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Dennis, They do not coinside at all ! CNS decreases at a 1.5 hr half time rate. (every 1.5 hours the percent decreases by 1/2 - after 6 half times you should be clear - 9 hours) Being clear means that you can start from 0 again. OTU's remain constant for 24 hours by which time you are considered to be cleared out (I would extend this if I were running close to the limits to about 36 hours). It is reccommended that shallow dives within this 24 hour period not be counted as time off of oxygen meaning that these exposures should be counted also (probably as at least .5 PPO2 even if actually less) and they restart the 24 hour count down to clearing. If you dive again within 24 hours of the end of your last dive then you should be calculating for a 2 day mission which would restrict you to a max of 700 units average daily dose (1400 units divided among all dives made in the 2 day period but still adhering to the 850 max daily limit). The OTU clock is giving the tissues of the lungs and other organs time to recover from the effects of high O2 on many of the somatic cells and to give the lungs time to rebuild a layer of surfactants without interferance. That is why it is called Whole Body or Pulmonary Toxicity. The CNS clock gives the body time to expell and recover from effects of high PPO2 that are indeed toxic in every sense to the nervous system . These are two totally different reactions to high doses of oxygen. The CNS is what will kill you quickly and suddenly. The OTU whole body or pulmonary toxicity usually puts you in the hospital with lung damage. On short single dives OTU's rarely need to be considered if you remain within CNS limits simply because the CNS will catch up to you first but over many daily exposures OTU's can become a concern as you have noted. On the OTU table : This table is telling you that if you intend to dive 7 days in a row your average daily dose of O2 should be no more than 380 units meaning that you should not exceed 380 X 7 = 2660 OTU's over the 7 day period and that still, no single day's diving should exceed the 850 limit. If you are diving for 11 days the maxim number of units alowed for the period is 300 X 11 = 3300 OTU's. The 850 limit always applies to any single day (24 hour period). If you skip diving days (24 hr period) in between then technically you can restart from 0. BUT because you are basing your calculations on 7 days of intermittent exposure I would reccommend hedging on this and not reseting the clock till you get about 36 hours of no diving at all if you are running close to the daily limits, which is closer to what you will actually get if you skip a day anyway. If the indicated daily average is 380 (2660 total), as for the 7 day period, you can spread and divide that 2660 up any way you want over the period as long as you do not exceed the 850 individual daily limit. If you decide to extend your expedition from 7 to 11 days in mid stream you will have to change your allowed average daily dose accordingly immediately PLUS reconsider the exposures you already have incurred into the newly calculated plan. NOTE: It appears from your wording that you may believe that the table is telling you to reduce the daily exposure from 850 max on the first day to progressively lower exposures on subsequent days. As you can tell from above this is not the case. Nothing regarding either CNS or OTU's has any effect at all on the other. Each must be considered completely independently. Take a calculator ! Does all this make sense ? Chuck -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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