Quote: (Originally Posted by Genesis)
Now where there MAY be controversy is if you disallow override (e.g. during decompression.) I'm not sure that is nearly as open-and-shut as it is for the "working" portion of the dive.
Well that is where the interesting bit comes in. You see, from the two US Navy based trials referenced (Butler, Central Nervous System Oxygen Toxicity in Closed Loop SCUBA Divers II and Harabin, A Model for Predicting Central Nervous System Oxygen Toxicity from Hyperbaric Oxygen Exposures in Humans), the predicted death rate if you decompress on a PPO2 of 1.3 and run your CNS to 75% is quite high - far higher than the 1 in a billion requirement set in European Law.
As a quick check on whether the calculated figure of 1 in 10,000 to 1 in 100,000 risk is correct, we looked at this forum and accident lists. There were roughly 500 members of this forum when we looked, not all of whom have rebreathers. Their experience seems to be on average about 3 years. There are six incidents reported of O2 convulsions. This suggests the convulsion rate is below the 1 in 100,000 hour level.
Possible reasons for this are also cited. CO2 and Helium both increase the risk of O2 toxicity. See again the references cited.
Finally, there is the issue of error in calibration or failure of multiple sensors causing the PPO2 to be out by enough to have an impact on O2 toxicity.
Given these factors, it is necessary to either put a safety margin around the measured PPO2 or some other conservatism factor.
The margin around PPO2 seems the best. Given the Naval studies, simply doing the sums says that to get the risk below 1 in a billion hours, one needs a padding factor of 0.25. So we can put in 0.2 and 0.3 for aggressive and conservative use.
This would mean you can decompress on 1.4 even, but your CNS clock would tick fast causing you to need low PPO2 breaks.
Comments more than welcome. I do emphase this is a mathematical thing rather than anything subjective. We need to back decisions by hard facts and prove probabilities: this again is a requirement for EC approval - a Safety Assessment is required and various standards stipulate the requirements these must meet.
There is an update already to the paper, from internal feedback since January. The change is a minor one, reported on the front page, relating to simplifying the integer calculation. There is no change in results for the data set presented.
Cheers
Alex