CNS% Calculations

Hi Scubaboard.

I am wondering if anybody can give me a formula for calculating CNS% loading for a given pressure and a given O2 content. I know I only have to look up the tables to find the answer, but I am curious to know how these figures where arrived at... Can anybody help?

I wrote a spreadsheet that did this, but it was very much a 'table look-up' format, rather than a mathematical formulae. If there is a formulae, I wasn't able to reverse engineer it from the NOAA tables.

I will be surprised if you don't get the correct answer today, lots of smart intelligent people read these forums.

Easiest way is just to dive to an average of 1.2PPO2 as a maximum.
Then you can get an excellent approximation by dividing your bottom time by 2

I suspect that the limits are empirically derived.

There is an interesting article that describes how oxygen limits have changed over time, but it doesn't go into too much detail about the work that was done in defining the limits:

http://archive.rubicon-foundation.org/dspace/bitstream/123456789/3986/1/15233156.pdf

This may be of interest:

Need an excel formula that will calculate CNS% [Archive] - ScubaBoard

Personally I use decoplanner

Not entirely sure what you're asking. Since your handle implies you're into tech diving, it could be one of several things:

Are you using a fixed maximum PO2 such as 1.4? If so, perhaps it's simply the ratio of the pressure at your depth to the pressure at MOD.

Are you taking into consideration that CNS also has a time element (which isn't always mentioned in rec-level courses)? If so, perhaps you're looking for the ratio of your time at that PO2 to the time limit for that PO2.

Are you perhaps actually asking about pulmonary toxicity? If so, this article:
ftp://downloadfiles:decompression1@ftp.decompression.org/Baker/Oxygen Toxicity Calculations.pdf
has relevant equations and should be of interest. In fact, it's interesting whatever you're asking.

tenerifetechdiving:

I know I only have to look up the tables to find the answer, but...

Click to expand...

Which tables are you referring to? From what source?

Rhone Man:

I wrote a spreadsheet that did this, but it was very much a 'table look-up' format, rather than a mathematical formulae. If there is a formulae, I wasn't able to reverse engineer it from the NOAA tables.

Click to expand...

I did a order curve fit and backed out an annoyingly super high polynomial that works okay-ish. But looking at the table is better.

I had the same issue a few weeks ago, with some research I came up with this.
There is also a NOAA CNS Exposure chart
Have a look at my thread, there might be some info from me or other posters that could help you
http://www.scubaboard.com/forums/advanced-scuba-discussions/331389-trouble-understanding-nitrox-po2-2.html

To calculate this we use a CNS table:

PO2 Maximum Single Exposure
(minutes) Maximum 24 hour Exposure
(minutes)
1.6 45 150
1.5 120 180
1.4 150 180
1.3 180 210
1.2 210 240
1.1 240 270
1.0 300 300
0.9 360 360
0.8 450 450
0.7 570 570
0.6 720 720

To calculate the CNS% do this:
CNS% = Time spent / Maximum Single Exposure (for used PO2)

For example let's say we did a dive to 21msw/70fsw for 50 minutes using EAN32. What's the CNS%?

Let's start by finding our our PO2.
PO2 = FO2 * Pata => PO2 = 0.32 * 3.1 ATA = 0.99 ATA
We always round the ATA up and look it up in the table.

Our maximum single exposure time is 300 minutes.
CNS% = 50 min / 300 min = 16.7%
So our accumulated CNS% is 16.7%

If we do multilevel diving we have to calculate CNS% for every segment of the dive and add them together.

Also doing a surface interval longer then 90 minutes will lower the CNS clock with 50%.

Scubadives:

I had the same issue a few weeks ago, with some research I came up with this.
.

Also doing a surface interval longer then 90 minutes will lower the CNS clock with 50%.

Click to expand...

Or you could just assume a "maximum average" PPO2 of 1.2 for recreational dives and divide your bottom time by 2, and then reduce the CNS residual percentage according to the 90 min halftime rule ....

I know which I'd rather do