CNS% Surface Interval Credit & NOAA Oxygen Exposure Limits

[exposure]

Hi,

I am looking at formulas that can explain the CNS% Surface Interval Credit and the NOAA Oxygen Exposure Limits. I use to use a table or decompression software while planning a dive but basicaly I'd like to know what is the formula that can produce these two tables.

I have reproduced all tables that I learned during my courses (Deep Stops, Trimix END, SAC Conv., MOD, PO2, OTUs and CNS%). But I can find anything for CNS% Credit and the NOAA one.

If some of you are interested by these tables, they are here.

Thanks in advance for your help.
Yvan

 

[exposure]

Nobody has that info

 

[PfcAJ]

IIRC, its a 50% reduction in the CNS% every 90mins.

So if you were at 100%, after a 90min SI, you'd be down to 50%. Another 90mins, and you're at 25%, and so on. That lines up with your table, I believe.

How that number was derived has eluded me, and I think its probably highly questionable (as is the entire CNS% idea).

 

[gkndivebum]

ResidCNS = exp(-0.0077 * SIinMin) * CNS

In other words, assuming a 90 minute halftime, you can calculate your residual CNS exposure by the exponential function listed above, where SIinMin is your surface interval in minutes. Exponential means e​ raised to the power of x: Euiler's number (or Napier's Constant) [not to be confused with Euiler's Constant], the transcendental and irrational number 2.71828...

e = lim (1 + 1/n) ** n
n->infinity

Alegbra lesson over for today.

 

[exposure]

Thank to both of you, that was exactly what I was looking for
ANything for the NOAA part?

 

[gkndivebum]

​From: Hamilton, R.W., Kenyon, D.J., Peterson, R.E., Butler, G.J., & Beers, D.M. REPEX: Development of Repetitive Excursions, Surfacing Techniques, and Oxygen Procedures for Habitat Diving. NURP Technical Report 88-1A, 1988.

PO2	%CNS/min	OTUs/min
0.6	0.14	0.26
0.7	0.18	0.47
0.8	0.22	0.65
0.9	0.28	0.83
1.0	0.33	1.00
1.1	0.42	1.16
1.2	0.48	1.32
1.3	0.56	1.48
1.4	0.67	1.63
1.5	0.83	1.78
1.6	2.22	1.93

OTU: Oxygen Tolerance/Toxicity Unit: an estimation of pulmonary ("whole body") oxygen exposure (loss of vital capacity; the Lorrain Smith effect)

OTUs = (((2 * PO2) - 1) ** 0.833) * time.

 

[exposure]

Thanks gkndivebum. My question was not properly presented. In fact, I am looking for some more detail of the daily (or multi day) limits set by the NOAA, like 850 OTUs for 1 day, 1400 for 2 days and so on....

 

[gkndivebum]

[...] I am looking for some more detail of the daily (or multi day) limits set by the NOAA, like 850 OTUs for 1 day, 1400 for 2 days and so on....

Ah - where do those magic numbers come from, anyway?

Turns out according to NOAA you're allowed 1440 OTUs in a day (O2 at 1.0 ATA for a 24 hour period). The recommended maximums in the tables come from leaving enough OTUs to tolerate a USN TT6 recompression session.

A TT6 is approximately 600 OTUs; 1440-600 = 840, rounded up to 850 for the first day. As far as I know the reduction in allowable exposure for subsequent days is based on a SWAG.

 

[exposure]

Thanks gkn, that really makes sense actually.