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NOAA CNS Percentage Exposure Table
NOAA CNS Oxygen Exposure limits: Table gives percentage of NOAA "allowable" limits for a single dive. At the 1.6 atm exposure level the "CNS Clock" runs almost 4 times faster as at a PO2 exposure level of 1.4 atm. An exposure to a PO2 1.4 atm is the maximum "recommended" limit for the working portion of any dive. Exposure levels above a PO2 of 1.4 atm are shown for contengency planning and for calculations concerning the decompression (resting) portion of stage decompression dives.
The NOAA CNS Oxygen Exposure Limits chart is used for avoiding CNS Oxygen Toxicity problems while conducting multiple dives on one day. The first columm represents the maximum oxygen partial pressure exposure in atmospheres. The second column gives the maximum duration, in minutes, a diver may safely remain in the water at that exposure during a single dive. The subsquent columns represent the percentage of the maximum exposure limit used during a single dive, in five minute intervals.
A diver having made the first enriched air nitrox dive of the day would find the CNS exposure percentage on the table. By subtracting that number from 100% they could determine the maximum exposure for their next dive. Following the second dive, the CNS exposure percentages from each dive would be added, then subtracted from 100% to determine the maximum allowable exposure for a third dive. While many authorities suggest that a 90 minute half-time is applicable for surface interval credit when considering oxygen exposures, such credit is seldom taken into account for planning recreational dives.
This table is commonly used by divers trained in the use of Enriched Air Nitrox and/or Trimix. Diver's who have not completed training in the use of these specialized gases should not attempt to execute dives using them. The NOAA CNS Oxygen Exposure Limit chart is provided for informational purposes only.
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