Musings on CCR set point

I had an interesting conversation last week with another diver about what set point to use for a specific dive profile on a rebreather.

The other diver sometimes used 1.2 to cut down on the amount of oxygen exposure during longer dives. My “argument” against this was that you would just accumulate more decompression time by using 1.2 vs 1.3 and that the O2 exposure would be about the same due to the longer deco required with 1.2 (Note: this all assumes you performing decompression dives).

I wasn’t sure if this was going to be true or not, but it seemed like a reasonable assumption. I decided to test my assumption by running a few profiles at different set points and then comparing the CNS and OTU exposures.

OTU exposure is related to pulmonary oxygen toxicity and CNS exposure is related to central nervous system oxygen toxicity. I won’t wade into the debate about which is “better” or “more important” other than to say that my computer (Shearwater) only measures and tracks CNS exposure. More info can be found in Erik Baker’s paper on Shearwater’s website: https://www.shearwater.com/wp-content/uploads/2012/08/Oxygen_Toxicity_Calculations.pdf

Generally accepted guidelines are to stay under 300 OTUs in a day and below 80% CNS.

Here is an example dive with all parameters held static except for set point. For those who want to calculate on their own, I used MultiDeco on a 200′ dive for 40 minutes with a gas of 15/50 and a gradient factor of 45/80 (again, let’s not digress into comparing GFs – that isn’t the point of this).

  • SetPoint 1.2 : OTUs 203, CNS 72.2%, Runtime 153 minutes
  • SetPoint 1.3 : OTUs 207, CNS 78.3%, Runtime 140 minutes
  • SetPoint 1.4 : OTUs 208, CNS 82.8%, Runtime 128 minutes

Next, I decided to run a deeper, longer dive of 250 feet for 45 minutes with 10/50

  • SetPoint 1.2 : OTUs 335, CNS 119.3%, Runtime 253 minutes
  • SetPoint 1.3 : OTUs 336, CNS 127.1%, Runtime 227 minutes
  • SetPoint 1.4 : OTUs 337, CNS 134.1%, Runtime 207 minuters

Conclusions?

  1. For any given profile, OTUs are about the same regardless of SetPoint. I find this pretty interesting.
  2. For any given profile, CNS will increase as the SetPoint increases but it isn’t as drastic as I would have expected it to be
  3. For any given profile, runtime is pretty drastically reduced as SetPoint increases. I think it is worth considering the “wear and tear” on your body from oxygen toxicity vs. duration in the water by using a different SetPoint.

Disclaimer: I’m not a doctor but instead just a regular old rebreather diver and you should make your own informed decision about what SetPoint to use.

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