For those novice divers reading along with this thread, @doctormike brought up an important concept: rock bottom gas. That's the pressure at which you ABSOLUTELY MUST head up. Let's do the math, to see why this dive just doesn't ring true, especially for well-trained divers. And what likely really happened wasn't just bad judgment, it was absolutely crazy. If you're so inclined, ride along with me as we do the math...
You want to finish with 200psi absolute min, with a 3000# 80cu ft tank. Worst case scenario.
An AL80 has 38.8 psi per cubic foot. (38.8 x 77.4 = 3000). That's right - an 80 only has 77 cu ft.
Let's say your nominal gas consumption is 0.7 cu ft/min. The diver being quoted said that their gas consumption was greater than expected, so let's call it double, or 1.4 cu ft/min.
At their stated 40m/131 feet (5 atm), that becomes 1.4 x 5 = 7 cu ft/min.
You want enough gas to ascend normally from 131 ft.
Ascending at a constant rate of 60fpm from 131’ (5 atm) to 60' (3 atm), and then 30fpm to 15’ (1.5 atm) takes 1.2 min at an average of 4 atm, and 1.5 min at an average of 2.25 atm.
(1.2 min x 4 x 1.4) + (1.5 x 2.25 x 1.4) = 11.4 cu ft. Remember this amount.
Now, safety stop:
3 min x 1.5 atm x 1.4 cu ft/min =
6.3 cu ft. Remember this one, too.
Now fifteen feet of final ascent at 30fpm = 0.5 min x 1.25 atm x 1.4 = 0.9 cu ft.
Add the three together:
11.4 + 6.3 + 0.9 = 18.6 cu ft. Call it 20.
20 cu ft x 38.8 psi per cu ft = 776 psi. Call it 750.
750 psi required ascent gas + 200 psi ending tank pressure =
950 psi rock bottom gas pressure at 131 ft, (not including buddy gas for a shared ascent, which is the true definition of Rock Bottom - so let's think of this as solo rock bottom).
Using 500psi for descent means they have 1500 psi usable gas at 131 ft (5 atm).
1500/ (38.8 x 1.4 x 5) = 1500/272 = SIX minutes to hang around at 131 ft! For a bounce dive to test themselves, this is plenty. And if each overstayed their limit and hit their solo rock-bottom? A nice leisurely ascent. And if ALL FOUR INSTRUCTORS ignored their rock bottom, well, they could skip their safety stop on this supposedly NDL dive.
But they ran out of gas? Two, then four of them? It only requires 350 psi to make a straight 60 fpm ascent from 131'. FOUR (countem', four) INSTRUCTORS didn't notice anything before their SPG read less than 350?
Nah. Something's fishy.
Now what if these divers went to 200 ft?
Add 60 fpm ascent to 131 ft at avg of 5.5 atm:
5.5 x 1.4 x 1.1min x 38.8 = 325 psi
That means their solo rock bottom was 1275 psi.
And using 750 for descent, they'd still have TWO minutes at 200 feet (and if they only stayed 2 minutes, they could avoid doing a deco dive requiring more gas and time on ascent). Nah! Even at double consumption rates, there's still enough gas to bounce to 200 ft. Just tell me that four INSTRUCTORS didn't think they could stay more than 2 minutes at 200 ft on a single 80 cu ft tank each.
What if these divers went to 300 ft?
Add 60 fpm ascent to 200 ft at avg of 7.6 atm:
7.6 x 1.4 x 1.7min x 38.8 = 700 psi
That means their rock bottom at 300’ was 1975 psi.
If they bounce dived to 300’, they had <1 min of gas at that depth before they hit rock bottom (NOT INCLUDING deco gas). But that's irrelevant because after only 10 sec at 300 ft they have already incurred 10 minutes of decompression obligation on the way up. Now we're finally seeing a reason to run out of gas. But at normal consumption rates there would be just enough gas even for the 10 min of deco at crazy liberal settings. Did they maybe think they could do this?!!
You can see why so many posters are calling BS. Only 40 meters? Really?
Something's fishy about this story.
You want to finish with 200psi absolute min, with a 3000# 80cu ft tank. Worst case scenario.
An AL80 has 38.8 psi per cubic foot. (38.8 x 77.4 = 3000). That's right - an 80 only has 77 cu ft.
Let's say your nominal gas consumption is 0.7 cu ft/min. The diver being quoted said that their gas consumption was greater than expected, so let's call it double, or 1.4 cu ft/min.
At their stated 40m/131 feet (5 atm), that becomes 1.4 x 5 = 7 cu ft/min.
You want enough gas to ascend normally from 131 ft.
Ascending at a constant rate of 60fpm from 131’ (5 atm) to 60' (3 atm), and then 30fpm to 15’ (1.5 atm) takes 1.2 min at an average of 4 atm, and 1.5 min at an average of 2.25 atm.
(1.2 min x 4 x 1.4) + (1.5 x 2.25 x 1.4) = 11.4 cu ft. Remember this amount.
Now, safety stop:
3 min x 1.5 atm x 1.4 cu ft/min =
6.3 cu ft. Remember this one, too.
Now fifteen feet of final ascent at 30fpm = 0.5 min x 1.25 atm x 1.4 = 0.9 cu ft.
Add the three together:
11.4 + 6.3 + 0.9 = 18.6 cu ft. Call it 20.
20 cu ft x 38.8 psi per cu ft = 776 psi. Call it 750.
750 psi required ascent gas + 200 psi ending tank pressure =
950 psi rock bottom gas pressure at 131 ft, (not including buddy gas for a shared ascent, which is the true definition of Rock Bottom - so let's think of this as solo rock bottom).
Using 500psi for descent means they have 1500 psi usable gas at 131 ft (5 atm).
1500/ (38.8 x 1.4 x 5) = 1500/272 = SIX minutes to hang around at 131 ft! For a bounce dive to test themselves, this is plenty. And if each overstayed their limit and hit their solo rock-bottom? A nice leisurely ascent. And if ALL FOUR INSTRUCTORS ignored their rock bottom, well, they could skip their safety stop on this supposedly NDL dive.
But they ran out of gas? Two, then four of them? It only requires 350 psi to make a straight 60 fpm ascent from 131'. FOUR (countem', four) INSTRUCTORS didn't notice anything before their SPG read less than 350?
Nah. Something's fishy.
Now what if these divers went to 200 ft?
Add 60 fpm ascent to 131 ft at avg of 5.5 atm:
5.5 x 1.4 x 1.1min x 38.8 = 325 psi
That means their solo rock bottom was 1275 psi.
And using 750 for descent, they'd still have TWO minutes at 200 feet (and if they only stayed 2 minutes, they could avoid doing a deco dive requiring more gas and time on ascent). Nah! Even at double consumption rates, there's still enough gas to bounce to 200 ft. Just tell me that four INSTRUCTORS didn't think they could stay more than 2 minutes at 200 ft on a single 80 cu ft tank each.
What if these divers went to 300 ft?
Add 60 fpm ascent to 200 ft at avg of 7.6 atm:
7.6 x 1.4 x 1.7min x 38.8 = 700 psi
That means their rock bottom at 300’ was 1975 psi.
If they bounce dived to 300’, they had <1 min of gas at that depth before they hit rock bottom (NOT INCLUDING deco gas). But that's irrelevant because after only 10 sec at 300 ft they have already incurred 10 minutes of decompression obligation on the way up. Now we're finally seeing a reason to run out of gas. But at normal consumption rates there would be just enough gas even for the 10 min of deco at crazy liberal settings. Did they maybe think they could do this?!!
You can see why so many posters are calling BS. Only 40 meters? Really?
Something's fishy about this story.
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