The whole area more than a bit fuzzy. Depending on how you interpret the NOAA statements, the CNS exposure for a daily limit doesn't seem to exhibit the 90 minute half-life, but miraculously resets itself after a 24 hour period.. .
CNS O2 exposure
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The whole area more than a bit fuzzy. Depending on how you interpret the NOAA statements, the CNS exposure for a daily limit doesn't seem to exhibit the 90 minute half-life, but miraculously resets itself after a 24 hour period.. .
The NOAA manual actually DOES mention a 90-minute half-life... BUT ONLY for exposures at 1.6 bar... otherwise the 24-hour limit for 1.6 would be 45 minutes the same as the single dive... and it clearly is not
And I suspect you do not understand the way the 24-hour limit works if you feel it "miraculously" resets itself. It is a rolling limit. ANY DIVES within ANY PERIOD of 24-hours count towards the total... in other words, Saturday's exposures do not disappear at midnight. A dive conducted at 16:00 on day one continues to "contribute" to CNS loading until 15:59 on day two... etc.
Perhaps you might search my blog if you want to learn more...
OP
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So, the conservative thing to do is to use the NOAA table stictly as 24 hour exposures. The less conservative thing to do is to take into account the oxygen elimination half life to decrease the magnitude of oxygen exposure. I believe the latter is more closely related to reality, I wish I knew how close.
I wasn't terribly clear. Let's consider two scenarios (lets remove the restriction (as you do in your blog examples) about recreational diving, for the sake of simplicity:
Scenario #1
Day 1: 20:00 Dive with 25% CNS exposure
Day 2: 9:00 Dive with 25% CNS exposure
12:00 Dive with 25% CNS exposure
15:00 Dive with 25% CNS exposure
By NOAA recommendations at 20:01 you're clear to do another dive (note 5 hours since last dive)
Scenario #2
Day 1: 9:00 Dive with 25% CNS exposure
12:00 Dive with 25% CNS exposure
15:00 Dive with 25% CNS exposure
20:00 Dive with 25% CNS exposure
Day 2:
9:00 Again by NOAA recommendations, you're clear to do another dive (note 13 hours since last dive)
Would you care to explain the physics behind this, or why these are equivalent?
In my area recreational dives are conducted with single tanks varying in size from 63 cuft to 130 cuft. Some people even do recreational dives with doubles. The most common size I see used is a HP 100 with a few buddies using the HP 120s or 130s. It is amazing how long you can dive on one of these tanks with my SAC rate of .37 cuft/min or even theirs of .70. The double divers MAY (or may not) be Tech trained, I know the singles divers are not.
DevonDiver
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Yes, but that'd break the protocols/recommendations they were taught as a recreational nitrox diver....
The argument seems moot, when one rule has to be broken for the other rule to become an issue...
Sure: I can recommend a couple of books too!
You seem hung up by the time since last dive... which is probably an artifact of thinking in half-time decay. Try to forget that and focus rather on sort of revolving 24-hour picture. If it makes things any easier, think of the body's capacity to deal with CNS loading as a bucket placed on the ground. In both the half-time and 24-hour models, water is added to and taken from the bucket every time we dive. The half-time model describes a leak, and in the 24-hour model a ladle is used to remove water. Bill Hamilton, who worked on the NOAA limits and recommendations, saw CNS exposure over multiple dives/days as a bucket and ladle.
Anyhow, here's the Hamilton analogy. Water flows into the bucket from a spigot that represents the loading from a dive. in Scenario #1 at the finish of dive 4 (a dive that began at 15:00 on day 2), the bucket is full. At 20:01, a magic ladle dips into the water and removes 25% of its contents... sure, the diver can dive again, but a dive netting more than a 25% loading means the bucket will overflow. Potentially making the floor wet and ruining his or her day.
in scenario #2, same mechanics but the magic ladle reappears at 9:01.
OK, so with that in mind, it becomes clear that after the fifth dive in each series, the buckets are full again. The wrinkle is the necessary elapsed time for each scenario for the diver to be able to conduct dive six. Using Hamilton's suggestions, and your examples of dives netting 24% of the allowable limit each, there must never be more than four dives in each 24-hour period... the time between dives is "immaterial" to the daily limits.
There is no physics behind this... no pure maths. The physiology, as with all physiology, is as much alchemy as it is science because by definition, these guidelines are based on data collected from NOAA divers on working dives and wet and dry chamber runs. I understand that having a bucket and magic ladle is worrisome, but the model works and has worked in the recreational arena since the mid-1980s, so it's what we have to work with by default and best practice. Is it logical? Of course not. It's physiology and there is little logic involved with the workings of the human body. Exceptions are the norm.
What Hamilton was striving for was a simple system that was workable and 'safe.' His concern (shared by a lot of experienced technical divers) is that CNS has too many variables to be ****ed around with.
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Steve's explanation is very useful and helped me with my initial question about how various dive computers track oxygen exposure. These are models to allow for diving with safe oxygen exposures over time. The models do not neccessarily have to be accurate physiologically to accomplish that goal.
The bucket and ladle analogy reflects the absolute 24 hour oxygen exposure without interval loss. I think the ladle dips in 24 hours after the end of the applicable dive rather than at the beginning though.
The bucket with a spigot that opens when the pO2 drops below a certain level reflects a model including a half life of elimination for oxygen. Though 90 minutes is the most commonly used half life, I've also seen an estimate of somewhere betwween 90-120 minutes. I don't know how certain this value is. When does the spigot open? The only thing I've seen is the section from the ScubaPro computer manuals that states this is at a pO2 of 0.5. I don't know the basis of this value. Assuming both models use the same duration of exposure, and I'm not sure they do, this model would be more liberal with oxygen loss during the 24 hour period.
Now, back to my initial question concerning my Oceanic computer. My Oceanic computers are bucket and ladle model. Shearwater, ScubaPro, and likely some others, are bucket and spigot model. We know quite a bit about various decompression algorithms and have made detailed comparisons beteen them. Personally, I would like to know more regarding the models for handling oxygen and comparisons between them. At least I know quite a bit more now than when I started this thread.
Good diving, Craig
Something I neglected to mention that is important (and my excuse is that it is covered in my blog entries on this topic). NOAA recommends that if a diver achieves the 24-hour CNS limit, he or she should stay out of the water for 24-hours... Some technical instructors have also discussed teaching that reaching 80 percent of the daily allowance should require the diver to take a break for 12 hours. Your mileage may vary but as odd as the 24-hour limit may seem, it's easier to apply than playing around with half-times and is backed up by empirical evidence and data from NOAA.
I don't pretend to be an expert in this area, and am mostly thinking out loud with this. My understanding is that exceeding the NOAA 24 hour 02 exposure limits puts the diver at a higher risk of developing an oxygen toxicity reaction. This reaction can be sudden seizures. If an Ox Tox event happens it would happen at depth. If the reaction was a seizure, then death is likely. Whereas exceeding the nitrogen loading limits increases the risk of DCI. Most cases of DCI do not result in fatalities. As a result I would be more concerned with exceeding the NOAA 24 hour oxygen clock than the nitrogen loading limits. Not that I advocate exceeding either - just a thought process on which has a worse probable outcome.
What I was taught was that if you kept the PP02 to 1.4 or less and you respected the 24 hour CNS clock that your risk of having an Ox Tox seizure was very small. If you exceed either of these limits the risks rise (how quickly?). Of course nothing is quite that simple, nor cut-and-dried, so "individual mileage may vary".
What I was taught was that if you kept the PP02 to 1.4 or less and you respected the 24 hour CNS clock that your risk of having an Ox Tox seizure was very small. If you exceed either of these limits the risks rise (how quickly?). Of course nothing is quite that simple, nor cut-and-dried, so "individual mileage may vary".
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