diving thirds vs rock bottom

[boulderjohn]

Just to clarify, rock bottom is part of recreational 1 UTD course (basic open water), apology accepted.

I'm confused. I think you actually name two different UTD courses in your answer. By Recreational I, do you mean Essentials of Recreational Diving, the class you take before Recreational II? If so, that is not the Basic OW course. Here is the description of the first three courses in the UTD recreational path:

Your path begins with Open Water, UTD’s primary scuba certification class. Here you’ll learn the basics of diving – proper buoyancy, kicks that do not stir up the environment, ascent planning, and emergency procedures. This class generally takes four days and will certify you to dive to 60’/18m.

After a few dives, it’s time for Essentials of Recreational Diving. This is a personal skills class that builds on your Open Water certification. Here you’ll refine your buoyancy skills, add more kicks, including the backward kick, and continue to work on ascents and emergency procedures.

Then it’s time to go deeper. UTD’s Recreational 2 class will train you to depths of 100’/30m using an advanced gas called Nitrox. Nitrox has less nitrogen than air, so you are able to do longer bottom times. In Rec 2 you will also experience a night dive and more advanced emergency training.​

What percentage of of the world's OW divers do you think are trained by UTD? (BTW, I hold 5 UTD certifications, so I am not a stranger.)

 

[GrumpyOldGuy]

Thirds takes into account sac rates. Example (assuming same size cylinders)....

You have a good point there, but SAC takes in to account a constant SAC rate for the longest possible duration.

Rock bottom is usually done using a theoretical stressed SAC rate (typically 2X normal) for a shorter duration (just enough to time ascend with stops).

If you are truly in a dive where you must return to your entry and you have to share air from the most distant point, chances are both you and your buddy will have a somewhat increased rate of gas use compared to the swim out and the 2/3rd won't be enough.

 

[bluekieran]

"Rock Bottom" is merely (!) about the concept that a diver should always have enough gas to get him and his buddy back to the surface safely while making all planned stops. It is a dynamic number (that is, RB for 100 ft is different from RB for 30 ft) and is based on certain assumptions as to how much you and your buddy will consume (stressed SAC rate). The "Rock Bottom" number is a safety margin number.

Out of interest, how do you deal with rock bottom calculations during a dive?

For a no-deco dive, I figure it's pretty simple: you could just have "RB-10m: 60bar, RB-20m: 85bar, RB-30m: 100bar" (made up numbers) among the notes on your slate. But throwing in deco obligations that increase throughout the dive must complicate that horribly... unless you just assume for rock-bottom calculation purposes that you'll need to make all the deco your full dive plan calls for?

 

[Adobo]

You have a good point there, but SAC takes in to account a constant SAC rate for the longest possible duration.

Rock bottom is usually done using a theoretical stressed SAC rate (typically 2X normal) for a shorter duration (just enough to time ascend with stops).

If you are truly in a dive where you must return to your entry and you have to share air from the most distant point, chances are both you and your buddy will have a somewhat increased rate of gas use compared to the swim out and the 2/3rd won't be enough.

Divers with only recreational dive training should really not be doing any dives where they *must* return to the entry point. Such a dive requires more than knowing how to divide by 3. It requires guideline and failures management.

---------- Post Merged at 11:51 AM ---------- Previous Post was at 11:44 AM ----------

Out of interest, how do you deal with rock bottom calculations during a dive?

For a no-deco dive, I figure it's pretty simple: you could just have "RB-10m: 60bar, RB-20m: 85bar, RB-30m: 100bar" (made up numbers) among the notes on your slate. But throwing in deco obligations that increase throughout the dive must complicate that horribly... unless you just assume for rock-bottom calculation purposes that you'll need to make all the deco your full dive plan calls for?

You don't do minimum gas calculations during the dive. You do those calculations on your kitchen table days before you do the dive. You establish the parameters and limits for the dive and you obey those parameters and limits so that you know that whatever resources you are bringing for the dive will be sufficient.

If you don't establish any boundaries for a dive (depth, time, no deco obligation, etc), you can't know what resources will be required and sufficient in either an uneventful dive or a dive wherein you experience a *common* emergency.

This approach is applicable to a recreational dive and it is applicable to a tech/cave dive. The steps for the latter are slightly more involved but the mind set is the same.

 

[PfcAJ]

Out of interest, how do you deal with rock bottom calculations during a dive?

For a no-deco dive, I figure it's pretty simple: you could just have "RB-10m: 60bar, RB-20m: 85bar, RB-30m: 100bar" (made up numbers) among the notes on your slate. But throwing in deco obligations that increase throughout the dive must complicate that horribly... unless you just assume for rock-bottom calculation purposes that you'll need to make all the deco your full dive plan calls for?

RB for deco dives is pretty straight forward. Think of RB as a limit. Your other limit for the dive is time. When either time or rock bottom is reached, the thumb comes out and everyone heads home. If you have to leave the bottom early, you will either have an excess of gas (this is good) or be at your rock bottom value, but before the end of your planned bottom time. This is ok, too, since everyone has enough gas to get their buddy to the first gas switch.

It all comes down to solid dive planning, and executing the plan.

 

[GrumpyOldGuy]

Divers with only recreational dive training should really not be doing any dives where they *must* return to the entry point. Such a dive requires more than knowing how to divide by 3. It requires guideline and failures management.

Yup!

There is still value though in understanding the different techniques of gas planning (the good, the bad and the ugly) and when when each is appropriate even if someone (like me) is a recreational diver.

 

[decompression]

I'm confused. I think you actually name two different UTD courses in your answer. By Recreational I, do you mean Essentials of Recreational Diving, the class you take before Recreational II? If so, that is not the Basic OW course. Here is the description of the first three courses in the UTD recreational path:

Your path begins with Open Water, UTD’s primary scuba certification class. Here you’ll learn the basics of diving – proper buoyancy, kicks that do not stir up the environment, ascent planning, and emergency procedures. This class generally takes four days and will certify you to dive to 60’/18m.

After a few dives, it’s time for Essentials of Recreational Diving. This is a personal skills class that builds on your Open Water certification. Here you’ll refine your buoyancy skills, add more kicks, including the backward kick, and continue to work on ascents and emergency procedures.

Then it’s time to go deeper. UTD’s Recreational 2 class will train you to depths of 100’/30m using an advanced gas called Nitrox. Nitrox has less nitrogen than air, so you are able to do longer bottom times. In Rec 2 you will also experience a night dive and more advanced emergency training.​

What percentage of of the world's OW divers do you think are trained by UTD? (BTW, I hold 5 UTD certifications, so I am not a stranger.)

Yes, John, I know you are affiliated with UTD,so this should just be a reminder for you. Essentials of rec is the intro course for qualified divers (like GUE fundies). Rec 1 is the entry level course (open water equivelant), Rec 2 is similar to advanced (includes deep and Nitrox), Rec 3 expands on the previous and includes heliox. And as I mentioned, part of the curriculum for Rec 1 includes rock bottom, gas selection, gas planning, dive planning and situational awareness.

As UTD has only been around for a few years I'd say the percentage is quite low. But you know this already. As you went through the considerable effort to write your own course to include these things I can only assume you consider them important as well. You must also see something in UTD having taken 5 courses.

Unless I'm mistaken, i find Your tone condescending and I find that unfortunate as I've often read your posts and find them insightful and informative. The curriculum for the Rec 1 course isn't detailed in the course description online but it is in the instructor guide. Safe diving.

 

[lowviz]

Divers with only recreational dive training should really not be doing any dives where they *must* return to the entry point. Such a dive requires more than knowing how to divide by 3. It requires guideline and failures management. ...//....

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...

 

[boulderjohn]

Yes, John, I know you are affiliated with UTD,so this should just be a reminder for you. Essentials of rec is the intro course for qualified divers (like GUE fundies). Rec 1 is the entry level course (open water equivelant), Rec 2 is similar to advanced (includes deep and Nitrox), Rec 3 expands on the previous and includes heliox. And as I mentioned, part of the curriculum for Rec 1 includes rock bottom, gas selection, gas planning, dive planning and situational awareness.

As UTD has only been around for a few years I'd say the percentage is quite low. But you know this already. As you went through the considerable effort to write your own course to include these things I can only assume you consider them important as well. You must also see something in UTD having taken 5 courses.

I'm sorry you found the tone condescending. It was not intended to be so. My question was genuine. The descriptions and names for the courses you have in your post do not match the descriptions on the UTD web site, so I honestly didn't know which ones you are talking about.

 

[decompression]

I'm sorry you found the tone condescending. It was not intended to be so. My question was genuine. The descriptions and names for the courses you have in your post do not match the descriptions on the UTD web site, so I honestly didn't know which ones you are talking about.

Whew, thanks John. Ill talk to AG and see if we can put a better description of the courses online. Cheers!