Nitrox MOD and time

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Foxfish

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If you stay at or below the maximum operating depth (MOD) for a nitrox mix more than 45 minutes then you can get oxygen toxicity. I'm interested to know how many divers venture below the MOD for shorter periods of time. If you do dive below MOD, how do you determine when it is time to ascend to avoid problems with oxygen toxicity?
 
Do you understand how and why the MOD of a given mix is determined?
 
Dr Lecter, I have done the nitrox specialty course. The MOD is based on an allowable partial pressures of oxygen in you blood.
 
There is no question that time is a factor--just dipping below the MOD and coming back up won't do it. Further, the MODs are intentionally conservative so that momentary strays are not going to be fatal.

But no one can tell where the dividing line might be. It is probably different for different people, and it could be different for the same person on different dives. So, knowing what I wrote in the first paragraph, I generally do not violate MOD except for straying a few feet below it on occasion. If I want to go deeper than a given mix's MOD, then I get a different mix. Why chance it when there is such an easy solution?
 
The MOD is based on an allowable partial pressures of oxygen in you blood.


Actually, the MOD is based on the partial pressure of oxygen in the gas breathed. The most common limits (highest accepted partial pressures) used in OC are 1.4 atm for the working part of the dive and 1.6 atm for deco.

As you noted, however, the risk of CNS hit from oxygen exposure depends both on PPO2 and exposure time. Therefore, even breathing a PPO2 of 0.8 atm can lead to a oxygen toxicity hit if breathed long enough. Tables such as NOAA's are used to control exposure. Normally, a diver aims at not going above 80%.
 
I should add that my Suunto Cobra computer has a bar graph that indicates your oxygen partial pressure ie. oxygen exposure. In theory you could 'ride the graph' like you can when approaching the no decompression limit ie stay at a depth that keeps you just under the limit. To date I've just avoided going below the MOD. Interested to know if other divers base their depth limit on the oxygen exposure shown on the computer display.

---------- Post added August 29th, 2013 at 10:44 PM ----------

There is no question that time is a factor--just dipping below the MOD and coming back up won't do it. Further, the MODs are intentionally conservative so that momentary strays are not going to be fatal.

But no one can tell where the dividing line might be. It is probably different for different people, and it could be different for the same person on different dives. So, knowing what I wrote in the first paragraph, I generally do not violate MOD except for straying a few feet below it on occasion. If I want to go deeper than a given mix's MOD, then I get a different mix. Why chance it when there is such an easy solution?

Reducing the Nitrox mix will reduce the time to NDL which is why you use Nitrox in the first place.

Your approach sounds sensible. I wonder if this is a common approach or whether many divers ride the limit. It seems similar in principle to your nitrogen exposure limit. Okay so the consequences of oxygen toxicity are pretty dramatic and potentially terminal.
 
Your approach sounds sensible. I wonder if this is a common approach or whether many divers ride the limit. It seems similar in principle to your nitrogen exposure limit. Okay so the consequences of oxygen toxicity are pretty dramatic and potentially terminal.

When you say "ride the limit," what do you mean? If you mean dive near the MOD without violating it, then I think a lot of people do that. If you mean intentionally violating it as you described earlier, then I don't know anyone who does that.
 
When you say "ride the limit," what do you mean? If you mean dive near the MOD without violating it, then I think a lot of people do that. If you mean intentionally violating it as you described earlier, then I don't know anyone who does that.

As in using the computer indication of your oxygen exposure rather than the MOD. I tried to explain this previously when I said, "I should add that my Suunto Cobra computer has a bar graph that indicates your oxygen partial pressure ie. oxygen exposure. In theory you could 'ride the graph' like you can when approaching the no decompression limit ie stay at a depth that keeps you just under the limit."
 
As in using the computer indication of your oxygen exposure rather than the MOD. I tried to explain this previously when I said, "I should add that my Suunto Cobra computer has a bar graph that indicates your oxygen partial pressure ie. oxygen exposure. In theory you could 'ride the graph' like you can when approaching the no decompression limit ie stay at a depth that keeps you just under the limit."

I understood that, and I used a Cobra for years, but in this case you are really talking about two different things.

1. Violating the MOD as you described earlier is risking CNS oxygen toxicity, which brings on a seizure and almost certain drowning.

2. When people are talking about the CNS clock that is being checked by a graph on your Cobra, they are talking about total oxygen exposure over time, what some people measure via OTUs (Oxygen Toxicity Units). The danger in this case is pulmonary oxygen toxicity, essentially an irritation of the lung. Back when I took the PADI nitrox course, that was a big part of the class, and we were given a special table to help us keep track of it. When you took the class, that was barely mentioned, which is why you are probably confused. The reason it is barely mentioned today is that it is pretty darn hard to get near those limits on a day of multiple recreational dives. It really isn't going to happen, and if it does, it isn't life-threatening. You can ride your Cobra this way if you want, but it won't be all that exciting--you won't get anywhere near the limits.
 

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