Shearwater CNS Metric

[Jeff Hester]

I'm trying to wrap my head around the CNS value that is being measure on my Shearwater Petrel 2. I'm on a rEvo rebreather, we did a dive at 1.3 ppO2 for 150 minutes, roughly 3 hours on the surface, then a 60 minute dive at 1.3 ppO2. After dive 1, the CNS value was 72, after dive 2, it was right around 60. If I look at the NOAA tables that I thought the algorithm was based on, 210 minutes is the max for a 24 hour period at 1.3 ppO2, and I know most say go to 80% of that for a safety factor. I've also seen the CNS value over 100 before so my original assumption was that it's not a percentage, but now that I type this out, would it just be saying that I was over 100% of my CNS allotment? And then in the case of the 2 dive day at 1.3, does the clock not take into account the 24 hour period column on the NOAA table? Thanks for any help with clarification!

 

[KevinNM]

Nobody has ever been able to explain the science behind the CNS clock. The CNS partial pressure of O2 and the pulmonary clock have underlying science, but I’m not sure the CNS clock does.

 

[Jeff Hester]

Thanks Kevin, appreciate the response. I found the other thread with folks debating the merits of the clock. The interesting part for me is that I'm a working diver so because the NOAA CNS clock exists, the companies that hire me feel the need to abide by it, even though I can't really find a basis for it.

 

[OTF]

Here's what Shearwater itself has published on that question: Shearwater and the CNS Oxygen Clock - Shearwater Research

For what it's worth people doing deep exploration diving will blow WAY past the limits that these computers recommend for CNS, surfacing with like 300% or much more. Not that I'm advocating for that at all, it's definitely adding risk, but some people are tolerating those risks. This stuff is getting firmly into the realm of unknown unknowns, our science still doesn't fully understand the mechanisms and potential long term hazards of extreme O2 exposure. Personally I play it safe by the numbers, and pop antioxidant pills lol.

Out of curiosity what commercial work are you doing on a CCR? Scientific? I would assume that most employers who know (or worry) enough to care about CNS would be using hardhats not rEvos.

 

[Jeff Hester]

Thanks for that response @OTF, I read that article but it doesn't seem to address the 24 hour time period that the NOAA table notes, just a single dive exposure, so I'm also wondering if that's in the calculation or not? It doesn't seem to be based on comparing the dives we did against the table itself as we should be pushing to 100% after the second dive.

I work as a wildlife cameraman.

 

[inquis]

After dive 1, the CNS value was 72, after dive 2, it was right around 60.

Lots of debate, of course, but the question is one of decay in the model when on the surface. NOAA uses a 24 hr rolling window. Others will use a half-time decay with 2 hr time constant. Still others use a 90 minute half-time, including Shearwater as evidenced by my logs.

At PO2 of 1.3, you accumulate at 0.556% per minute. After 150 mins, that's 83%. After a 180 minute surface interval, it will have decayed to 0.5^(180/90) = 0.25 of its value, down to about 21%. Then dive 2 adds another 0.556% * 60 mins = 33%, bringing the total to about 54%.

Seems to be in line with your readings, with the variation explained by non-constant PO2 on actual dives. Whether or not it makes any sense is a different question, but hopefully that helps you understand the calculations.

 

[Jeff Hester]

Really appreciate that @inquis, now the readings make sense! Thanks for taking the time.