cns clock
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blacknet
Contributor
Hello,
They didn't cover this in your nitrox class?
This IANTD article will answer your questions on the cns clock.
Ed
They didn't cover this in your nitrox class?
This IANTD article will answer your questions on the cns clock.
Ed
Charlie99
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Actually Blacknet, the article you referenced calculates the CNS for repetitive dives quite differently than NOAA, and which is what is taught in PADI Nitrox class (and probably NAUI, SSI, etc.) What did your first nitrox class teach, Blacknet?
Doc6464 --- Many assume that CNS loading has a 90 minute halftime. This is true of IANTD, and computers such as Oceanic. Others use an even shorter halftime --- Suunto uses 60 minute IIRC.
NOAA uses a simpler method where the CNS loading from a earlier dive just hangs around for 24 hours, then instantaneously disappears after 24 hours. Obviously, that doesn't make sense from a physical standpoint, but it makes calculation easier.
Personally, I just use a 2 hour halftime, but it isn't all that relevant since I'm not doing accelerated deco and rarely run my clock as high as 50%, even though my MOD limit is based on 1.6ppO2. My computer, meanwhile, is decaying the CNS clock at 90% halftime, but only while surfaced.
So by my calculations: 60% goes to 15% after 4 hours.
By NOAA: 60% is still 60%
By 90 minute rule: 60% is about 10%
by 60 minutes rule (too agressive for me): 4%
Doc6464 --- Many assume that CNS loading has a 90 minute halftime. This is true of IANTD, and computers such as Oceanic. Others use an even shorter halftime --- Suunto uses 60 minute IIRC.
NOAA uses a simpler method where the CNS loading from a earlier dive just hangs around for 24 hours, then instantaneously disappears after 24 hours. Obviously, that doesn't make sense from a physical standpoint, but it makes calculation easier.
Personally, I just use a 2 hour halftime, but it isn't all that relevant since I'm not doing accelerated deco and rarely run my clock as high as 50%, even though my MOD limit is based on 1.6ppO2. My computer, meanwhile, is decaying the CNS clock at 90% halftime, but only while surfaced.
So by my calculations: 60% goes to 15% after 4 hours.
By NOAA: 60% is still 60%
By 90 minute rule: 60% is about 10%
by 60 minutes rule (too agressive for me): 4%
The Iceni
Medical Moderator
I am not really convinced that there exists a true "CNS oxygen exposure clock" since the sensitivity to a CNS hit is so variable and it is an all-or-nothing phenomenon, rather than an accumulative effect as is so clearly seen with pulmonary oxygen toxixity.
As a rule of thumb, as has been posted elswhere, with a ppO2 above 1 bar there is a statistical risk of a CNS hit which increases with increasing ppO2, until a hit is almost guaranteed with even short exposures above 3 bar ppO2.
Reference to this thread tends to support this opinion.
As a rule of thumb, as has been posted elswhere, with a ppO2 above 1 bar there is a statistical risk of a CNS hit which increases with increasing ppO2, until a hit is almost guaranteed with even short exposures above 3 bar ppO2.
Reference to this thread tends to support this opinion.
Dr. Paul has hit the nail squarely on the head... a habit of his.
While we still teach and calculate CNS exposure for decompression or for multiple dives using reference to a CNS clock calculation, the half-time issue is up for debate right now. Using NOAA daily rates and sticking below 80% of the total is commonly considered best practice. With multiple dives, consider the daily dosage as sacrosanct.
With the example given above (60% four hour SIT) the best option would be to keep total exposure to less than 20%!
In practical terms, a dive delivering 60% of the allowable CNS loading would be 1/ a kick a** deco dive 2/ a "poorly" planned dive with nitrox... you may have been better served to have used a lower pO2 for bottom mix.
While we still teach and calculate CNS exposure for decompression or for multiple dives using reference to a CNS clock calculation, the half-time issue is up for debate right now. Using NOAA daily rates and sticking below 80% of the total is commonly considered best practice. With multiple dives, consider the daily dosage as sacrosanct.
With the example given above (60% four hour SIT) the best option would be to keep total exposure to less than 20%!
In practical terms, a dive delivering 60% of the allowable CNS loading would be 1/ a kick a** deco dive 2/ a "poorly" planned dive with nitrox... you may have been better served to have used a lower pO2 for bottom mix.
Dr Deco
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Dear Readers:
Oxygen clock
I suspect that the clock is a lot like limits. A nice concept in theory but something that does not exist in reality. Everyone is different in DCS tolerance and different in oxygen tolerance.
One problem with decompression is that as the process departs for ideal, the number of growing gas bubbles increases. This will radically change the decompression characteristics since the nitrogen is no longer all in solution.
The possibility exists also with oxygen toxicity. As the deleterious effects of oxygen increase, it might well be that the oxygen-scavenging enzymes are not only overwhelmed but are also themselves destroyed. One could easily see how this could vastly change the picture in some cases. I have not experimental evidence of this and simple use it as an example to indicate how irreversible processes can alter the picture.
Dr Deco :doctor:
Readers, please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm
Oxygen clock
I suspect that the clock is a lot like limits. A nice concept in theory but something that does not exist in reality. Everyone is different in DCS tolerance and different in oxygen tolerance.
One problem with decompression is that as the process departs for ideal, the number of growing gas bubbles increases. This will radically change the decompression characteristics since the nitrogen is no longer all in solution.
The possibility exists also with oxygen toxicity. As the deleterious effects of oxygen increase, it might well be that the oxygen-scavenging enzymes are not only overwhelmed but are also themselves destroyed. One could easily see how this could vastly change the picture in some cases. I have not experimental evidence of this and simple use it as an example to indicate how irreversible processes can alter the picture.
Dr Deco :doctor:
Readers, please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm
blacknet
Contributor
Hello,
Does this mean we can ignore the cns % that we are taught to keep tabs on? Will be interesting to look at cns accidents and find out how many was with in the 'clock' and how many was over.
Ed
Does this mean we can ignore the cns % that we are taught to keep tabs on? Will be interesting to look at cns accidents and find out how many was with in the 'clock' and how many was over.
Ed
Charlie99
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It would also be interesting to know how many cns accidents ocurred while the diver was breathing the gas he thought he was breathing. It seems to me like the most common causes of oxtox hits are
1) improper gas switches (or simply descending on the wrong bottle), and
2) mixing problems or labelling problems compounded by failing to analyze the gas.
No. NO. NO!!!
I do not think anyone here is suggesting that you ignore the CNS loading... you have to track it and until some reputable source tells us otherwise, I would strongly suggest you do it exactly as you were taught (NOAA limits I imagine).
I think we are talking more about CNS decay, and the fact that it may need a rethink.
Continue to track CNS both per dive and per day!
Doppler
Dr Deco
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Dear Readers:
CNS Clock
I did not mean to imply that the CNS limits are without value. Only that you might exceed them one day and have a serious problem on another. The limit is basically the lowest level for safety for all divers. When you exceed the limit, the situation begins to compound, possibly becoming more irreversible. [Not exactly the correct wording - - more of a shortcut.] This might have a parallel in decompression where crossing the limit means that more and more of the dissolved nitrogen finds its way into gaseous bubbles. Thus, the decompression becomes increasingly an irreversible process.:boom:
Dr Deco :doctor:
Please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm
CNS Clock
I did not mean to imply that the CNS limits are without value. Only that you might exceed them one day and have a serious problem on another. The limit is basically the lowest level for safety for all divers. When you exceed the limit, the situation begins to compound, possibly becoming more irreversible. [Not exactly the correct wording - - more of a shortcut.] This might have a parallel in decompression where crossing the limit means that more and more of the dissolved nitrogen finds its way into gaseous bubbles. Thus, the decompression becomes increasingly an irreversible process.:boom:
Dr Deco :doctor:
Please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm
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