NOAA CNS exposure limits

[Padowan]

What are peoples feelings here on RB diving and the CNS limits? Looking at next week I am likely to be doing 3 or 4 dives per day on the new 'box and if I run the SP at 1.3, the daily limit (single dive limit not really an issue for me at the moment) is 210 mins. If I drop the PO2 SP to 1.2bar, this increases to 240 minutes, meaning I can do 4x 1 hour dives in a day without violating the limits.

BUT, at levels over 80% of the limits (ie ~190 minutes at 1.2bar) are you not supposed to start looking at air breaks (something I know about from my Trimix diving, but have not yet had to do, as I've always been within the limits on OC)

Am I being over cautious? Would a 1.2bar SP for the week be a safer option (I can still likely laugh at my OC buddies on deco!)? Any thoughts or suggestions on a suitable way to manage the exposure (perhaps running some of the shallower dives on the 0.7bar SP?)

Cheers.

[Juz]

Assuming you're diving no deeper than 40m, you could even go for a PPO2 of 1.1 or perhaps 1.0, you'd still be using a higher PPO2 than everyone else on the boat at the deep part and still having a much higher PPO2 than them on most of the dive.

Obviously I've not done the course yet, but seems to make sense to me.

Juz

[Phil Ennis]

Remember, you have a 90min halftime on your CNS. You will come out eg with 30% cns but after a 90min SI you will go back in with a residual of only 15%.

It will take a lot to rack up your 80% unless you are cutting your SI's down considerably. If you plan properly then there shouldn't be a problem.

[Padowan]

Originally Posted by Juz

Assuming you're diving no deeper than 40m, you could even go for a PPO2 of 1.1 or perhaps 1.0, you'd still be using a higher PPO2 than everyone else on the boat at the deep part and still having a much higher PPO2 than them on most of the dive.

Obviously I've not done the course yet, but seems to make sense to me.

Juz

Yeah. At the moment I'm thinking that dropping the SP to 1.2bar is conservative enough, and will allow me to get in the 4x 1 hour dives. I've modified my logsheets to include some spaces for me to track CNS loadings and scrubber hours too.

Do you have a date yet for the course?

[Padowan]

Originally Posted by Phil Ennis

Remember, you have a 90min halftime on your CNS. You will come out eg with 30% cns but after a 90min SI you will go back in with a residual of only 15%.

It will take a lot to rack up your 80% unless you are cutting your SI's down considerably. If you plan properly then there shouldn't be a problem.

But aren't the NOAA limits kind of halftime irelevant - surely thats why they have a column for "per dive" and another column "per day"?

If I can include a halftime decay, then 1.3bar would probably also be fine.

Is the decay curve not for pulmonary OTU, as opposed to CNS? Perhaps that's where the difference lies.

I rememder in my AdvNx course that basically "if you look after the %CNS, the OTU pretty much look after themselves", which would make sense if OTUs include a decay co-efficient too?

[Phil Ennis]

Sorry, I didn't read your post properly. Yes you are limited to 210mins, so as said just knock your SP down to fit your diving schedule. Sp 1.2 would just allow your 4hrs.

[wreckweasel]

Originally Posted by Padowan

But aren't the NOAA limits kind of halftime irelevant - surely thats why they have a column for "per dive" and another column "per day"?

If I can include a halftime decay, then 1.3bar would probably also be fine.

Is the decay curve not for pulmonary OTU, as opposed to CNS? Perhaps that's where the difference lies.

I rememder in my AdvNx course that basically "if you look after the %CNS, the OTU pretty much look after themselves", which would make sense if OTUs include a decay co-efficient too?

The halftime decay applies to CNS. Essentially, on OC, provided you don't splurge your PO2's, you can near enough forget OTU unless you're doing serious diving.

On CCR you'd do well to observe single day exposure limits, there have been a few documented cases of Hyperoxic myopia (one of which was in the seychelles!)

Still, tis fun eh?

[Padowan]

Originally Posted by zaksherlock

Still, tis fun eh?

Oh yes indeed!!!

[Juz]

Originally Posted by Padowan

Do you have a date yet for the course?

Yeah, not til the end of Nov, but that's the trade off for wanting to do it with Jack, I could have gone elsewhere and done it immediatley, but I know and trust Jack, so Nov it is.

Juz

[Padowan]

Originally Posted by Juz

Yeah, not til the end of Nov, but that's the trade off for wanting to do it with Jack, I could have gone elsewhere and done it immediatley, but I know and trust Jack, so Nov it is.

Juz

Do you know if you're doing TDI or IANTD with Jack?