I figured I would ask this question because I cannot get a definitive answer from anyone. Here it goes.....I realize that you can dive for a maximum of 45 minutes @ PO2 of 1.6. The TDI chart then explains that the daily time spent at the PO2 of 1.6 is 150 minutes. But is that true? I thought that O2 has a half life of roughly 90 minutes on the surface. Does this at all effect the maximum daily amount of time that one can spend at a particular PO2? The other question I have stems from the same area. Can one spend a maximum daily time at one PO2, then move to a shallower depth and spend the maximum daily time there too? For example, let's say someone was at PO2 of 1.0 for 300 minutes, could they hypothetically move shallower to PO2 of 0.9 and have 360 minutes to spend? Thanks.
The truth about Oxygen pressure time limits
- Thread starter diverryan
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The TDI and other NOAA derived charts do not give any direct credit for half life effects so they are pretty conservative, although they do require a 2 hour surface interval if you exceed 80% of the single dive limit on any dive.
Even with the conservatism, you still have to dive pretty hard to exceed the limits unless you are doing serious deco at a 1.6 PO2.
To figure the % of the total CNS clock used on dives of different depths, you'd figure each dive using the %CNS/min numbers for the given PO2.
For example, if the max PO2 or your first dive was 1.4 and you stayed for 20 minutes, you would multiply the %CNS/Min (.67%) by 40 minutes to get 26.8%. If the second dive had a max PO2 of 1.1 for 60 minutes, you would multiply .41 times 60 minutes to get 24.6% and add the two to get 51.4% for the day so far.
Similarly on a deco dive where the PO2 for the deep portion is, for example, 1.4 for 40 minutes, you would have .67% times 40 minutes for 26.8%. Then you'd add in the deco portion with the PO2 at each stop (for simplicities sake we'll say the PO2 was 1.6 at each stop for a total of 30 minutes - unrealistic unless you are using a CCR, but it makes the example shorter.) This would be a %CNS/Min of 2.22% times 30 minutes for a total of 66.6%. Adding both together you get a total of 93.4% of the daily (24 hour) CNS limit for the total dive.
So basically the chart is hard to exceed unless you start spending considerable time at a PO2's greater than 1.4 as the %CNS/Min climbs from .67% per minute at 1.4 to .83% per minute at 1.5 to 2.22% per minute at 1.6.
Even with the conservatism, you still have to dive pretty hard to exceed the limits unless you are doing serious deco at a 1.6 PO2.
To figure the % of the total CNS clock used on dives of different depths, you'd figure each dive using the %CNS/min numbers for the given PO2.
For example, if the max PO2 or your first dive was 1.4 and you stayed for 20 minutes, you would multiply the %CNS/Min (.67%) by 40 minutes to get 26.8%. If the second dive had a max PO2 of 1.1 for 60 minutes, you would multiply .41 times 60 minutes to get 24.6% and add the two to get 51.4% for the day so far.
Similarly on a deco dive where the PO2 for the deep portion is, for example, 1.4 for 40 minutes, you would have .67% times 40 minutes for 26.8%. Then you'd add in the deco portion with the PO2 at each stop (for simplicities sake we'll say the PO2 was 1.6 at each stop for a total of 30 minutes - unrealistic unless you are using a CCR, but it makes the example shorter.) This would be a %CNS/Min of 2.22% times 30 minutes for a total of 66.6%. Adding both together you get a total of 93.4% of the daily (24 hour) CNS limit for the total dive.
So basically the chart is hard to exceed unless you start spending considerable time at a PO2's greater than 1.4 as the %CNS/Min climbs from .67% per minute at 1.4 to .83% per minute at 1.5 to 2.22% per minute at 1.6.
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