Filmmaker Rob Stewart dies off Alligator Reef

[The Chairman]

You don't need a drysuit. You need a brain.

Actually... you need experience. That's the problem. Training doesn't make it automatic. Your brain has a hard time functioning when you're exhausted, hypoxic, hypercapnic or panicked while drowning. Experience enables the brain to shortcut to a good solution. There's no substitute for it.

 

[RichardC]

The "Dead Bug" (dropping like a stone after taking out an open DSV on the surface with immediate loss of buoyancy) is a very real thing. Rigs with over the shoulder counterlungs don't suffer nearly as badly as the rEvo, which dumps it's gas in just a few seconds if you do this. It's almost instantaneous.

John, you seem to be taking every opportunity to bash rEvos, I guess it internally validates your choice to change unit, but don't be so complacent about OTS lungs. The loss of buoyancy comes from the fact that the water pressure squeezes the air out of the lungs from below (not flooding as some have claimed). This is true of both back mounted lungs (which are fitted to many units other than the rEvos) and OTS lungs too. The air will be squeezed out of OTS lungs up as far as the connection to the loop, reducing the buoyancy significantly, and just as fast as in the rEvo; what happens then depends on your other buoyancy devices, attitude in the water etc.

The choice of back mounted vs OTS lungs is not a one sided deal, what I like about the rEvo is the very efficient and fast flushing of the loop that I get and I would not trade that for some supposed benefit on the surface. The bottom line on ALL rebreathers is that you shouldn't be reliant on the lungs for buoyancy on the surface.

Keep safe

Rich

 

[mderrick]

It's Occam's razor: The simplest solution is usually the correct one.

NetDoc's fits all the widely known (and the less widely known) facts, and passes my personal smell test when looking at CCR accidents that almost always have some forever unknowns... Occam's Razor.

 

[Jeffmedic]

The current theory is that they ran out of O2 for the RB. Because of this, they would have had only the dil to increase the loop PPO2 which with a hypoxic dil doesn't work too great. Thus at the deco stop they would have had a steadily reducing PPO2 until eventually they would be breathing a hypoxic mix. This is fine at 20 ft but from there to the surface gets all kinds of fun all kinds of fast.

If they were doing a bounce to get the anchor as has been postulated, its entirely possible that they grabbed a bottom bailout before splashing, not necessarily thinking this through. In that case they would have had a hypoxic bailout also no help.

Just thinking out loud, in aviation we have a phenomenon where it is REALLY hard to abandon a "serviceable" piece of equipment even when its the best thing to do. CCR training is all about staying on the loop, mostly I suspect from the cave diving roots of much of the training where every meter longer on the loop is more OC gas reserve.

In a situation where a diver is at 20 feet with a perfectly functional (though hypoxic) RB and a bailout of e.g. 32% or such, I wonder how much subconscious resistance there would be to bailing out. In the Rec CCR world the mindset is "when in doubt, bailout" i.e. there is NO pressure to stay on the loop at all. Obviously staying on the loop is an important factor when deep in a cave etc etc but something to consider when diving more forgiving gas-situations.

The ccr trimix course i taught encouraged students to perform a drill i recommended a few years after AP valves brought inspiration to market. It proved that a diver could have complete loss of electronics and still perform exit from cave and ALL deco on that same rebreather. I wrote a paper on it and not a single agency asked for more info or asked me to go through the procedure. EVERY student that attempted the procedure was able to perform it. My other issue w the dive community; most get SO smart they know it all and quit learning.

 

[St John the Diver]

John, you seem to be taking every opportunity to bash rEvos, I guess it internally validates your choice to change unit, but don't be so complacent about OTS lungs. The loss of buoyancy comes from the fact that the water pressure squeezes the air out of the lungs from below (not flooding as some have claimed). This is true of both back mounted lungs (which are fitted to many units other than the rEvos) and OTS lungs too. The air will be squeezed out of OTS lungs up as far as the connection to the loop, reducing the buoyancy significantly, and just as fast as in the rEvo; what happens then depends on your other buoyancy devices, attitude in the water etc.

The choice of back mounted vs OTS lungs is not a one sided deal, what I like about the rEvo is the very efficient and fast flushing of the loop that I get and I would not trade that for some supposed benefit on the surface. The bottom line on ALL rebreathers is that you shouldn't be reliant on the lungs for buoyancy on the surface.

Keep safe

Rich

For my own edification I tested it.

Five seconds to total loss of gas with rEvo

Infinite time to total loss of gas on the rig im diving now. It stagnated at about a 30% loss after twenty seconds

The scrubbers of the rEvo were soaked. The other rig had a dry scrubber, and was cleared and diving continued.

Done in a swimming pool. We pulled the cells out of the rEvo as we suspected the results in advance.

Data is data. There's no comparison in the results.

 

[Capt Tom McCarthy]

The ccr trimix course i taught encouraged students to perform a drill i recommended a few years after AP valves brought inspiration to market. It proved that a diver could have complete loss of electronics and still perform exit from cave and ALL deco on that same rebreather. I wrote a paper on it and not a single agency asked for more info or asked me to go through the procedure. EVERY student that attempted the procedure was able to perform it. My other issue w the dive community; most get SO smart they know it all and quit learning.

Care to elaborate?

 

[St John the Diver]

The ccr trimix course i taught encouraged students to perform a drill i recommended a few years after AP valves brought inspiration to market. It proved that a diver could have complete loss of electronics and still perform exit from cave and ALL deco on that same rebreather. I wrote a paper on it and not a single agency asked for more info or asked me to go through the procedure. EVERY student that attempted the procedure was able to perform it. My other issue w the dive community; most get SO smart they know it all and quit learning.

Smithers came up with the fin-kick counting method for no cells bailout. It works: Nobody seems to remember it though. For certain there aren't any courses at the Phd level of rebreather diver, but there ought to be.

Constant buoyancy method, fin kick count method, etc etc... None of its taught. Most "instructors" have no clue.

 

[RichardC]

Infinite time to total loss of gas on the rig im diving now. It stagnated at about a 30% loss after twenty seconds..

John, I don't dispute your figures, but 30% loss in 20 seconds is significant and fast. If you were relying on that it would matter. I also imagine it relies on the exhale valve holding. I still wouldn't trade it for the flushing.

 

[Capt Tom McCarthy]

View attachment 395950

Yea, Occam's Razor has flaws when it comes to real world application. Many diving related accidents are more often like a Rube Goldberg machine in how they lead up to an actual incident. Some of these leave evidence. Some of these leave nothing. Some of these reset or vanish as soon as they happen.

 

[St John the Diver]

John, I don't dispute your figures, but 30% loss in 20 seconds is significant and fast. If you were relying on that it would matter. I also imagine it relies on the exhale valve holding. I still wouldn't trade it for the flushing.

Exhaust valve is at the bottom of the loop (unlike the rEvo) so that's not a player. Ever see Leon cut the bottom out of his counterlungs with scissors? My rig has a similar layout. And the flush is just as fast.

But "whatever".... My point is simply that on the rig this diver was wearing, it's very very possible that loss of all loop buoyancy after a DSV removal on the surface was a factor. That particular rig vents all of its gas very VERY quickly when that's done. And that's not subject to much debate. It's just physics.