Virginian diver dead at 190 feet - Roaring River State Park, Missouri

[steinbil]

Theory is helium is really cheap on a rebreather as you use so little, so there’s no reason to dive CCR without helium.

Obviously. I’m just puzzled how a gas consisting of oxygen and nitrogen can be more narcotic on a CCR than on OC, it seems to defy logic…

 

[ChuckP]

Obviously. I’m just puzzled how a gas consisting of oxygen and nitrogen can be more narcotic on a CCR than on OC, it seems to defy logic…

My argument on that is that it is not actually the gas being breathed but the increased WOB of the rebreather verses OC - the increased WOB is actually causing a lower CO2 flush from your lungs - CO2 is 10x more narcotic then Nitrogen or Oxygen.

 

[steinbil]

My argument on that is that it is not actually the gas being breathed but the increased WOB of the rebreather verses OC - the increased WOB is actually causing a lower CO2 flush from your lungs - CO2 is 10x more narcotic then Nitrogen or Oxygen.

That makes perfect sense. I didn't consider increased WOB from the loop. All the more reason to use helium, then.

PS. @Nitrogenius
I still don't understand how that increased WOB and CO2 retention can be quantified in any meaningful way to say that the equivalent depth would be 60m instead of 58m, and I don't see how the O2 setpoint makes a difference?

 

[kensuf]

Obviously. I’m just puzzled how a gas consisting of oxygen and nitrogen can be more narcotic on a CCR than on OC, it seems to defy logic…

Increased work of breathing = increased CO2 retention in your body. CO2 is the devils gas.

 

[L13]

...CO2 is the devils gas.

Amen!

 

[Jack Hammer]

Obviously. I’m just puzzled how a gas consisting of oxygen and nitrogen can be more narcotic on a CCR than on OC, it seems to defy logic…

On a rebreather your lungs are powering the gas movement, not the pressure in the reg hose, there is resistance on both inhale and exhale. You essentially suck in the gas to your lungs then blow out to keep the gas moving through the scrubber media. This action makes the gas 'feel' denser and makes your body produce more CO2 compared to breathing the same gas on OC.

 

[Vicko]

On OC once you expel CO2 trough respiration that gas is gone, while on ccr that gas goes trough the scrubber that takes out the CO2, with a denser gas the scrubber has more chance of "missing" some of the CO2 molecules and sending them back to your lungs, starting a cycle where every time you exhale the scrubber misses more and more CO2, leading to hypercapnia, leading to more CO2 in the gas you are breathing, leading to death.
So even if the gas density is the same on OC and CCR, and you being in perfect fitness and capable of breathing that gas density without over exertion, on a CCR there are more factors affecting the CO2 elimination.

 

[steinbil]

I appreciate the explanations on extra WOB on a RB, compounding the effect of CO2 retention, I didn't consider that at first, which made my reply a little less precise than I intended. I also learned that dense gas makes the scrubber less effective, good to know.

However, I'm still confused by the post I responded to, which seemed to imply that somehow the gas got more narcotic because of the O2 setpoint of the RB. I still don't see how that is possible.

 

[formernuke]

I appreciate the explanations on extra WOB on a RB, compounding the effect of CO2 retention, I didn't consider that at first, which made my reply a little less precise than I intended. I also learned that dense gas makes the scrubber less effective, good to know.

However, I'm still confused by the post I responded to, which seemed to imply that somehow the gas got more narcotic because of the O2 setpoint of the RB. I still don't see how that is possible.

O2 is also narcotic according to some studies. So with that rich of a blend and no helium he had to have been extremely "high"

 

[stuartv]

You're not looking at how standard gases fit into the overall safety system, how they can be used to remove pressure from the divers to do the dive without the right gases, and how they can enable other members of the group to speak up when they see something they view as unsafe happening.

By choosing to dive standardized gasses the team is agreeing before the "heat of the moment" on what is an acceptable pp02, END, maximum allowable gas density, and other considerations that go into gas selection.

If he'd had been diving standardized gasses and using a team approach to diving the conversation on the pre-dive checks could have gone something like this.

We're planning a dive to 190ft and the standard gas chosen by the team for this dive as dilout is 15/55. This could have enabled the deceased to speak up and say that he only had a tank of nx26 right then and there and he was outside of the safety parameters discussed by the team beforehand and prevented this incident, or if he choose to continue the dive during in water checks he would have shown his team his gas analysis sticker during the bubble check, and then during the pressure checks reported to his team his pressure and gas selection. This would have enabled the team to say something like. I'm not comfortable doing this dive with you on Nx26 when you should have 15/55.

To summarize, some of the benefits of standard gasses are:

• The gas selection parameters are chosen beforehand, not when you're dealing with logistical or cost issues and avoids debates around acceptable risk once people have already invested considerable time and money and feel external pressures to complete the dive
• Simplifies project logistics
• Enables the diver to have SOPs to fall back on when making decisions that might compromise safety
• Enables the team to have SOPs to fall back on when someone is committing unsafe acts.

I make a distinction between standard gases (maybe I should write Standard Gases) and standardized gases. For team projects, I fully support standardIZED gases. That is not the same as supporting Standard Gases.

I appreciate the explanations on extra WOB on a RB, compounding the effect of CO2 retention, I didn't consider that at first, which made my reply a little less precise than I intended. I also learned that dense gas makes the scrubber less effective, good to know.

However, I'm still confused by the post I responded to, which seemed to imply that somehow the gas got more narcotic because of the O2 setpoint of the RB. I still don't see how that is possible.

I think what was written was simply a shortcut for writing out that the mix itself is not more narcotic, but the person is more susceptible to narcosis because of the various factors involved in CCR diving versus OC.