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Scott - The "oxygen clock" is a useful concept to think about the body's accumulation of oxygen toxicity over time. There are two principal type of oxygen toxicity - pulmonary and CNS. Pulmonary oxygen toxicity starts to occur with long-term breathing (over 24 hours) of oxygen at a partial pressure over 0.5. The higher the partial pressure of oxygen, the shorter exposure time required before pulmonary oxygen toxicity begins to develop. Pragmatically, a recreational "technical" diver generally doesn't have to worry about pulmonary oxygen toxicity unless he/she needs to undergo extensive recompression therapy (more than a Navy Table VI). CNS toxicity is more critical. The higher the partial pressure, the "faster" the oxygen toxicity clock runs. Exercise, carbon dioxide retention, and other factors increase the susceptibility to CNS oxygen toxicity. The NOAA limits I cited are "conservative" limits, based on some "wet" diving human experimentation plus industry experience. In addition to the limits I included (which are single dive limits), there are also 24-hour limits for repetitive dives. Staying within these limits exposure a diver to a very low risk of CNS oxygen toxicity. Exceeding a partial pressure of 1.6, even for a few minutes, exposures a "working" diver to a very high risk of an oxygen seizure. My specific example (125 minute exposure to a ppO2 max of 1.4)is just an example of the use of the tables. If you're going to be exposed to O2 over 0.5 partial pressure on a dive for 125 minutes, you should limit the maximum ppO2 at any point of the dive to 1.4. Martin > -----Original Message----- > From: ScottBonis@ao*.co* [mailto:ScottBonis@ao*.co*] > Sent: Tuesday, February 08, 2000 6:42 PM > To: quiglem@ib*.ne* > Cc: techdiver@aquanaut.com > Subject: Oxygen Limits > > > Hi Martin, > > On 2/7/00 you wrote; > << ... The 3rd Edition of the NOAA Manual (4th due out this month > - but I don't expect changes) has time/pO2 limit tables for > oxygen exposure for working divers as follows: > SINGLE EXPOSURE Maximums: > PPO2 ATA Minutes > 1.6 45 > 1.5 120 > 1.4 150 > 1.3 180 > 1.2 210 > (i.e., if your underwater time is planned for 125 minutes, the > max. you can breath at any point in the dive is 1.4).... >> > > Could you please explain a little more about the parenthetical > statement. How does this correlate with the concept of an "Oxygen clock?" > > Thanks a lot for your help. > > Scott > > -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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