What is really Rockbottom?

[Twiddles]

Okay here goes, somehow I think this is wrong based upon what I have read so far but I am uncertain so here goes.

Rockbottom is defined as the minimum amount of gas required for you and your buddy to ascend from depth safely to surface breathing off one supply?

Using a 99' as the depth I have come up with a safe ascent rockbottom of 930psi or 1130 with spg buffer. I used the following information. The requirements listed on the RDP and information provided by Lamont Granquist and Bob Bailey. Ascent rate is no more than 30' per minute, no safety stop as per the RDP, none is required on dives less than 100' and this is after all an emergency situation.

RMV of 2.0
AL 80

Identifying OOA situation time 2 * [99/33 +1] * 1 = 8 Cu ft.
Ascent Time at 30 feet per minute = 3.2 minutes
Ascent Gas (Avg depth is used) = 2 * [ (99/2) / 33 + 1 ] * 3.2 = 16 Cu ft.

Total 24 cu ft.

PSI conversion for AL80 24 * (3000/77.4) = 930.2 psi

Safety margin for SPG 200psi

Total rockbottom gas required 1130.2 psi

Now for the questions, why would you sit at 80' to offgas for a minute during an emergency? Why would you sit at 15 or 20' for 3 minutes to offgas during an emergency? Is not the safest course during an emergency under water to get the OOA diver to the surface as soon as safely possible? If PADI only recommends but does not require you to stop why perform a safety stop during an unsafe situation? All the numbers for rockbottom are based upon the OOA situation happening at exactly the time you plan your ascent and not earlier than that time. If the situation occurs earlier than your planned ascent your only adding buffer to your time. Why be concerned about a 200psi inaccuracy at empty from your SPG (unless your saying that your SPG is always off by 200psi?)? How much buffer is enough?

 

[Cheekymonkey]

because the bends can be just as dangerous as other problems

 

[Twiddles]

We are ascending at a rate half the limit of the rdp over a period of more than three minutes from a depth of 99'? If the bends are seriously a concern under this situation then the RDP the open water manual and the AOW manual need to be changed dramtically?

Or is PADI really cutting a fine line where ascent rates are concerned? Based upon posts I have read here I would think they were very cushioned?

 

[H2Andy]

of course they are a concern. NDL's are gray areas, not hard and fast lines. the RDP's 60 fpm is much too fast anyway.

this is a very conservative (read safe) way to do it. if you're going to plan for an emergency, plan for an emergency -- don't plan to possibly create another one on the way up

also, the 30 fpm ascent is good to keep any baurotrauma issues at bay (even a few feet can hurt you), plus to give you a relaxed pace at which to ascend to keep good bouyancy control and prevent an accidental runaway ascent given the stress and task load of the situation

 

[ae3753]

1 minute at 80' is not to off gas, but it's the time required to handle the situation - donate alternate air source, get positioning, communicate, etc. For some people, 1 minute might not be enough time to get everything situated.

The safety stop, as you say, is not required. However, if you can do it safely, then it should be performed. Once the out of gas diver has been donated to, the direct emergency is solved. In this case, I would prefer doing a proper ascent than getting out of the water as soon as possible.

The 200psi guage buffer is just that. If you feel don't want a buffer, that's your team's perogative.

The amount of buffer needed is dependent on your teammates' and your comforts level.

 

[Thalassamania]

Okay here goes, somehow I think this is wrong based upon what I have read so far but I am uncertain so here goes.

Rockbottom is defined as the minimum amount of gas required for you and your buddy to ascend from depth safely to surface breathing off one supply?

Using a 99' as the depth I have come up with a safe ascent rockbottom of 930psi or 1130 with spg buffer. I used the following information. The requirements listed on the RDP and information provided by Lamont Granquist and Bob Bailey. Ascent rate is no more than 30' per minute, no safety stop as per the RDP, none is required on dives less than 100' and this is after all an emergency situation.

I’d say no, it’s a plan to keep it from becoming an emergency situation and when I calculate Bingo Air (what you call Rock Bottom) I include gas for both divers at all stops that I’d normally make.

RMV of 2.0
AL 80

Identifying OOA situation time 2 * [99/33 +1] * 1 = 8 Cu ft.
Ascent Time at 30 feet per minute = 3.2 minutes
Ascent Gas (Avg depth is used) = 2 * [ (99/2) / 33 + 1 ] * 3.2 = 16 Cu ft.

Total 24 cu ft.

PSI conversion for AL80 24 * (3000/77.4) = 930.2 psi

Safety margin for SPG 200psi

Total rockbottom gas required 1130.2 psi

Now for the questions, why would you sit at 80' to offgas for a minute during an emergency?

Because it’s not an emergency … it’s a contingency that you’re planning for and thus one should not make that into an emergency.

Why would you sit at 15 or 20' for 3 minutes to offgas during an emergency?

Because it’s not an emergency

Is not the safest course during an emergency under water to get the OOA diver to the surface as soon as safely possible?

Once the OOA diver is on the long hose it is no longer an emergency, now it’s just a thumbed dive.

If PADI only recommends but does not require you to stop why perform a safety stop during an unsafe situation?

Once the OOA diver is on the long hose it is no longer an unsafe situation, then it’s just a thumbed dive.

All the numbers for rockbottom are based upon the OOA situation happening at exactly the time you plan your ascent and not earlier than that time.

’Cause that’s the worst possible case.

If the situation occurs earlier than your planned ascent you’re only adding buffer to your time. Why be concerned about a 200psi inaccuracy at empty from your SPG (unless your saying that your SPG is always off by 200psi?)? How much buffer is enough?

200 psi is kind of an agreed to number to cover both work loss at the regulator as well as SPG error.

Now don’t get me wrong, I have no problem with you saying that you’ll make a sixty foot per minute no stop ascent ‘cause that’s what you normally do (on a 99 foot dive that’s what a lot more people than you know actually do) but the key is that it’s not an emergency, any more than flooding your mask is an emergency, so don’t treat it as one. It’s just another situation that you plan and train for.

And I think your RMVs a bit high.

 

[Charlie99]

Many divers calculate rock bottom such that they can do their normal ascent, deep stops, and safety stop even with two divers breathing at an elevated rate.

Other divers, including me, choose to not double worst case everything and use a rock bottom number that is closer to the true absolute limit. With an AL80, the numbers work out reasonably close to "100psi per 10' of depth, but never less than 500psi".

With the 1000psi limit you have chosen, if you and your buddy are indeed breathing at a combined rate of 2cfm, and you do indeed take a full minute at depth to get organized and start the ascent, then you won't have enough gas left for a safety stop. The question is whether you are willing to take that risk. Since you will be using an NDL well within the old USN No stop limits, and will be ascending at 30fpm rather than the USN table recommendation of 60fpm, and since the USN table does not assume a safety stop, in an air sharing ascent with a highly-excited hoovering buddy, you will be diving a bit safer than someone diving the USN table to the limit. Sounds good to me.

http://www.scubaboard.com/showthread.php?t=30754 is a thread from 2003 where I went through my assumptions and calculations ---

1. One limit assumes sharing air with an excited buddy. 3cfm, but with relatively fast ascent rate of 50fpm. Added to this is 1 minute of getting our act together on the bottom. If my buddy is someone I know and trust, I may drop this down to 2cfm. There is no "psi" fudge factor in this limit. Hopefully, 3 cfm has enough fudge. The ascent is halted at 30' to 20' range, and a safety stop executed if the situation is under control and there is enough gas.

#2. The other one reflects my desired gradual ascent. 20-30fpm, a couple of stops for a couple minutes in 40-50 and 25-35' ranges, then a safety stop that start 15-20' and transitions to 8-10' by the end of 4 minutes. All told, from 60' to surface this requires the same gas as would 20 minutes on the surface.
For this I use an estimate of 0.6cu ft/min (50% more than my normal 0.4cfm SAC), a bit more if cold, or if I've been using more gas than normal so far in the dive.

 

[Ben_ca]

Because it’s not an emergency
Once the OOA diver is on the long hose it is no longer an emergency, now it’s just a thumbed dive.

Twiddles,
Like Don said the 1 min is to donate gas and then stabilize... How long does it take a typical diver to do an OOG share? How long did it take you the last time you practiced one?

Once you are on the long hose the emergency is over... you just have to clean up and then move to the exit.... Not an emergency anymore like Thas said... but be careful not to dilly dally and sit there... remember the big picture... is it an Out Of Air or an Out Of Gas? Fixable or not? ... there's a whole lot more to the exercise... The math part is but a small piece...

ps... too tired to check your math...

 

[hudson]

Another possible approach is to use a 60fpm ascent rate for deeper depths and 30fpm for shallower depths although I'm not sure if it'll change the gas requirements that much - you can run the numbers and find out. Other than that, what is published by lamont makes perfect sense to me. In fact, in addition to the SPG buffer, which may depend on your particular SPG, I also leave a buffer of 100-150psi to avoid potentially bleeding the tank dry. This works out to about 200psi for me as my AI comp has an error margin of 60psi.

One thing to keep in mind with these rock bottom calculations is, the max depth reserve only holds true at that depth and as you ascend to shallower depths your reserve requirements will decrease - once people realize that, I think the amount reserve gas stops being a big deal.

 

[MauiScubaSteve]

A couple Q's;

When using tables don't we round up? 91' require's ss by the book.

For me, a 99' deep drift dive (minimal effort) could easily result in mandatory ss (within 3 pg's) even without rounding up (computer). In your calculations how fast is your descent and what is your bottom time to reach bingo?

If I'm diving a computer, do I blow through the saftey stop it asks for on every ascent? Will I then be computer penalized on the second dive? I know what my regs feel like approaching empty. If both buddies are in control, why not stay until 150psi then creep up to 10' until empty or until ss completed, whichever comes first?

Seems to me below 90', bingo should have a ss. Stay healthy!