Need an excel formula that will calculate CNS%

[John C. Ratliff]

I did a bit of looking, and see that CNS% is actually the percentage of oxygen in partial pressures that will affect the central nervous system (CNS) to potentially cause oxygen toxicity, potentially resulting in convulsions and death. While this is a diving forum, it never hurts to define your terms. My earlier post was assuming you had some new designation for determining decompression tables. Being an air diver all my life, oxygen toxicity is not something I have ever been concerned with, as that would happen on air at somewhere around 297 feet depth.

SeaRat

 

[addohm]

FWIW, the OTU calculation is:
OTU/min = (2 * PPO2 - 1)^(5/6).

=DURATION*((PO2-0.5)/0.5)^0.83
This is the formula I use in my planning worksheet.

 

[addohm]

I am trying to write a excel spreadsheet that will figure CNS calculations based on an entered time & PO2.

However, I am not having any luck making it work.

I need a formula that will look at the PO2 column make a decision based on the PO2 then take the time from the time column and divide the time by the maxim exposure for that PO2.

example... if the PO2 in the column is 1.5 and the Time in the TIME column is 20 then I need the formula to divide 20 by 150.

example... if the PO2 in the column is 1.6 and the TIME column is 20 I need 20/45.

example....if the PO2 in the column is 1.4 and the Time column is 15 I need 15/150.

example... if the PO2 is 1.5 and the Time is 30 then I need the formula to divide 30 by 120.

and so on....

thanks!

OTUs = EXPOSURE_TIME*((PO2-0.5)/0.5)^0.83

 

[LFMarm]

Shearwater and the CNS Oxygen Clock

The question of what CNS oxygen exposure is safe can spark heated debate among technical divers. This reflects the seriousness of CNS oxygen toxicity when diving. This article sidesteps that debate, and focuses solely on how Shearwater dive computers calculate the CNS value. The CNS display is a...

shearwater.com

Shearwater uses linear interpolation of the NOAA table

 

[pig]

I should have read to the end of this thread before I started, so I need to read that Shearwater article in more detail, but I had another crack at an excel formula:

=IF(ppo2 < 1.4, 0.3971 * ppo2^2 - 0.146 * ppo2 + 0.0848, 0.0205 * EXP(21.82 * (ppo2 - 1.4)) + (0.651 * 1.4 - 0.284 - 0.0205))

Which fits pretty well:

The blue dots are NOAA, the orange ones are from that equation. It under counts a tiny bit on the 1.4 dot, but the rest seems to be pretty close.

PO2	NOAA	piecewise	Δ
0.55	0.12​	0.12​	0.00​
0.60	0.14​	0.14​	0.00​
0.65	0.16​	0.16​	0.00​
0.70	0.18​	0.18​	0.00​
0.75	0.20​	0.20​	0.00​
0.80	0.22​	0.22​	0.00​
0.85	0.25​	0.25​	0.00​
0.90	0.28​	0.28​	0.00​
0.95	0.30​	0.30​	0.00​
1.00	0.33​	0.34​	-0.01​
1.05	0.37​	0.37​	0.00​
1.10	0.42​	0.40​	0.02​
1.15	0.44​	0.44​	0.00​
1.20	0.48​	0.48​	0.00​
1.25	0.51​	0.52​	-0.01​
1.30	0.56​	0.57​	-0.01​
1.35	0.61​	0.61​	0.00​
1.40	0.67​	0.63​	0.04​
1.45	0.72​	0.67​	0.05​
1.50	0.83​	0.79​	0.04​
1.55	1.11​	1.15​	-0.04​
1.60	2.22​	2.22​	0.00​

What are your general thoughts?

 

[nadwidny]

I should have read to the end of this thread before I started, so I need to read that Shearwater article in more detail, but I had another crack at an excel formula:

​

What are your general thoughts?

Worrying about CNS clock is a total waste of time. People tox at 75%. People tox at 100% and a lot don't at 110%+. It's never been proven to have any scientific validity.

 

[tursiops]

I believe the NOAA tables came directly from the US Navy tables, and those were based on the work of Kenneth Donald in the early 1940s, and on work by Lambertsen and Lamphier and others at the NEDU post-war and in the early 1950s.. I've attached the original two papers by Donald. His work was summarized in a 1992 book (Oxygen and the Diver). Much of this is reported in a 2004 article by Frank Butler which I have also attached.

 

[inquis]

What are your general thoughts?

Stay under 1.4, and i's practically impossible to have an issue doing recreational dives.

 

[crofrog]

What are your general thoughts?

The CNS clock seems to be more myth than reality, based on the number of dives that routinely exceed it by staggering amounts

 

[boulderjohn]

The CNS clock seems to be more myth than reality, based on the number of dives that routinely exceed it by staggering amounts

That is a problem for tech instructors who are required to teach the CNS clock and keep dives within those limits. What do you do when a student asks about those dives that "routinely exceed it by staggering amounts"? For those who don't know, I am not sure the word "staggering" does it justice. The problem is not keeping the dives within the limits--it takes a dive well beyond typical training dives to do that. You have to tell the students that the people doing those mammoth dives do not believe the CNS clock matters, and so far they have been OK.