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***ScubaDadMiami*** · 23rd September 2007, 20:48

I think you are mixing and (mis)matching CNS clock and OTU related issues.

The CNS O2 clock, which deals with oxygen exposure to the central nervous system (toxing issues), does have a surface interval credit feature for less than 24 hours. Even if you are at 100 percent, after a 6 hour surface interval, you are all the way back down to 7 percent. Also, by using intervals of lower PO2 during dives (such as the equivalent of back gas breaks on open circuit), divers have managed to complete dives in which they theoretically have gone well above the 100 percent mark.

Now, I am in no way encouraging anyone to dive to anywhere near the maximum, and I certainly am not doing so for above the mark. However, I am just putting this information out for the sake of discussion and comparison.

One thing that has been drilled into me during training (thank you, John) is that you really do have to start paying attention to OTU exposure (pulmonary exposure) when you start doing longer dives and multiple diving days. There is no credit for intervals less than 24 hours when tracking OTUs like there is for CNS O2.

Keep in mind that the maximum exposures suggested contemplate being able to complete a chamber treatment after. You do not want to limit your options in that regard by pushing it in your diving.

Treating things in a conservative way, I just go with the long-term exposure limits of 300 per day, which one can tolerate pretty much forever. This is pretty easy to do if you keep the PO2 for your dives in the conservative range.

One other related issue: Myopia. You do not want to start pushing high PO2 for several days because this has led myopia issues. It seems that some divers can tolerate the exposures better than others. So, you do not want to get close to the mark by experimenting on yourself during these outings.

Really, you will find that there will not be a significant difference in run time using a lower PO2 when compared to a higher one (within reasonable limits). However, you will find that your O2 exposures will be siginicantly different. I tend to run at lower PO2 during the dive and then bump up higher during portions of decompression, and this often makes the run time on the dive just a few minutes longer than it would were I to run at a consistent PO2 throughout the entire dive.

23rd September 2007, 20:48	#10 (permalink)
*ScubaDadMiami* Moderator Current Rebreather/s: Optima Other Rebreather/s: Join Date: Sep 2005 Location: Florida, USA Posts: 2,220	Re: Oxygen Clock Question I think you are mixing and (mis)matching CNS clock and OTU related issues. The CNS O2 clock, which deals with oxygen exposure to the central nervous system (toxing issues), does have a surface interval credit feature for less than 24 hours. Even if you are at 100 percent, after a 6 hour surface interval, you are all the way back down to 7 percent. Also, by using intervals of lower PO2 during dives (such as the equivalent of back gas breaks on open circuit), divers have managed to complete dives in which they theoretically have gone well above the 100 percent mark. Now, I am in no way encouraging anyone to dive to anywhere near the maximum, and I certainly am not doing so for above the mark. However, I am just putting this information out for the sake of discussion and comparison. One thing that has been drilled into me during training (thank you, John) is that you really do have to start paying attention to OTU exposure (pulmonary exposure) when you start doing longer dives and multiple diving days. There is no credit for intervals less than 24 hours when tracking OTUs like there is for CNS O2. Keep in mind that the maximum exposures suggested contemplate being able to complete a chamber treatment after. You do not want to limit your options in that regard by pushing it in your diving. Treating things in a conservative way, I just go with the long-term exposure limits of 300 per day, which one can tolerate pretty much forever. This is pretty easy to do if you keep the PO2 for your dives in the conservative range. One other related issue: Myopia. You do not want to start pushing high PO2 for several days because this has led myopia issues. It seems that some divers can tolerate the exposures better than others. So, you do not want to get close to the mark by experimenting on yourself during these outings. Really, you will find that there will not be a significant difference in run time using a lower PO2 when compared to a higher one (within reasonable limits). However, you will find that your O2 exposures will be siginicantly different. I tend to run at lower PO2 during the dive and then bump up higher during portions of decompression, and this often makes the run time on the dive just a few minutes longer than it would were I to run at a consistent PO2 throughout the entire dive. __________________ Howard Packer IANTD CCR Instructor Miami Beach, Florida CCRDiveTraining.com Last edited by ScubaDadMiami : 23rd September 2007 at 20:50.
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