CNS Daily O2 limits and Rec only diving.

Continuation of other threads on this subject with a specific application focus.

lets just take NOAA 1.6 at 45min/150 daily, if I dive 32% at 132 ft(ppO2 1.6), in a square profile single tank dive this would take a sac of .4ish so within reason but rare.

however lets say my dive is to 132 for 3 min, and then meander back up the reef and end my dive after 45min, most of this dive would have been about 40-50ft.

assuming I want to make 4 of these dives a day for a week how do I track my CNS % because if I use the table I will be exceeding limits due to the few min at 132 and if I use my computer I will be fine.

I leave tonight for a week in Bonaire, and I am planing a few deeper air/nitrox dives.
on my first trip I tried to be aware of the cns% limit but after tracking a few dives I realised I was wasting time. and my computer never shows much of any loading even with 5 dives a day.

assuming I track as well as possible the different depths and times can I use the CNS table and calc 3 min at 130, 5 min at 90, 20 min at 50, 20 min at 40 and add them all up? or am I measuring an inch with a thumb?

Do a multilevel calculation, this is similar to what your computer does:
1. estimate how much time you are spending at the different depth ranges
(with a resolution of e.g. 1m/3ft - this correlates nicely with the resolution of
my DSAT table, which is 0.1bar ppO2)

2. add up the results like this:
1.6bar, 3min =5%,
1.5bar, 6min = 5%,
1.4bar 8min = 5%,
1.3bar, 18min = 10%,
1.2bar, 20min= 10%,
1.1bar, 10min= 5%
<1.0bar, 15min= 5% total: 45%.

I wouldn't choose to dive 32% at 130 feet.

Better to get the training,use 2 tanks of air and do some deco.

If you stick to 32%, do not go deeper than 110 feet, and do not go into deco, you can completely forget about CNS issues.

ianr33:

I wouldn't choose to dive 32% at 130 feet.

Better to get the training,use 2 tanks of air and do some deco.

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I'd rather take EANx30. It gives you a ppO2 = 1.5bar at 130', where the
CNS clock is ticking slower than half the speed as at 1.6bar.
(50% saturation reached after 60min as opposed to 23)

I know, 30% doesn't seems much of an improvement compared to air, but
provided it is a multilevel dive with a slow ascent, one could spend quite a
lot of time at the bottom and off-gas in the shallows.

SailNaked:

assuming I want to make 4 of these dives a day for a week how do I track my CNS % because if I use the table I will be exceeding limits due to the few min at 132 and if I use my computer I will be fine.

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I recently did a weeks diving overseas and was using Nitrox three to four times a day close to the MOD limits for short periods of time. Hence the interest in the topic. My dive computer gives a graph of the oxygen exposure and I'd expect if you have one that would be the easiest ways to track the exposure.

As others have said Id stick with the MOD for a PPO2 of 1.4 b.

4x130' dives a day is quite a lot... For the 3rd one I'd pick air, to give a little chance to the O2 to clean. For the last I'd be back to nitrox just to avoid the bends. Besides, I'd maximize the surface intervals - start diving as early as reasonable and the last dive could be a night dive.

My computer (Subgear xp10) is bonusing a 90min half-life to the CNS load while on the surface.

really interested in the calculations and what the current theory is. and what the justification of the theory is. not what others would or would not do. the example was only to put real numbers out there not suggest a dive profile.

in reality I will be going to 50-70ft for 40% of the dive and then 40-30 until about 60-65 min with a 20min safety stop at 20-15ft. and I do not plan to track CNS % at all. after 5 trips and over 100 dives doing this I am not expecting any changes.

would still like to know how cns % is tracked over many days of multiple level diving.

seems to me this is the same as trying to compare dive tables to computers. you really cant. you have two choices that i see. you either follow what your computer says or you use the o2 tables in the manor you were trained to do in your nitrox class. if the tables unfairly penalize you because in reality you were multilevel diving, oh well, thats the price you pay for having to use the tables. it will be very conservative. so if you don't feel you want to do that, then follow the computer data instead.
perhaps hooking up with an advanced instructor to take an advanced deco class and/or an advanced nitrox class might get you the answers you are looking for

SailNaked:

would still like to know how cns % is tracked over many days of multiple level diving.

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The accepted half-life for O2 clearing is 90 minutes (at least according to NOAA). If you assume you're taking an overnight break of 12 hours (so your last night-dive is at 9:00 PM and you splash the next morning at 9:00 AM) -- [This is a pretty hard-core profile for multiple days and leaves very little time for the mandatory post-dive beer interval], you would have 12*60/90 = 8 half-lives, so your O2 level would be (1/2)^8 = 0.004 which is close enough to zero to call your system entirely cleared.

As has been pointed out in multiple threads, unless you seriously exceed a pp of 1.4 for extended periods of time, it is extremely difficult to exceed your daily Ox-Tox threshold for recreational dives

SailNaked:

really interested in the calculations and what the current theory is. and what the justification of the theory is.

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You might want to download a trial version of V-Planner and play with some profiles. It will give you a % CNS loading at the end of each dive in a series of dives.

You would most likely be fine dropping to 130 feet on 32% for a few minutes. Unless you are not, in which case you are dead.

---------- Post added September 13th, 2013 at 12:58 PM ----------

tamas970:

I'd rather take EANx30.

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I think EANx28 would be even better, but the OP is not likely to get the choice. Off the shelf choices in Bonaire will be air or 32%