Mr Chen, I set the oxygen partial pressure to 1.4 b which is conservative for recreational diving and typically use a 32% oxygen mix. On a recent dive trip I did get close to the NDL but we were doing multiple dives a day and the dives were relatively deep. Generally it wasn't a problem and having this mix made a big difference in the time I could spend at depth without getting close to the NDL. Not surprising since that is why you use Nitrox. :blinking: With a 32 % oxygen blend I can get down to 33 m. In this case the simple solution for me was not to exceed 33m. If I ever did want/need to go deeper I'd probably just go with straight air.
Nitrox MOD and time
- Thread starter Foxfish
- Start date
Please register or login
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
- Ability to post and comment on topics and discussions.
- A Free photo gallery to share your dive photos with the world.
- You can make this box go away
I checked the course book for the Nitrox specialty course and it was adamant, don't exceed your MOD. It is okay to dive at that depth for a period up to 45 minutes but should not exceed the depth. As a recreational diver I'm happy to dive within that limitation.
Simon, thanks for your comments. The Nitrox course doesn't go into calculating exposure limits for the different modes of oxygen toxicity. I've had my Nitrox specialty for a while and have done a number of dives on Nitrox but generally in areas where MOD is not an issue until recently when it was. I guess the lesson here is that over time it is easy to forget some of the things you learn on the courses. If in doubt check. That was the point of this thread!
I have a problem with blanket statements. It is ok to dive at a depth for 45 mins but don't exceed the depth? What is the optimum blend for a dive to 35m and what would that give you as an NDL? Trust me, it will be nowhere near 45 mins.
Higher oxygen content in blends certainly requires OTU's to be covered in the course.
If the NDL comes up first on a dive then the dive time will be constrained by that requirement. Rules about duration at the MOD are given by the various diving agencies. In my case it was SDI. There are also restrictions on the percent oxygen in the mix and the allowable cumulative time at the MOD over a 24 hour period. People who have done the Nitrox specialty course should be familiar with them. I've forgotten some of them already so being prompted to review them was a good thing. My computer also provides an indication of both CNS/P oxygen toxicity - which to date I've never monitored.
On a slightly different note, I've found personally and have observed in others that one of the big challenges we face as divers is remembering all the important things we learned during our dive courses. Like the things that we've just been discussing. They can be the difference between life and death. Maybe you don't have that problem Simon.
This was bought home recently while on a diving holiday. I spoke to a diver who was keen to go for a dive but felt appprehensive because although she'd done the course it was some time since her last dive. We talked a bit about the things you need to keep in mind on a dive. I said "You do remember the dive tables that tell you how long you can stay at a certain depth before ascending". She thought for a moment. Her face went blank. Then she said "No"! My guess is that this is probably a lot more common that you might expect.
On a slightly different note, I've found personally and have observed in others that one of the big challenges we face as divers is remembering all the important things we learned during our dive courses. Like the things that we've just been discussing. They can be the difference between life and death. Maybe you don't have that problem Simon.
This was bought home recently while on a diving holiday. I spoke to a diver who was keen to go for a dive but felt appprehensive because although she'd done the course it was some time since her last dive. We talked a bit about the things you need to keep in mind on a dive. I said "You do remember the dive tables that tell you how long you can stay at a certain depth before ascending". She thought for a moment. Her face went blank. Then she said "No"! My guess is that this is probably a lot more common that you might expect.
I don't know everything, no-one does, but I know some bits
As for your scenario about a blank expression, it is far too common. Students finish their open water course, buy a computer and totally ignore dive planning. The problem also lies with dive centres who give the "50 mins or 50 bar" as part of the briefing. How many actually consider that for some divers it might be the 3rd dive of the day, for others their first. Differing NDL's (surface interval dependant).
Too much complacency sadly in recreational diving.
As for your scenario about a blank expression, it is far too common. Students finish their open water course, buy a computer and totally ignore dive planning. The problem also lies with dive centres who give the "50 mins or 50 bar" as part of the briefing. How many actually consider that for some divers it might be the 3rd dive of the day, for others their first. Differing NDL's (surface interval dependant).
Too much complacency sadly in recreational diving.
docmartin
Contributor
In the real world OTUs are primarily of concern for saturation divers and the treatment of patients in recompression chambers. Covering them in a recreational Nitrox course is of no practical relevance. It is a form of oxygen toxicity that occurs on VERY long exposures at relatively low PO2. It is impossible for an open circuit diver to attain an OTU level of any practical relevance. You will always exceed CNS limits long before and not live to worry about OTUs.
Nirvana
Contributor
That is why I said normally: the usual recommendation is not to go above 80%, but nothing guarantees someone will drop dead going to 100% or 200%.
For you to have gone above 200%, I guess you must have been doing a deco dive and stayed for quite some time doing stops breathing a mixture with a high O2%. And I believe you managed the risk of O2 tox by taking "air breaks" (or back gas breaks) after reaching a certain CNS% or after some time on O2. This brings us then to the discussion of how can an air break significantly lower the risk of O2 tox. As far as I know, there is no clear answer for this, but these breaks do seem to allow one to go beyond what would be 100% or, in your case, 218% CNS toxicity.
Did I pass the quiz?
To the OP: Oxygen toxicity is actually quite scary. Studies which have been done have shown that the time to symptoms or seizures can vary wildly between people . . . and in a given person on different days. There is no bright line, and the 1.4 limit was chosen as being a level where no toxicity had ever been reported, although I am aware of at least one case where it seems likely that the diver's seizure was oxygen toxicity at prolonged exposure to 1.4.
Can you dive below the MOD, if you do it only briefly? Quite likely. But since underwater seizures rarely have survivors, I don't push it, and I don't recommend that anyone else do it, either.
Can you dive below the MOD, if you do it only briefly? Quite likely. But since underwater seizures rarely have survivors, I don't push it, and I don't recommend that anyone else do it, either.
its funny to come across this discussion today, since today we finished of a few dm's on their final dive and most people on the dive were on a EAN 33 mix but since i was side mounting i had one EAN 33 and one AIR. and the dms were supposed to take cars of us i switched to air and dove down to 40 mts and saw what they would do. to make a long story short after the dive we ended up discussing what each different instructor would do. and all 3 instructors on the boat agreed they would dive well below MOD for a few minutes if it was to pull up a diver going to deep. they all mentioned there was a risk, but it was one they were willing to take to bring a student out of risk. my question would be what would You have done?
i switched to air and dove down to 40 mts and saw what they would do. to make a long story short after the dive we ended up discussing what each different instructor would do. and all 3 instructors on the boat agreed they would dive well below MOD for a few minutes if it was to pull up a diver going to deep. they all mentioned there was a risk, but it was one they were willing to take to bring a student out of risk. my question would be what would You have done?
Similar threads
