Rock Bottom?

[MHK]

SARmedic once bubbled...
A static chart like that will not work, too many dynamic variables. Too many variables...

Not a very good idea if you ask me.

Part of the complexity of explaining issues such as Rock Bottom on a scuba forum is that one simple example, such as the original poster posted as an example, will not satisfy everyone. I try to avoid discussions of this nature over usernet forums for that very reason..

Rock Bottom, when applied properly is a concept that we teach in every class we do, and it does in fact work well and it is in fact a very good idea.

However, the problem is that the initial post only covered the salient details, not the primary concepts behind the WHY.. It is perhaps possible to explain the HOW's on a scuba forum, but I find that explaining the WHY's is more important and more difficult because all too many want to find the one-in a million that could perhaps prove the negative. For example your running into a sea urchin on the ascent, while probable isn't something that should be considered when planning Rock Bottom in the practical world.

In order to be in the situation where Rock Bottom applies, you would have to have one member of a unified team run OOA [ hopefully a very unlikley scenario], then you would have to have at the same time a wild sea urchin run a amock that causes such extruciating pain so as to result in blowing off what would amount to 7 minutes.. These scenario's while theortetically possible, are just to immaterial to discuss with any credibility. BTW, if you are planning for those kind of contingencies, I would say use doubles for every dive or a rebreather because Rock Bottom is the soundest course of action. It certainly beats the " be on the boat with 500 psi" scenario which doesn't tell you much, it doesn't tell you turn pressure, gas reserve or when to leave the bottom. Planning for a run away sea urchin is just beyond logic, in my view.

BTW using 1 CuFt make perfect sense, most have a .5 rate so even if during the emergency your breath is eleveated, certainly during the ascent and safety stop your rate will level off..

However, I'm open to other ideas if you'd care to offer one.

Thanks

 

[MikeFerrara]

Scubaroo once bubbled...
At what pressure does a regulator stop delivering air? Can a regulator deliver gas below it's set IP? Never drained a tank that low to find out.

So would you have to add the IP of your regulator to rockbottom, because anything below that IP just isn't available? ie about 140psi? Or would you have to add 14.7psi, which is ambient pressure at the surface, because you couldn't go below that?

Not being smart or anything, but if a reg won't deliver gas below it's IP, 140psi can make a big difference to your calculations.

Exactly what will happen depends on the reg design but in order to work as intended you need at least a presure of IP plus ambient in the tank.

At lower presures most first stages will stay open. The second stage works against ambient.

 

[Big-t-2538]

Scubaroo once bubbled...
At what pressure does a regulator stop delivering air? Can a regulator deliver gas below it's set IP? Never drained a tank that low to find out.

Nor have I...but I am going to take a stab here...I don't totally agree with mike in saying that the pressure has to be at least IP plus ambient....that doesn't make total sense to me...

What does make sense to me is this:
A reg designed to stay open is open...it is only allowing 140psi to get through....noting over that. It is a step down of pressure, not a pressure required to let air through. So I would say a reg will deliver air until the ambient pressure is reached (on the surface 1atm...at 33 feet, 2atm or 29.4 psi)

<edit>...I just came up with an analogy...the IP is like a bouncer at the door of a club....3000 people want to get in, but he can only allow 140 in at a time...as people leave, there is eventually no-one for the bouncer to regulate. <edit>

 

[Scubaroo]

I think the key to Mike's response was "works as intended". Rick Murchinson answered the question in another thread - the reg will continue to deliver air, but will degrade in ease of breathing.

So you might want to add a 50 or 100psi fudge factor to your rockbottom, otherwise you won't be able to access the last couple of cubic feet in your rockbottom plan.

 

[Spectre]

Scubaroo once bubbled...
So you might want to add a 50 or 100psi fudge factor to your rockbottom, otherwise you won't be able to access the last couple of cubic feet in your rockbottom plan.

I'm not totally sure that's necessary. You don't plan your rock bottom numbers with an agressive SAC rate, you calculate it with some room to deal with stressful situations... A lot of things need to go wrong to be burning a high SAC rate while doing a safety stop after hitting your OOA at the last second before ascent...

Personally the fudge factors I put in my rock bottom need should compensate in the event that we hit that OOA at exactly the last second. I plan my ascent for 20 ft/sec, I plan my safety stop for 5 minutes @ 20. I use a SAC rate that is at the high end of my working SAC rate. I also calculate my estimated bottom time with the gas remaining, so I know if I'm consuming faster than expected, and adjust my ascent pressure accordingly. I also round up in different places in my calculations which makes for some more fudge...

In order to have a true problem, the OOA has to occur near the point of rock bottom; which is unlikely timing for an unlikely event. You _then_ need to combine that with running at the high SAC rate that you used in planning. If you have concerns that you can't deco/savety stop hang and relax while doing that; you probably need to practice your OOA drills. You should be comfortable breathing off someone elses reg, and you should be comfortable with someone breathing off your reg. When doing your safety stop in an OOA ascent, you should still be able to relax enough to keep your SAC rate at least at the high end of your resting SAC rate...

There already is enough built in fudge factor that you probably don't need to add more to account for that last 1/2 cuft in the bottle...

Jonnythan. Sorry about the 'no SAC calculated' comment. I missed it in your page [and I apologize for not looking back to confirm when I responded]. As far as "what would you think is reasonable". 1 is probably fine, or you can just give 'em that 500 psi that is taught

 

[Norda]

I just recently I had my eyes opened on just how poorly this subject was covered during certification, which led me down the path of proper gas management and dive planning. After much research and several hours I managed to put together a several page addition to my dive log tailored to address proper planning. My dive buddies and I are now on the path of better dive planning!

Anyway, to get back to the subject, here is what I have been using for Rock Bottom:

Definition: The gas needed for you and your OOA buddy to make it to the surface (or to first gas switch) doing all stops and with a safe ascent rate. Based on SAC rate of 1 cf/minute for both divers.

Computation:

Ascent Time Minutes * SAC @ 2 cu/ft * ATA = Rock Bottom.

Situation Assessment: 1 minute (ATA at depth)
30' - 20' Ascent: 1 minute (ATA 2.0)
20' - 10' Ascent: 1 minute (ATA 1.5)
10' Safety Stop: 1 minute (ATA 1.5)
10' - 0' Ascent: 1 minute (ATA 1.5)
Each 30' of Ascent below 30': 1 minute (ATA 3.0, 4.0, 5.0, etc).

So rather than go through complex calculations I just round off the ATA for each 30' increment (except for the top 30') and have built it into a table with ascent time and RB requirements already calculated for depths of 30', 60', 90' and 120'. Makes the dive planning much faster when everyone is in a hurry to get wet.

Bill.

PS. Credit is due to UP for helping me unravel my confusion over this subject. I believe the definition above is in his own words.