Gotta love passive voice:
Teaching Oxygen Tracking Article 95-2
1. CNS%/minute
Again based on the NOAA single exposure times, the CNS% load for every minute of exposure at that PPO2 is extrapolated and found in tabular form in the Advanced Manual.
aEx. If a diver spends 45 minutes at a PO2 of 1.6 bar (ref. NOAA single exposure limit), then they can be said to have used 100% of their Oxygen Clock. Hence 1 minute of the total allowed exposure as a % would be:
1/45 x 100 - 2.22% for every minute at a 1.6 bar PPO2
The table in the manual shows a %/minute for most PPO2s. This %/minute is multiplied by the Decompression Bottom Time to give the % CNS for the dive (assuming no decompression gas with a high FO2 (above 40%) is used at the decompression stops). Any decompression stops conducted on an elevated FO2 compared to bottom mix must have their CNS and pulmonary load calculated for separately and added to the bottom time oxygen exposure of the dive. As a rule, an 80% CNS exposure is the maximum planned for. If a diver reaches an exposure of 80% on a single dive then they must take a two hour interval at the surface, breathing normoxic air.
Multiple dives may be conducted providing that the final CNS exposure in a 24 hour period does not exceed 80%. If the 80% figure is reached, then a 12 hour surface break breathing normoxic air must be taken.
Where repeat dives are conducted with a surface interval, a 50% surface reduction in the CNS load (breathing normoxic air) is assumed for every 90 minutes at the surface. Ex. Dive 1 generates a load of 40%. After 90 minutes this falls to 20%. After a further 90 minutes this falls to 10% etc. This Residual CNS load is added to the next dives CNS load to give the two dive total at the end of the second dive.
Agreed - everyone assumes it, but no authoritative source quoated.
