Nitrox MOD and time
- Thread starter Foxfish
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I think part of the problem is that people get the idea for MOD, like they do with no-deco limits, that there is a hard and fast line. Stay on this side of it, and you will always be safe; cross the line by even a little bit, and you are in terrible trouble.
For NDLs, that's obviously not true -- people do technical dives every day, and my guess is that people go a minute or so over the NDLs from time to time, too
And for oxygen toxicity, it is almost exactly the same . . . as you push the limits, the RISK of an adverse outcome goes up. The risk is proportional to both the degree to which you violate the MOD and the time you stay at that depth, but oxygen toxicity is even more unpredictable than DCS and far more deadly. This is what makes it so scary, and why agencies that teach Nitrox or technical diving tend to be so conservative. As I said above, scientific studies have shown almost incredible variations in time to symptoms or seizure between subjects, or even in the same subject on different days, which means it's very difficult to say, "Well, I'm tolerant to oxygen, so I can do thus-and-such."
The limits that are in place are sort of statistical exercises in likelihood, but they aren't very valid because the data are so bad. But if you take a zero tolerance approach, and say, "We are going to set the working MOD at a ppO2 where NO event that was even probably oxygen toxicity has ever taken place," you are unlikely to encounter an oxygen toxicity event in anyone who has respected those limits. 1.4 seems to be a pretty safe place to set it, although some agencies (at least mine) recommend an even lower ppO2 for dives with significant exertion or very long dives.
Will you tox if you dip five feet below your MOD for a moment, to look at a fish? Probably not. 10 feet? 10 minutes? Nobody can answer the question, and nobody really wants to try. But anybody asking the question the OP asked should know that, as far as we can tell, a LOT of oxygen seizures occur without ANY of the CONVENTID premonitory symptoms we are taught. So if you are depending on detecting the onset of oxygen toxicity so that you can get shallower and prevent the eventual seizure, you are truly playing Russian roulette.
For NDLs, that's obviously not true -- people do technical dives every day, and my guess is that people go a minute or so over the NDLs from time to time, too
And for oxygen toxicity, it is almost exactly the same . . . as you push the limits, the RISK of an adverse outcome goes up. The risk is proportional to both the degree to which you violate the MOD and the time you stay at that depth, but oxygen toxicity is even more unpredictable than DCS and far more deadly. This is what makes it so scary, and why agencies that teach Nitrox or technical diving tend to be so conservative. As I said above, scientific studies have shown almost incredible variations in time to symptoms or seizure between subjects, or even in the same subject on different days, which means it's very difficult to say, "Well, I'm tolerant to oxygen, so I can do thus-and-such." The limits that are in place are sort of statistical exercises in likelihood, but they aren't very valid because the data are so bad. But if you take a zero tolerance approach, and say, "We are going to set the working MOD at a ppO2 where NO event that was even probably oxygen toxicity has ever taken place," you are unlikely to encounter an oxygen toxicity event in anyone who has respected those limits. 1.4 seems to be a pretty safe place to set it, although some agencies (at least mine) recommend an even lower ppO2 for dives with significant exertion or very long dives.
Will you tox if you dip five feet below your MOD for a moment, to look at a fish? Probably not. 10 feet? 10 minutes? Nobody can answer the question, and nobody really wants to try. But anybody asking the question the OP asked should know that, as far as we can tell, a LOT of oxygen seizures occur without ANY of the CONVENTID premonitory symptoms we are taught. So if you are depending on detecting the onset of oxygen toxicity so that you can get shallower and prevent the eventual seizure, you are truly playing Russian roulette.
gcarter
Contributor
There are constraints on the time you can spend at ANY depth - there is nothing magical about the MOD in that regard. Every dive has time constraints.
Your references to "up to 45 minutes" and "in a 24 hour period", which you have hooked to MOD, relate to oxygen exposure limits, not depth limits. Within those limits can easily be - and very often are - dives done on multiple mixes at multiple MODs, and at multiple actual depths. On multi-level dives, the limit will depend on oxygen exposure at multiple depths - NOT the MOD. The way you are using MOD is incorrect.
Whether the original question was answered is not relevant to this point.
Your exposure to oxygen toxicity and your NDL are different. Be careful not to confuse the two.
Once you dial in the gas mix the computer automatically computes the increased time you have till your NDL because of the increased oxygen fraction in the mix. :cool2:
My Suunto Cobra has a bar graph on the left of the dial that gives a measure of your exposure to oxygen. It sounds like yours has the same thing.
I haven't seen anything in the Suunto Cobra manual suggesting the calculated time to NDL includes the time limit on your oxygen exposure.
If you follow the guidelines taught in the Nitrox course you will probably never become over exposed to oxygen.
This makes me laugh.
I'm referring in this thread to diving under the protocols set out in the Nitrox course taken as a specialty after you have done your advanced diving course. There are some comments on the thread such as this one that relate to the use of Nitrox in circumstances beyond the limits of the course. ie. using higher gas mixes and having supplies of both air and Nitrox on the same dive.
When I first started diving I wondered why people would bother with all the fuss of using Nitrox. As I started diving a bit deeper and bottom times got longer I found that running into NDL became a problem. Dives had to be cut short or I kept having to ascend partway through the dive as I got close to the NDL's. Using Nitrox made a big difference in getting around this constraint.
For me the guidelines set out in the course are clear and relatively simple. However, Nitrox is not something that I use all the time and I found over time the different requirements becomes blurred or forgotten. I do recommend hanging onto the course book and reading it from time to time until the requirements become burned into your scone!
Tigerman
Contributor
They DONT seem to be clear and simple to you as you keep confusing NDL, O2 clock and MOD all through the thread.
The MOD is just that - the MAXIMUM OPERATING DEPTH. Its the max depth you will go to on the gas youre breathing, you will not violate this other than if you have set a contingency mod for "emergencies". The MOD will depend on a) the gas you breahte and b) the max ppo2 you consider to be safe. By most (all?) agency standards thats 1.4 for the dive 1.6 for emergencies. If you break the contingency MOD youre to consider your o2 clock at 100% (atleast if youre with PADI).
The O2 clock or the total oxygen exposure is a function of how long you have been exposed to a certain pO2 and the only relation it has to the MOD is that the O2 percentage in the gas affects both the pO2 of any given depth and the MOD of the gas. Youre unlikely to be able to "clock out" unless you get yourself a deco obligation, which is a whole different set of training..
The NDL will change with the O2 percentage in the gas and thats why we like nitrox in the first place, isnt it?
If you stay at the MOD for 45 minutes (suggested in the OP) you will for any mix leaner than than 36% o2 (by inaccurate PADI tables - lots of rounding up) be going over the NDL and have a deco obligation.
That said, the computer give a more accurate NDL, but the fact remains that staying for 45 minutes at the MOD wont be very likely on a rec dive in the first place.
This is of course in a rec nitrox setting. How youre willing to bend and mend (pun intended) the different limitations after tech training is for you to decide - after tech training..
Edit: BTW also, if youre with PADI the nitrox specialty requires nothing in the way of previous courses and can even be done as part of your OW class, just as the drysuit specialty.
gcarter
Contributor
Nirvana
Contributor
Foxfish,
I think I was not clear on what I tried to say in my first post, and that I may have subsequently added some confusion to the debate you proposed. I will try now to tell you what I intended in a more thorough and, hopefully, understandable fashion.
The effects of oxygen toxicity on the central nervous system (CNS) depended on the partial pressure of oxygen breathed and duration of exposure. Divers track the degree to which their CNS gets compromised during a dive through tables (such as NOAA's) or computer algorithms, both of which give them a result in terms of percentage exposure in relation to the maximum recommended (which is usually referenced as %CNS).
The susceptibility to the effects of oxygen intoxication, however, is not uniform from diver to diver or for a same diver in different days. This fact has led to the adoption of a few practices. The first is to plan to limit the exposure to 80%. You can verify that the Suunto Cobra, which has been mentioned in this thread, gives an alarm sign when the exposure reaches 80%. The second practice adopted is to recommend divers to limit the partial pressure of O2 in their breathed gases during the working phase of a dive (any part of a dive for a rec diver) to 1.4atm.
The inconsistency in the effects of O2 exposure, though, has made it impossible to set a hard line on what is the maximum ppO2 breathable without incident. There is only the concept of risk of adverse outcome. Agencies, then, recommend a certain ceiling on ppO2, but a diver may decide there is another value of ppO2 that they think they shouldn't go beyond and plan their dive accordingly. Therefore, many divers choose to operate with a limit of 1.3 atm or 1.2 atm.
On the other hand, some divers have decided in the past (and I am NOT advising anyone to do it) that they can operate with a ppO2 higher than 1.4atm. The book "Shadow Divers" (great book, by the way) deals with the discovery of a wreck 240ft bellow sea level. The people who first dove it did so on air - the ppO2 was as high as 1.7atm.
In any event, when a diver plans their dive, they decide what is the maximum ppO2 that they will tolerate. This value, together with their selection of gas, will dictate the depth at which the ceiling ppO2 would be attained. This depth is called the maximum operating depth or MOD. After identifying their MOD for a dive, then, there is no reason to go beyond it meaningfully but for an emergency. Not because they will suddenly die, but because they will expose themselves to a risk they are not willing to take.
So, to answer your original question: no thinking diver ventures in an appreciable manner beyond their MOD except for an emergency. What the MOD for a given gas is, however, may vary from diver to diver, and the MOD is only one of the parameters used to control the risk of an oxygen toxicity CNS hit.
I'm sorry for the long post and for repeating some of what others said before in this thread.
I think I was not clear on what I tried to say in my first post, and that I may have subsequently added some confusion to the debate you proposed. I will try now to tell you what I intended in a more thorough and, hopefully, understandable fashion.
The effects of oxygen toxicity on the central nervous system (CNS) depended on the partial pressure of oxygen breathed and duration of exposure. Divers track the degree to which their CNS gets compromised during a dive through tables (such as NOAA's) or computer algorithms, both of which give them a result in terms of percentage exposure in relation to the maximum recommended (which is usually referenced as %CNS).
The susceptibility to the effects of oxygen intoxication, however, is not uniform from diver to diver or for a same diver in different days. This fact has led to the adoption of a few practices. The first is to plan to limit the exposure to 80%. You can verify that the Suunto Cobra, which has been mentioned in this thread, gives an alarm sign when the exposure reaches 80%. The second practice adopted is to recommend divers to limit the partial pressure of O2 in their breathed gases during the working phase of a dive (any part of a dive for a rec diver) to 1.4atm.
The inconsistency in the effects of O2 exposure, though, has made it impossible to set a hard line on what is the maximum ppO2 breathable without incident. There is only the concept of risk of adverse outcome. Agencies, then, recommend a certain ceiling on ppO2, but a diver may decide there is another value of ppO2 that they think they shouldn't go beyond and plan their dive accordingly. Therefore, many divers choose to operate with a limit of 1.3 atm or 1.2 atm.
On the other hand, some divers have decided in the past (and I am NOT advising anyone to do it) that they can operate with a ppO2 higher than 1.4atm. The book "Shadow Divers" (great book, by the way) deals with the discovery of a wreck 240ft bellow sea level. The people who first dove it did so on air - the ppO2 was as high as 1.7atm.
In any event, when a diver plans their dive, they decide what is the maximum ppO2 that they will tolerate. This value, together with their selection of gas, will dictate the depth at which the ceiling ppO2 would be attained. This depth is called the maximum operating depth or MOD. After identifying their MOD for a dive, then, there is no reason to go beyond it meaningfully but for an emergency. Not because they will suddenly die, but because they will expose themselves to a risk they are not willing to take.
So, to answer your original question: no thinking diver ventures in an appreciable manner beyond their MOD except for an emergency. What the MOD for a given gas is, however, may vary from diver to diver, and the MOD is only one of the parameters used to control the risk of an oxygen toxicity CNS hit.
I'm sorry for the long post and for repeating some of what others said before in this thread.
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