Instructor bent after running out of air at 40m

[Bob DBF]

Seems like the 4 divers would have had to collude to make sure that there was only one consistent story. The actual profile was certainly captured by any dive computers they may have been carrying. I would think that this information is available somewhere in the recompression medical record and/or in the accident investigation.

Collusion is not hard to do, and the outcome reinforces the reason to continue. A computer is not needed for the dive, and would be detrimental to the cover story if they were not looking at nudibranchs at 40m and something went wrong.


Bob

 

[soggybadger]

I'm tempted to have a peek on some of the UK forums to see what is being said. I know that the comments will extremely harsh but very funny.

 

[CuzzA]

@scubadada alluded to the biggest issue here. The journalist didn't know what questions to ask. Had this guy posted his story on here and had the balls to stick around I'll bet this story changes quite a bit. It just doesn't add up.

 

[EdC]

After reading through this thread I’m thinking two of the divers stayed a little “shallower” and were waiting as “safety” divers for the deeper bounce pair. That would explain two cases of DCS and two divers with no symptoms. Just a guess.

 

[IncreaseMyT]

After reading through this thread I’m thinking two of the divers stayed a little “shallower” and were waiting as “safety” divers for the deeper bounce pair. That would explain two cases of DCS and two divers with no symptoms. Just a guess.

That makes the most sense. It would also explain the "miscalculation". The "safety" divers were breathing too much.

 

[Kevrumbo]

That's a fair speculation -two "safety divers" and the other pair bouncing down for a short excursion to break the 100 meter mark. A questionable and obviously dangerous endeavor just to reach the three significant figure depth in meters, or double digits in ATA, and at the human physiological limit for ambient pressure Scuba diving on Air (i.e. Hypercapnia, ppO2 Oxygen Toxicity and/or extreme Narcosis leading into unconsciousness). . .

@ 90msw depth (10ATA);
The ppO2 atm of Nitrox 21% (Air) @ 10ATA: 2.1 bar;
Gas Density @ 10ATA: 12g/L (approx 10x more dense WOB than Air at surface);
Depth Consumption Rate: a whopping 20 to 25 bar per minute (roughly 300 to 375 psi/min)!

 

[KeithG]

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.

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 they overstayed their limit and hit their 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 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. 40 meters? Really?

Something's fishy about this story.

Agreed. When I started reading the thread I wondered how can someone get severe DCS on a 40m dive with an al80?

Your calculations above give them enough air to spend 6 minutes at 131 feet which is within NDL limits. I am bad at math: how much time would it take to drain the tank at 131 feet and how much of a deco obligation would they have?

 

[Bob DBF]

Agreed. When I started reading the thread I wondered how can someone get severe DCS on a 40m dive with an al80?

Your calculations above give them enough air to spend 6 minutes at 131 feet which is within NDL limits. I am bad at math: how much time would it take to drain the tank at 131 feet and how much of a deco obligation would they have?

@rsingler was assuming their normal air consumption at .7cf/m and calculated at a elevated rate of 1.4. If they were breathing at .7 or less my wag would be 15 min, or 5 min of deco more or less. If he likes crunching numbers, he will be by to correct me, and deservedly so as I'm too lazy to do my own math.


Bob

 

[MichaelMc]

Fishy.

2 ran out at 40m, they slowly ascend no panic, then totally run out at 30m?
"""
RICH - It's not routine but you're prepared for it. From there we were breathing breath for breath off of the two remaining tanks that had air in them, and we were coming up slowly as you have to do with scuba diving, so we were coming up slowly.
BETH - No element of panic or anything at this point?
RICH - No, no element of panic. And then we got to about 25, 30 metres safely, and then from then we were all out of air.
"""
Did these four instructors not check SPGs after the first two OOAs??

I like the two divers deep, 2 divers shallow theory. Or even one pushing that extra bit deeper.
"""
RICH - Two of them were absolutely fine. There was one individual that had a little bit of nitrogen build up in his elbow; a little bit of oxygen sorted that out. This guy who doesn't do things by halves gets a full on spinal bend.
"""

Sad.

 

[rsingler]

Agreed. When I started reading the thread I wondered how can someone get severe DCS on a 40m dive with an al80?

Your calculations above give them enough air to spend 6 minutes at 131 feet which is within NDL limits. I am bad at math: how much time would it take to drain the tank at 131 feet and how much of a deco obligation would they have?

Lol! Bob's WAG is close enough for government work.
But to answer your question, if they DIDN'T have double gas consumption, their ascent requirement would only be 9 cu ft/350 psi (NOT counting deco). That gives them almost 2000 psi usable gas, which at 5 atmospheres and an RMV of 0.7 (as Bob WAG'ed) is 15 min!! But unfortunately, that leaves you with a 9 minute deco obligation at GF 80/90, and the entire tank is down to zero right around the end of the run. Missing some of that deco might give us what we see with this kid especially with aggressive GFs. I don't really know...you'd have to ask @boulderjohn .

With more conservative GF's, say GF 50/80, but at an RMV of 1.0, if they stayed at 131 for 11 minutes, and ran themselves out of gas, they again would have had a 8-9 min deco obligation. That much missed deco might give us what we see with this kid. They may have planned for an RMV of 0.7, which would have left them with 500 psi reserve after 12 minutes at depth as well as doing 9 min of deco. But they didn't follow a rock-bottom ascent trigger when their RMV was higher, and instead maybe stayed by the clock, only to be surprised by their gas remaining. But all four of them???? Darwin in action, I guess.

Yeah, maybe I was too severe in running those earlier numbers. Maybe this is just four knuckleheads who didn't even look at their SPG. Maybe they didn't bounce to 200 feet or more. I dunno. Don't know if we ever will. But @doctormike had it right. Whatever the reason, they didn't ascend when they hit min gas.