One dead, one missing (since found), 300 foot dive - Lake Michigan

[RainPilot]

Do you have specific information the rest of us don’t have access too how dwell time had a cause in this accident?

Nothing specific to this incident, but a known factor in deep CCR diving is the relation to gas density and dwell time. The very short, probably wrong in all the details, version is:

Higher density leads to higher work of breathing. These two then lead to great difficulty in flushing CO2 from the lungs. This leads to higher rate of breathing, which in turn can lead to too short a dwell time in the scrubber, adding to the CO2 issues. It really is a cascade of things at this stage, each one interconnected, but it all leads to a CO2 event.

It's the most likely cause of the Dave Shaw accident, and (my guess) several other "unexplained" deep diving incidents.

 

[Johnoly]

.. but a known factor in deep CCR diving is the relation to gas density and dwell time. ...

It's illegal for me to dive a CCR so I have limited knowledge. Is the corrective action while deep underwater to increase O2 injections or are you just screwed because there's no way to purge the CO2 without causing further gas mix problems at depth?

 

[doctormike]

It's illegal for me to dive a CCR

I want to know the backstory on this....

 

[RainPilot]

It's illegal for me to dive a CCR so I have limited knowledge. Is the corrective action while deep underwater to increase O2 injections or are you just screwed because there's no way to purge the CO2 without causing further gas mix problems at depth?

Disclaimer: I am not a deep CCR diver, nor do I play one on ScubaBoard...

My understanding is that, once you have the gases you are using, you are stuck with the density at depth. If the WOB gets too much, your only option is to reduce your CO2 production and try to flush as much as possible to stop the cascade. Use of an OC bailout may assist, as you can breathe that as fast as you like without creating scrubber issues, but the key is recognizing the issues and then acting appropriately. If you watch the Dave Shaw video, the time frame from him starting to exert himself till he passes out is REALLY short. It also doesn't help that CO2 excess leads to a reduction in thinking ability either.

Adding O2 won't help, in that O2 is more dense than a high helium diluent so replacing He with O2 will exacerbate the problem, as well as all the other high PPO2 issues that may arise.

I would guess that a dil flush might help,IF the dil PPO2 is significantly lower than the setpoint your unit is maintaining, by replacing some of that O2 with He but that is just a guess. No idea on how practical that may be.

 

[beester]

It's illegal for me to dive a CCR so I have limited knowledge. Is the corrective action while deep underwater to increase O2 injections or are you just screwed because there's no way to purge the CO2 without causing further gas mix problems at depth?

Once there is an excess of CO² the only viable solution is bailing out to OC. It is still sketchy... because you will be breathing very rapidly and consuming your bail out at an alarming rate.

 

[Johnoly]

I want to know the backstory on this....

I'm a harvester,,,it's against the law.

 

[doctormike]

It's illegal for me to dive a CCR so I have limited knowledge. Is the corrective action while deep underwater to increase O2 injections or are you just screwed because there's no way to purge the CO2 without causing further gas mix problems at depth?

I don't dive deep either, but basically, there are two causes of hypercapnea: Overproduction and some sort of failure of the loop/scrubber system. The former would be things like excessive work, high gas density, etc... where your body just produces more CO2 than the system can handle. The latter would be things like channeling of the sorb, running the scrubber too long, or failure of a mushroom valve.

So you can definitely do a diluent flush to replace the loop contents with a known gas mixture that is safe to breathe and that has no CO2 in it. Sometimes, that's enough to make things better, especially if the problem is overproduction of CO2 and you can get that under control. Of course, if the problem is a scrubber failure, that will only buy a short amount of time, and you will need to bail out to open circuit.

 

[uwxplorer]

Two reasons why flushing diluent could be counterproductive at these depths (on a rEvo):
1) on a rEvo, the OPV is right next to the diluent/O2 injection points. So if you press injection and just wait, it will flush your exhale counterlung really good, but most of the gas will escape through the OPV. Flushing the loop on a rEvo involves actively breathing and then venting through the nose or loose lips. Not sure that is practical if you are hypercapniac and fighting for gas...
2) at 300 ft, the pressure is ~10 atm and flushing the loop (~5 L) will take several times 5×10 = 50 L of gas (remember, a lot of gas is wasted due to the strategic positioning of the OPV). Let's be conservative and say 3 times that. That's 25% of a full 3 L tank filled to 200 bar. How many times are you willing to try that (CO2 is still building up in this scenario)?
Of course this doesn't resolve anything if hypercapnia is the result of shallow breathing, due to overexertion or any other cause, as you will still be retaining CO2.

Of course, hypercapnia is a completely hypothetical scenario as we don't know if it had anything to do with this tragic accident.

On a different topic, I could picture one diver trying to haul back an inanimate buddy by inflating her BC, letting go halfway because of strain or slipperiness, and taking off uncontrollably to the surface.

 

[dmaziuk]

Higher density leads to higher work of breathing. These two then lead to great difficulty in flushing CO2 from the lungs. This leads to higher rate of breathing, which in turn can lead to too short a dwell time in the scrubber ...

Thank you, now I get how our built-in CO2 detector and high gas density play (or, rather, fail) together. Or remember: I did watch the talk of Dave Shaw accident (Simon Mitchell's?) a while back, but I forgot if this was spelled out in it.

 

[doctormike]

Two reasons why flushing diluent could be counterproductive at these depths (on a rEvo):
1) on a rEvo, the OPV is right next to the diluent/O2 injection points. So if you press injection and just wait, it will flush your exhale counterlung really good, but most of the gas will escape through the OPV. Flushing the loop on a rEvo involves actively breathing and then venting through the nose or loose lips. Not sure that is practical if you are hypercapniac and fighting for gas...
2) at 300 ft, the pressure is ~10 atm and flushing the loop (~5 L) will take several times 5×10 = 50 L of gas (remember, a lot of gas is wasted due to the strategic positioning of the OPV). Let's be conservative and say 3 times that. That's 25% of a full 3 L tank filled to 200 bar. How many times are you willing to try that (CO2 is still building up in this scenario)?
Of course this doesn't resolve anything if hypercapnia is the result of shallow breathing, due to overexertion or any other cause, as you will still be retaining CO2.

Yeah, I don't know much about rEvo... On the JJ a dil flush comes through the manual add button on the ADV, on the inhale loop, while the OPV is on the exhale counterlung. I just pull the dump valve while pushing the ADV, breathing to circulate.

It won't resolve anything if hypercapnia is the result of a loop or scrubber failure, but it might actually break the vicious cycle if you are working too hard, or if you are shallow breathing, increasing the proportion of dead space ventilation..

Basically, I agree with you, If you have a serious CO2 hit from a loop or scrubber failure, probably not best to waste time with a flush, better to just bail out. But if you are just starting to overbreathe the unit and don't feel great, sometimes that's a reasonable alternative. Taking a vent, as the commercial divers say. I guess that's unit specific, but unless you are in a cave with a lot of up and down to get back to the surface, you don't really need dil any more at that point.