CNS O2 exposure

[ajduplessis]

As mentioned in post #2, Shearwaters do give credit. From the Petrel's manual: "The CNS percentage is calculated continuously, even when on the surface and turned off." I know the Atomic Cobalt also gives SI credit for CNS clock, though the less detailed manual doesn't mention that. Like post #2, I have no idea what the half-life used is for either one.

That is why a large number of serious tech divers like the Shearwater computers so much. It is the benchmark IMO for dive instruments. I see they now also sell the NERD and I would love to hear what the CCR guys think based on actual experience.

 

[DivemasterDennis]

Sounds like a reason to maintain DSAT table familiarity. That is what I always use to calculate PPO2 when diving nitrox. I should everyone would do so.
DivemasterDennis

 

[kelemvor]

I'd still like to know if some computers give oxygen surface interval credit. It's my impression that this is generally not taken into account for recreational diving.

Scubapro Galileo does.

From the manual:

The CNS O2 clock increases when the oxygen partial pressure is higher than 0.5bar, and decreases when the oxygen partial pressure is lower than 0.5bar. Hence, while on the surface breathing air you will always be decreasing the CNS O2 clock. During the dive, the depth at which 0.5bar is reached for various mixes is as follows:

Air: 13m/43ft
32%: 6m/20ft
36%: 4m/13ft

This might just be a (significant) omission in oceanic computers. One might even be inclined to call it a software bug. Do dive computer manufacturers ever release updated firmwares?

 

[JohnN]

This might just be a (significant) omission in oceanic computers. One might even be inclined to call it a software bug. Do dive computer manufacturers ever release updated firmwares?

You're debating the number of angels that could dance on the head of a pin. You cannot hit your CNS O2 limit diving recreational profiles (as mentioned before, the half-life to clear O2 is 90 minutes).

If you're diving non-recreational profile (dives with a deco obligation), dive planning software (e.g. V-Planner) will calculate and track this for you. Unless your recreational computer locks you out when it thinks you've hit your CNS limit, it really doesn't make any difference

 

[dfx]

You're debating the number of angels that could dance on the head of a pin. You cannot hit your CNS O2 limit diving recreational profiles (as mentioned before, the half-life to clear O2 is 90 minutes).

That's not what agencies teach though.

 

[JohnN]

That's not what agencies teach though.

OK. . . please give me a series of recreational dives (no deco) where you think you reach your daily O2 CNS limit.

 

[Nirvana]

I agree with JohnN in that it is extremely unlikely for a recreational diver to reach the single exposure CNS O2 limit, once you accept the 90min half-life concept. In fact, according to V-Planner, even a series of 10 dives to 25m for 45min each on EAN40, separated by 1h SI would bring the diver to 78% only.

However, if we include monitoring for 24h exposure, as recommended, for example, in the NOAA diving manual, we would find a limit of 180min per 24h for 1.4 atm PPO2, which would have been clearly breached in this example and can be reached in far more realistic scenarios.

The conclusion is that perhaps it is better for a rec diver to monitor 24h exposure, as it is a more relevant piece of information. It seems that is what the OP's computer is doing.

 

[DevonDiver]

That's not what agencies teach though.

Not sure which agencies you refer to. The two largest agencies certainly don't...

Here's what PADI used to teach:

"It’s recommended that you have a surface interval of at least an hour between enriched air dives whenever possible, especially if you exceed more than 50 percent of allowable exposure. This is believed to further reduce the likelihood of oxygen toxicity.

1. There are methods for calculating surface interval credit for your oxygen exposure. Enriched air dive computers generally do this automatically.

2. In recreational no stop enriched air diving, the benefits of such credit are minor, yet make repetitive dive planning with tables unnecessarily complex. Therefore, you don’t calculate surface interval credits when determining oxygen exposure with tables.


3. Technical divers can benefi t from oxygen exposure surface interval credit and are trained to do so. [Refer interested students to the DSAT Tec Deep Diver Manual for more information, but emphasize that surface interval credit for oxygen exposure is not really an issue in recreational enriched air diving.]"


That's the 'old' in-depth EANx course. The new EANx course barely even mentions it..

Here is what SSI say:

"For recreational divers, as long as you do not exceed a P02 of 1.4 atmospheres and your single dive time does not exceed 150 minutes 12.5 hours}, or your 24 hour limit does not exceed 180 minutes 13 hours}, the oxygen limits won't be exceeded."

"...Requires extensive exposure at P02 levels above .5 atmospheres, which are well above those experienced by recreational divers".

"As this is a recreational Nitrox course, you should place primary emphasis on avoiding the need to perform complex CNS "Clock" calculations by remaining within a limiting PD2 of 1.4 atmospheres and keeping 24-hour accumulated Actual Bottom Time lABTI well within the 180 minutes NOAA recommends. Remind students that CNS "Clock" calculations are covered solely as a contingency in case they should accidentally exceed these limits."

 

[runez]

You're debating the number of angels that could dance on the head of a pin. You cannot hit your CNS O2 limit diving recreational profiles (as mentioned before, the half-life to clear O2 is 90 minutes).

If you're diving non-recreational profile (dives with a deco obligation), dive planning software (e.g. V-Planner) will calculate and track this for you. Unless your recreational computer locks you out when it thinks you've hit your CNS limit, it really doesn't make any difference

OK. . . please give me a series of recreational dives (no deco) where you think you reach your daily O2 CNS limit.

I was taught that it was not essential to track CNS O2 for recreation diving, as it is most likely that the NDL will be reached first. This thread got me thinking as to whether this statement is indeed true.

After some number crunching to answer whether NDL will alway be reached before CNS O2 limits. I arrived at some interesting findings:

1. Using high EAN% (36 & 40) and a high ppO2 (1.6ata) limit, it is theoretically possible to hit the NOAA single dive and daily limits before hitting NDL over multiple dives.
2. It is possible to hit the NOAA single dive and/or daily limits for some profiles, e.g, 4 dives using (EAN40, 30m, 45 minutes) will hit the NOAA daily limit.

Then I realised something else, would a recreational diver have enough gas to perform the dives with a high CNS O2 loading? Assuming a single AL80 tank like many divers in my region use:

1. Some of the theoretical high CNS O2 loading dive scenarios, also require a ridiculously low SCR. Probably unattainable by most recreational divers.
2. Adjusting the numbers for a reasonable SCR (say, 15L/min), it is difficult to reach the CNS O2 limit even for EAN 40 for ppO2 < 1.4

Hence, my conclusion is this: It is highly unlikely for a recreational diver to reach CNS O2 limits if ppO2 exposure is kept < 1.4 bar.

I hope these calculations help, any comments are welcome!

 

[dfx]

Not sure which agencies you refer to. The two largest agencies certainly don't...

Here's what PADI used to teach:

"It&#8217;s recommended that you have a surface interval of at least an hour between enriched air dives whenever possible, especially if you exceed more than 50 percent of allowable exposure. This is believed to further reduce the likelihood of oxygen toxicity.

1. There are methods for calculating surface interval credit for your oxygen exposure. Enriched air dive computers generally do this automatically.

2. In recreational no stop enriched air diving, the benefits of such credit are minor, yet make repetitive dive planning with tables unnecessarily complex. Therefore, you don&#8217;t calculate surface interval credits when determining oxygen exposure with tables.


3. Technical divers can benefi t from oxygen exposure surface interval credit and are trained to do so. [Refer interested students to the DSAT Tec Deep Diver Manual for more information, but emphasize that surface interval credit for oxygen exposure is not really an issue in recreational enriched air diving.]"


That's the 'old' in-depth EANx course. The new EANx course barely even mentions it..

Here is what SSI say:

"For recreational divers, as long as you do not exceed a P02 of 1.4 atmospheres and your single dive time does not exceed 150 minutes 12.5 hours}, or your 24 hour limit does not exceed 180 minutes 13 hours}, the oxygen limits won't be exceeded."

"...Requires extensive exposure at P02 levels above .5 atmospheres, which are well above those experienced by recreational divers".

"As this is a recreational Nitrox course, you should place primary emphasis on avoiding the need to perform complex CNS "Clock" calculations by remaining within a limiting PD2 of 1.4 atmospheres and keeping 24-hour accumulated Actual Bottom Time lABTI well within the 180 minutes NOAA recommends. Remind students that CNS "Clock" calculations are covered solely as a contingency in case they should accidentally exceed these limits."

I don't know what you're reading there, but I see the 24-hour allowed exposure clearly mentioned several times in the excerpts you posted.