Is my math right?

[skinerd]

ScubaBoard, I love you... *winks* Thanks guys!

 

[The Kraken]

I printed the post by Roakey and stuffed away in my diving library. Glad it had his name at the bottom!

the K

 

[cool_hardware52]

Ok so asuming that both my buddy and I have the same average SAC, an we are making a dive to 60Ft. In order for both of us to have enough air to ascened and have a 500psi reserve, I calculated that we would need 1157 to take 2.5min to ascend to 15ft with 2min saftey stop then surface. I came to that number by:


SAC: 0.5
ATA: 3

Time for ascent (TfA):
(ascent time + saftey stop) X 2
= (2.5+2)X2
=9min

Consumption at deapth (Cd):
SAC X ATA
=0.5 X 3
=1.5

Cuf used for ascent asuming breaths of 1.5 Cuf/min:
Cd X TfA
=1.5 X 9
=13.5

Tank vol - 13.5
=77 - 13.5
=63.5

Convert to a fraction of tank vol
63.5/80
=0.79375

0.79375 X (3000-500)
= 1984

There for, Rock Bottom Pressure is:
3000-1984
=1016

Ok phew, I think that is right... Is it? If you have any questions PM me plz!
Thanks

Rock Bottom is a volume of gas that is required to get you and your teammate safely to the surface, converted to PSI.

When I calculate RB I make a couple assumptions that you did not.

1. Emergency SAC rate will be higher than normal Sac rate. I usually assume 1.0 for both divers, unless the diver's known sac rate is close to 1.0, then picking a higher sac is a good idea.

2. Assume whatever emergency that lead to an OOG will require resolution, I assume 1 minute at depth.

3. I don't figure any reserve at the surface. If somebody's upset if I get back to the boat with less than 500 psi after successfully handling a OOG ascent, he and I will have to talk.

4. RB is never less than 500 psi.

This example assumes a 3 min 15ft stop, and no other stops. Other depths, and protocols might require more stops. The idea is the same however.

Emergency at depth of 60 ft

1 min(resolve the problem, and calm the OOG diver )x 2 divers x 1.0 sac x ~3ATA = 6 cuft

Ascent to 15 ft (60-15= 45 ft @ 30 ft / min = 1.5 minutes)
Average depth = (60-15/2)+ 15 = 37.5, call it 2 ATA

1.5 minutes x 2 divers x 1.0 sac x (average ATA ~ 2.0) = 6 cuft

3 Minutes @ 15 ft

3 minutes x 2 divers x 1.0 sac x ~1.5 ATA = 9 cuft

Total volume required 6 + 6 + 9 = 21 cuft.

Convert 21 cuft to PSI for your tank. Assume 3000 PSI AL80 (77 cuft actual)

3000/77 = ~ 39 psi / cuft 21 x 39 = 819, round to 900.


Regards,


Tobin

 

[cummings66]

Rock Bottom is a volume of gas that is required to get you and your teammate safely to the surface, converted to PSI.

When I calculate RB I make a couple assumptions that you did not.

1. Emergency SAC rate will be higher than normal Sac rate. I usually assume 1.0 for both divers, unless the diver's known sac rate is close to 1.0, then picking a higher sac is a good idea.

I've only had to deal with one ooa situation and the above is absolutely correct. At the time I didn't really do the rock bottom and in this case it was very lucky his SAC was much higher than mine, as is we ended up cutting it close. He sucked my air down so fast I couldn't believe my eyes, so I agree 100% that a higher than normal rate needs to be planned on.