Rebreather NSS/CDS report from death at Peacock earlier this year.

[Alekseolsen]

It might not be supported by literature but its clearly based in fact.


This tragedy would have been prevented if a build sheet had been followed and or a pre dive checklist followed.
The diver had multiple opratuitys to catch his mistake but chose to neglect his training.

I believe Crofrogs point is that people have done prebreaths without even noticing the scrubber is missing. It takes time to build enough CO2 in the loop to notice something is off relaxing at a bench at 1 atm.

HSE rules allows 15 minute exposure to 30000 ppm (3% CO2), thus I'd say we can't expect people to notice 30000ppm at 1atm.

3% of a loop (including canister, lungs) of say 20 liters if 0,6 liters.
Asleep we produce ~11 liters/hour --> .18 liters/min --> More than 3 minutes breabreathe just to get to a CO2 level where you are allowed to regulary carry out work for 15 minutes.

To top it off how much CO2 we tolerante is widely different between persons.

Suggest to have a look here:

https://petemesley.com/wp-content/uploads/2022/04/Deng-Five-minute-prebreath-in-evaluating-CO2-absorption.pdf

 

[fullytek]

I believe Crofrogs point is that people have done prebreaths without even noticing the scrubber is missing. It takes time to build enough CO2 in the loop to notice something is off relaxing at a bench at 1 atm.

HSE rules allows 15 minute exposure to 30000 ppm (3% CO2), thus I'd say we can't expect people to notice 30000ppm at 1atm.

3% of a loop (including canister, lungs) of say 20 liters if 0,6 liters.
Asleep we produce ~11 liters/hour --> .18 liters/min --> More than 3 minutes breabreathe just to get to a CO2 level where you are allowed to regulary carry out work for 15 minutes.

To top it off how much CO2 we tolerante is widely different between persons.

Suggest to have a look here:

https://petemesley.com/wp-content/uploads/2022/04/Deng-Five-minute-prebreath-in-evaluating-CO2-absorption.pdf

So both of you are advocating for skipping the pre-jump and build sheet?

 

[Alekseolsen]

So both of you are advocating for skipping the pre-jump and build sheet?

Nobody is, but believeing that is a 100% fix for everything is highly inaccurate and a dangerous belief

 

[fullytek]

You guys are looking for an equipment solution to the problem whare clearly the diver neglected to follow the 2 critical quality control checks that he was trained to do.

 

[inquis]

So both of you are advocating for skipping the pre-jump and build sheet?

The objection was to the perceived certainty of detection of scrubber error during a pre-breath step. That's it. That does not equate to scrapping the entire build/jump checklists.

If anything, the build checklist becomes MORE important since more than 25% of users will not notice a problem even if they did pre-breath.

 

[ChrisMBC]

The objection was to the perceived certainty of detection of scrubber error during a pre-breath step. That's it. That does not equate to scrapping the entire build/jump checklists.

If anything, the build checklist becomes MORE important since more than 25% of users will not notice a problem even if they did pre-breath.

Exactly - and I would say that the 75% detection rate (15/20 subjects noticing a completely absent scrubber) is an absolute BEST case for this kind of failure. Two additional things I would highlight from that study:

• The 'partially failed' canister (sealing o-ring between the canister and the head) was only detected by 10% (2/20) divers. I'd assume that the kind of misconfiguration in this accident would sit somewhere between the fully-absent and partially-absent canister situations tested in the study, so inferrring a detection percentage somewhere between 10% and 75% would be reasonable.
  
  
• In the test, "Subjects were briefed in a standardised manner prior totheir first prebreathe. They were reminded of the symptomsof hypercapnia, and it was emphasised that this was an experiment to determine whether the subjects could detect a scrubber problem if present". This means that people were on high alert for a potential issue, AND STILL, 25% of them couldn't spot that the canister was missing.

Based on that, I think relying whatsoever on the pre-breathe as a mechanism to detect subtle canister issues is just asking for trouble. I don't know what the solution is exactly, but I don't think you can say 'this accident wouldn't have happened if he had pre-breathed correctly'.

 

[ChrisMBC]

You guys are looking for an equipment solution to the problem whare clearly the diver neglected to follow the 2 critical quality control checks that he was trained to do.

Genuinely asking - is there reporting that shows that the diver neglected to pre-breathe?

 

[fullytek]

Genuinely asking - is there reporting that shows that the diver neglected to pre-breathe?

Clearly he did neglect a proper prebreathe . Had he walked from the parking lot to the water on the loop he would have experienced symptoms.

 

[ChrisMBC]

Clearly he did neglect a proper prebreathe . Had he walked from the parking lot to the water on the loop he would have experienced symptoms.

That may well be your personal experience - but bear in mind that individuals' sensitivity to CO2 varies quite a lot. The scientific paper above - which AFAIK is the only rigorous data on pre-breathes and their ability to pick up scrubber problems - suggests that some people are more capable than others at identifying a malfunctioning scrubber, even when they're prompted to look for the symptoms and in a lab setting.

Based on that, I think jumping to the conclusion that he "Clearly he did neglect a proper prebreathe" is very premature. The evidence is just not there that doing a prebreathe = picking up the problems 100% of the time.

 

[Snusmumrik]

This tragedy would have been prevented if a build sheet had been followed and or a pre dive checklist followed.

At least - that's a matter to discuss.

The five-minute prebreathe is an insensitive test for CO 2
scrubber function in a diving rebreather, even when the
scrubber canister is absent. A prebreathe is nevertheless
recommended for purposes such as checking the function
of the oxygen addition system before entering the water,
but a duration less than five minutes should be adequate
for that purpose.

If we assume that root cause is hypercapnia, IMHO empty DIL cylinder shows that diver completely lack knowledge of hypercapnia management and bailing out procedure.

Arguably the most important secondary
finding of our study is that partial scrubber failure in a
rebreather is a particularly insidious fault if divers rely on a
prebreathe to detect it. By modestly increasing ventilation,
subjects typically maintain normocapnia during a surface
prebreathe in this condition, resulting in a false negative
that is dangerous because normocapnia is much less likely
to be maintained during the dive itself. These findings raise
concerns around methods for testing and monitoring safe
CO2 elimination in rebreather circuits. Several manufacturers
offer CO2 analyzers in the inhale limb of the rebreather circuit
as an option, but these are not yet mainstream features. We
recommend that rebreather training courses emphasize
the importance of correct packing and installation of CO 2
scrubber canisters. There is mounting evidence that divers
are poor at recognizing the early symptoms of hypercapnia
(during both prebreathes and diving) and strategies for
avoidance of hypercapnia should be prioritized