Fiona Sharp death in Bonaire

[JohnnyC]

She made to 91m and back to 24m with air diluent. That means she was not OXTOX at PPO2 2.0 and as she accent to 24m, the PPO2 would steadily drop to 0.7, which would be in a normal range for diving with air. Why would she became unconscious at 24m with air diluent?

Could her CO2 scrubber be completely spent on the way up? If that was the case, she could also be in hypercapnia situation and became unconscious.

Not necessarily. She could certainly tox deep and happen to be positively buoyant, the loop venting as she ascends until it floods completely making her negative enough to settle on the reef at 24m. Not saying that's what happened, but it's a possibility.

Scrubber duration may or may not be a factor. Was she re-using a scrubber that already had time on it? Was it packed improperly leading to channeling? She would have to have purposely avoided bailing out, CO2 doesn't just go from fine to unconscious. And with a BOV, there would certainly be ample warning and opportunity to bail off the loop, first by switching to BOV, and then switching to her offboard gas. One of the benefits of a BOV is that you don't have to do the switch to a second stage while being physiologically incapable of holding your breath, so the idea that she took a CO2 hit and that's what killed her is kind of far-fetched. Not saying it's impossible, but fairly unlikely unless there were other contributing factors as well.

 

[Couv]

I don't know rebreathers so please bear with the ignorant question.
In a previous post, someone mentioned a technique of breathing the O2% down to arrive at depth with less O2 PP at depth. What are the chances the opposite happened? I.e. Dr. Sharp breathed down the O2% at depth, fail to add more into the loop, and when she arrived at 24m became hypoxic?

 

[Dan]

I don't know rebreathers so please bear with the ignorant question.
In a previous post, someone mentioned a technique of breathing the O2% down to arrive at depth with less O2 PP at depth. What are the chances the opposite happened? I.e. Dr. Sharp breathed down the O2% at depth, fail to add more into the loop, and when she arrived at 24m became hypoxic?

To my understanding she used air as diluent gas, which has 21% oxygen. So she would not be hypoxic. She would be the opposite, hyperoxic.

 

[Couv]

So she would not be hypoxic. She would be the opposite, hyperoxic.

Let me rephrase.
Could she have breathe the O2 level down at 91m-then ascended to 24m without adding any more air into the loop?

 

[Dan]

Let me rephrase.
Could she have breathe the O2 level down at 91m-then ascended to 24m without adding any more air into the loop?

To my understanding, every time you exhale you will lose 4-5% of the gas volume as some of the oxygen is metabolized and converted to carbon dioxide, which is further absorbed by scrubber. To keep from generating vacuum in the loop, a diluent gas must be added. If you used helium as the diluent gas or trimix with low oxygen concentration, then you can slowly reduce the oxygen concentration in the loop. She used air for the diluent gas, from what I read. So there is no way that she could bring the oxygen concentration down.

 

[tursiops]

Let me rephrase.
Could she have breathe the O2 level down at 91m-then ascended to 24m without adding any more air into the loop?

I'm not sure I'm understanding your point.
Let's say air got breathed down to PPO2=1.6 at 91m. That's about 17% O2 in the loop. The percentage will drop further as you ascend, due to continuing metabolism, while the PPO2 decreases as you ascend. if no more O2 were metabolized, at 25m the percentage would still be 17%, and the PPO2 would be 0.6. To be hypoxic at 25m, she'd need a PPO2 of less than about 0.17, which means about 5% O2 in the loop. So, to become hypoxic, she'd need to breath the 17% O2 down to 5% O2.
is this the kind of thing you mean?

 

[wedivebc]

To my understanding, every time you exhale you will lose 4-5% of the gas volume as some of the oxygen is metabolized and converted to carbon dioxide, which is further absorbed by scrubber. To keep from generating vacuum in the loop, a diluent gas must be added. If you used helium as the diluent gas or trimix with low oxygen concentration, then you can slowly reduce the oxygen concentration in the loop. She used air for the diluent gas, from what I read. So there is no way that she could bring the oxygen concentration down.

The diluent adds volume that result from pressure changes and losses through leaks in mask, mouthpiece etc. At a constant depth the inert gas volume is stable. Oxygen losses caused by metabolism are replaced by adding oxygen not diluent.

 

[Dan]

The diluent adds volume that result from pressure changes and losses through leaks in mask, mouthpiece etc. At a constant depth the inert gas volume is stable. Oxygen losses caused by metabolism are replaced by adding oxygen not diluent.

I understand that part. But adding 100% oxygen to make up the losses from metabolism would not helping in reducing the PPO2 in the loop, would it?

 

[The Chairman]

I worry about a depressed PPO2, or hypoxia, on every CCR ascent since it's a potential "shallow water blackout" situation. As you ascend, you have to vent, vent, vent in order to not start a runaway ascent. You must be sure to add enough 02 in order to drive your PPO2 up to counteract this.

 

[dmaziuk]

Not necessarily. She could certainly tox deep and happen to be positively buoyant, the loop venting as she ascends until it floods completely making her negative enough to settle on the reef at 24m. Not saying that's what happened, but it's a possibility.

Anything's possible but 24 msw on the reef is maybe 30 m from shore at Buddy's, possibly closer. 90 msw is 300 m out and the current is normally at right angle to required drift direction.