

This is "Basic Scuba Discussions" and thus, it may be appropriate to speak to the idea of "Rock Bottom".

Imagine two OW divers who are following their dive plan. Let's say one of them suffers a catastrophic loss of gas. The amount of gas that the second diver is carrying becomes all-important.

The absolute minimum amount of gas that will JUST get both of them to the surface, unhurt, and OUT OF AIR ON THE SURFACE (at the desired exit) is called the rock bottom gas reserve. This is not a "friendly" value. This is not a cushy, ideal gas load that will make life comfy. It the freaking minimum amount of gas that will just avoid bodily harm or death.

IJS...

 

[Peter Guy]

Bluek -- Adobo answered your question to me pretty well but I'd like to reitereate something -- the defintion of Rock Bottom (at least as I understand it). Rock Bottom is the amount of gas needed to get you and your buddy back to your next gas switch making all appropriate stops. For "Recreational Divers" their "next gas switch" is to that big tank in the sky -- aka the surface. For the "Technical Diver" their next gas switch would be whatever depth they had planned -- 20 feet (100%); 70 feet (50%); whatever.

For Recreational Divers RB can be, and should be, pretty simple so that you can, in fact, "refigure" RB "on the fly." Of course you are not REALLY refiguring it but, as you ascend up the wall or the reef, you just go to your next pre-calculated number -- 40 cu. Ft at 100 feet; 25 cu. ft at 60 ft; 500 psi at 30 feet. IF you know those three numbers, you can pretty much do any "Recreational Dive" with a safe RB -- remember, it ain't an exact science so you don't need exact numbers.

 

[TSandM]

Peter made the comment that RB and thirds address different aspects of the gas planning, but I want to correct that a little bit. It is my understanding (and I believe this is what the OP was asking) that some people teach RB plus gas apportionment, and others simply teach students to use thirds. As an example, for the diver doing a 60 foot reef dive using an Al80 filled to 3000, the way Peter and I would do the gas planning would be to take 20 cf of gas and set it aside as RB (that's 800 psi) and then divide the remaiing 2200 psi up into all available, halves or thirds, depending on the dive. Someone taught the other way would use 1000 psi out of the tank and then start the process of ending the dive. On a drift dive to 60 feet, that would significantly shorten the diver's bottom time (1000 psi versus 2200 our way) and the diver would surface with scads of unused gas. Now, I don't object to unused gas on principle, but I do look askance on a policy that mandates short bottom times for no particularly good reason.

Kevin, you are correct that the tank matching calculations are easy in metric. I don't live in metric, and neither do our students. Metric isn't big in Florida caves, either.

And with respect to the idea of thirds having no place in recreational diving . . . yeah, if I'm supposed to get back to the anchor line and the captain is going to be mad if I don't, or have trouble finding me, I'm better off planning gas on thirds. But in extremis, on such a dive, I am going to surface -- so long as I don't have a decompression obligation, I CAN, and I will do it rather than drown. In a cave, or with a major deco obligation, I don't have that option. That's what I mean by "true thirds" dives. On such dives, one MUST solve problems underwater, no matter what they are, because there is no other possibility. Doing that requires training and experience beyond that of the usual recreational diver.

 

[lowviz]

Lynne,

I tend to turn on thirds and finish on RB, if that helps...

 

[boulderjohn]

the defintion of Rock Bottom (at least as I understand it). Rock Bottom is the amount of gas needed to get you and your buddy back to your next gas switch making all appropriate stops.

... and others simply teach students to use thirds.

...
Now, I don't object to unused gas on principle, but I do look askance on a policy that mandates short bottom times for no particularly good reason.

I believe Rock Bottom has its origin in the GUE system. Many people who are used to the phrase will be surprised to learn that the term and concept are not universally known throughout the dive world, and I assure you there are technical diving instructors who have never even heard of it. I know some who never heard of it until I described it to them, and they were skeptical. They do, however, know the rule of thirds as they were taught it--begin your ascent with a third of your gas left. Lynne's description of how wasteful it would be on a 60 foot dive breathing air is accurate. Think how wasteful it is when you are at 260 feet breathing 55% super expensive helium and you reserve a full third of that gas (80 cubic feet) for the ascent to your first gas switch at 190 feet. If it takes you 2 minutes, you might use 7 cubic feet of it.

 

[Adobo]

Tell that to the captain. YOU WILL ascend the downline...

If you add waypoints to your planning you will know if you are staying on track. As Peter alluded:

1) NEVER violate rockbottom, especially at the "worst point" of your dive.
2) Following thirds is a brain-dead way to stay true to #1, but you have to verify this BEFORE the dive.

-so easy even a [-]caveman[/-] sorry, OW student can do it...

Two recreational divers go on this boat and are given those instructions. They descend the anchor line and proceed on their dive. Just when they are about to turn the dive (meaning, they are at their furthest point away from the anchor line), one of the divers experiences a catastrophic failure with his gas supply. They proceed to execute a gas share per their training.

Question: Is the captain expecting these two divers who are now sharing gas to return back to the anchor line before they begin their ascent? Or, is the expectation that they will begin their ascent immediately even though they are at their furthest point away from the anchor line?

Calculating thirds is really easy. Turning the dive when you have hit thirds is easy. Neither is sufficient to get a pair of divers to the anchor line with enough gas to get them to the surface in cases where they run into common failures.

 

[GrumpyOldGuy]

The absolute minimum amount of gas that will JUST get both of them to the surface, unhurt, and OUT OF AIR ON THE SURFACE (at the desired exit) is called the rock bottom gas reserve.

A proper rock bottom calculation includes factors to account for a stress related SAC increase, a minute or so to sort things on the bottom before ascending, a safety stop and tolerance for SPG accuracy. So should you need it, chances are you should have gas left over.

The obvious variable is knowing what your stressed SAC is, never mind you buddies, is a crap shoot no matter how precise your calculations are.

 

[boulderjohn]

A proper rock bottom calculation includes factors to account for a stress related SAC increase, a minute or so to sort things on the bottom before ascending, a safety stop and tolerance for SPG accuracy. So should you need it, chances are you should have gas left over.

The obvious variable is knowing what your stressed SAC is, never mind you buddies, is a crap shoot no matter how precise your calculations are.

Correct. Because this is the Basic Scuba forum, if we are talking about NDL dives, and if we have planned for all normal stops taken during an ascent, then we should have wiggle room in the form of the safety stop. It's a nice thing to have, and it should be included in calculations, but if we are really going through the air, it can be skipped.

Decompression diving is another issue, and that is one of the reasons that the training for decompression diving is as intense as it is, so that the divers are not stressed enough to blow through all available gas.

 

[CamG]

Great read to sort out the use of RB and 1/3rds pushing the limits a bit for the Basic Forum but still a good read.
I think even the novice reading this thread will obtain valuable information and certainly be able to research it further.
When applying these concepts to Rec. dives it will lead to more conservatism which in my opinion is a good thing.
I am not trying to pick on any agency or instructor just promoting independent research and growth is a positive.

CamG Keep Diving....Keep Training....Keep Learning!

 

[lowviz]

Two recreational divers go on this boat and are given those instructions. They descend the anchor line and proceed on their dive. Just when they are about to turn the dive (meaning, they are at their furthest point away from the anchor line), one of the divers experiences a catastrophic failure with his gas supply. ...//....

Could happen, so you plan for it. Stay close enough to the tie-in to manage this potential threat.

...//... Is the captain expecting these two divers who are now sharing gas to return back to the anchor line before they begin their ascent? ...//...

Yes.

...//... Calculating thirds is really easy. Turning the dive when you have hit thirds is easy. Neither is sufficient to get a pair of divers to the anchor line with enough gas to get them to the surface in cases where they run into common failures.

It can be done.