PADI Nitrox and CNS Clock

[scubadada]

What the formula or table to calculate the time at which a particular cns level will degrade over time?

That's always been my question, not certain I've ever heard the answer. Not absolutely certain, but seems as if my Oceanic computers do not give me credit until at 24 hours after an exposure

 

[rsingler]

What the formula or table to calculate the time at which a particular cns level will degrade over time?

The presumption is that you regain half of your CNS credit every 90 minutes, a time which was derived empirically, I believe.
If you really want to calculate it with a formula, it's: ResidCNS = exp(-0.0077 * SI) * CNS
There's a thread that discusses this from last year here:
CNS% Surface Interval Credit & NOAA Oxygen Exposure Limits [Archive] - ScubaBoard - Scuba Diving Forum - Diving Social Network

Frankly, it's just easier to Google "CNS Surface Interval Credit Table" , which will give you any number of downloads that have this table.

 

[wve]

Not absolutely certain, but seems as if my Oceanic computers do not give me credit until at 24 hours after an exposure

That's what my Sherwood computers do. When I spoke with their tech folks, they referred me to the articles mention previously, leading me to believe that you got credit for any time out of the water. But when we tracked our cumulative oxygen percentages, we found that the percent exposure would decrease by exactly the amount you accumulated on the dive you did 24 hours earlier. In other words, a dive's exposure remained for 24 hours, then just dropped off completely. I suppose that is a more conservative approach.

 

[DevonDiver]

Any decent Instructor will agree to this AND show you how to set your computer aswell..Otherwise what possible reason could they give for not teaching it???

I'm guessing if they sell a course of 'X' content, based on 'Y' duration, for 'Z' cost... then they would have a legitimate reason for not supplementing time/effort/material into that course for no financial reward.

Let's face it... the PADI Nitrox course is very basic in scope... but it's also very cheap.

Not saying I agree with that... just that it's understandable. Especially if that material is available in subsequent courses (it is), that you'd have to pay for.

There's very few scenarios where a recreational diver would have to be too concerned about OTUs and CNS%. It's primarily a concern for technical divers doing accelerated decompression on rich O2. That's why it's typically taught on entry-level tech. Courses do tend to teach you what you need to know at that level, rather than pre-empting future levels.

What the formula or table to calculate the time at which a particular cns level will degrade over time?

These are given in tables. PADI supply those on the Tec40 course. Other agencies supply at the equivalent level: Advanced Nitrox.

Like these: CNS Exposure Table

and here: Diving with Nitrox

If you want a true recreational course that teaches in-depth nitrox use, then look to the ANDI Complete SafeAir User (CSU) course. It's very comprehensive.

 

[pholife]

plain and simple answer for this one ( I believe this goes actually into one of the DM kr's)

In recreational nitrox, blends of 40% or less, CNS is disregarded because the o2 exposure is too low...

As for your exposure to o2, what the guys here are saying is 100% correct, use the NOAA o2 exposure tables

PADI has dumbed down nitrox, the new computer version is ridiculous, but a good majority of recreational divers the last thing they want to be exposed to again is tables... again where we meet sales and safety... but it sure isn't helping the diver move forward as a diver, just more proficient with their computer

grey area... personally I like the old PADI nitrox books for teaching, they actually teach the formulas, the best book I think for the student out there for recreational nitrox is the TDI understanding nitrox book