Questions about Rock Bottom Gas PLanning, Fundies

[Soggy]

But, DIR-Atlanta, wouldn't you agree that an actual calculated RB should be based on an actual SAC rate known for both (or all three) team members? By that criterion, any published "rule" is a rule of thumb.

IMO, it should be based on at least 1.5x a normal working SAC rate, which for most people brings it in the .75-1 cft/min range. 1 cft/min is incredibly easy to calculate, which is a benefit for on-boat quick calculations, so even though it is conservative, I use that pretty much regardless of who I am diving with because 1 cft/min for each diver for the entire range of the ascent is going to cover pretty much anything.

 

[H2Andy]

I know this may seem like nit-picking, but technically
speaking, those are not really "rules of thumb".

i don't recall what the DIR-F manual calls them, but they are essentially
pre-calculated rock bottom figures for specific tanks

 

[UnixSage]

This is the chart I use.. and dont yell at me about the depth that the chart goes to.. I just stopped where PPO2 is 1.4 for Air.

http://www.unixsage.com/scuba/rock_bottom.html

 

[cerich]

a diver in a panic will triple normal SAC easily. A 1.5 normal workin rate is fine for experienced divers in an OOA, both that keeps a cool head.
Whatever RBTM plans out to be I would add extra psi due to a strict calculation could leave both divers OOA.
RBTM is for single no deco diving and to be honest some may feel that RBTM calculation will take away any need of ever doing a ESA. If divers following RBTM planning get that in their head, they have throw away a "tool in the box" that could save their life at some point.

Best,

Chris

 

[TSandM]

We're arguing semantics here. The definition of "rule of thumb" -- an easily learned and easily applied procedure for approximately calculating or recalling some value, per Wikipedia -- implies approximation. Using 1cf/min is an approximation, but even using 2x known working SAC rate is an approximation, because you can't know if that's what will actually happen when the fan is getting hit. Certainly, a published set of values has no reference to any information known about the individuals, and is therefore by definition a rule of thumb.

 

[Soggy]

Ugh...

There is no way to know exactly what average SAC rate for an emergency situation will be.

But use your head for a moment. You have two divers, one goes out of air. Both SACs will spike initially, maybe to even 1.5-2 cft/min. When the OOG diver gets gas, he will settle down, at least some. The donating diver will likely settle down to near a normal rate. That is why 1 cft/min for BOTH DIVERS is very reasonable and conservative. If you have a better number or a good way of deriving it, feel free, but I am confident that number will get me to the surface along with a huge reserve.

 

[cerich]

No question with a diver of your skill Soggy, but I never assume that from divers.
RBTM is a good calculation to follow in dive planning, what it is not is absolute nor proven to work in every circumstance.

Some people are going to read RBTM as DIR, run the calculations and never consider that a situation may develope where by RBTM is not enough. If these divers don't consider the next move after RBTM fails then they will NOT be likely to be able to manage the situation.

Among other great and positive things, one thing that DIR does is give confidence and sometimes that confidence is without experience. There have been many cases where something that falls outside what "should" happen in an emergency while diving DIR that leaves the DIR Diver unable to cope because their belief in the completness of DIR is so high they can not comprehend that the snowball just got bigger, even though they Did It Right!

 

[lamont]

You're also applying a "rule of thumb" in that you're only going to spend a minute at depth, etc. There's a lot of fudging going on, and if your dive is getting more challenging you should start to pad the RB numbers out more.

Also, I've done the calculations a bunch of times, and for depths above 100 fsw, I typically now use "10 psi per foot plus 300 psi" for Al80 and "10 psi per foot" for 120s and 130s (with 500psi minimum on both). Its very close to doing the calcuations out. Then I pad for inexperienced divers, depth, current, etc...

 

[Soggy]

Agreed, Cerich. A delay due to entanglement or a truly panicking diver could cause all sorts of additional gas requirements. However, given someone who has at least an idea of how to handle an OOA situation, these are quite reasonable estimates. They also assume a direct ascent to the surface without any required underwater swim (to get back to the upline for example). On a large wreck in the north east where reaching the upline would be considered mandatory or when doing significant linear penetration, one might choose to plan the dive according to rule of thirds *plus* rock bottom (meaning, subtract your rock bottom from your available gas, figure thirds, then add your RB back to your turn pressure). Some might consider this too conservative, but knowing you have enough gas to get home is a very reassuring feeling.

 

[cmalinowski]

Attached is how I calculate it.

There is a simpler way just because you can always assume anything less than 100' is average 2 ata for the entire ascent and there will always be two divers. So, figuring out CF (or ft^3 if you so desire) needed is multiplying minutes needed times four. So, if you were going to 100', you'd need stops at 50, 40, 30, 20, 10 (5 stops = 5 mins). You'd need one min to figure out problem and two mins to reach the 50 stop from 100'. Total 8 mins * four = 32cf needed. You can guestimate from there.

ex:
- on an AL80, you'd need (32/77) * 3000psi = .4 * 3000 = 1200psi RB
- on an LP121, you'd need (32/121) * 2600 = .26 * 2600 = ~700
- on a 100cf tank you'd need right around 1/3 of the tank. If it's a 3000psi tank, then 1000 is your RB.
- etc, etc, etc

You can use that whole tank factor thing, but if you need something quick, you can figure it out with estimating pretty easily.

Chris