Nitrox MOD and time

[Foxfish]

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.

Like many divers, I dive simply because I enjoy exploring the underwater world. Using Nitrox enhances that enjoyment and with only a few simple rules it is relatively safe and hassle free. Divers should be encouraged to respect the limits and stay within them.

 

[boulderjohn]

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.

But it is very much possible, and very much allowed. The old PADI nitrox tables allow much more than 45 minutes at the MOD for some of the depths and mixes. It is interesting that these conservative tables allow you to do something that we are being told here will certainly kill you, and they allow it without even a hint of warning.

 

[Foxfish]

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.

Thankyou. What you say tallies well with what is taught on the Nitrox specialty course. I'd simply add that it is good to be aware of the risk factors associated with setting the oxygen partial pressure limits. These are covered in the course. I think the main reason for my initial ambiguity was to do with the way your oxygen exposure as reported by the computer (In my case as an OLF) relates to MOD. That has been clarified.

I am a little concerned that those who would really benefit by taking the Nitrox specialty course may be put off by the discussions on the complexities of monitoring oxygen exposure levels. The course teaches the different modes of toxicity poisoning and makes mention of the way the computer monitors your exposure. However it also points out that you will rarely if ever reach those limits.

It has been interesting to read comments by divers who appear to have a higher level of Nitrox certification, who are using higher Nitrox blends and maybe doing decompression diving. I understand that is such cases monitoring oxygen exposure becomes more critical.

A couple of comments in passing. It appears some divers are unfamiliar with the time limits placed on MOD specified by the Nitrox specialty course. I'll say it again. The learning from the Nitrox specialty course is that there is limit to the amount of time you can spent at the MOD. You'll have to work hard to exceed those limits but you should be aware of them if for no other reason than you will be asked questions about them in the course test!

---------- Post added September 1st, 2013 at 12:56 AM ----------

But it is very much possible, and very much allowed. The old PADI nitrox tables allow much more than 45 minutes at the MOD for some of the depths and mixes. It is interesting that these conservative tables allow you to do something that we are being told here will certainly kill you, and they allow it without even a hint of warning.

I just checked on my dive computer.

Using an oxygen partial pressure limit of 1.4 bar:

At EAN 32, MOD is 32.8 m, NDL is 27 mins (given @ 30 m).
At EAN 36, MOD is 27.9 m, NDL is 40 mins (given @ 27 m).
At EAN 40, MOD is 24.2 m, NDL is 62 mins (given @ 24 m).

The normal disclaimers apply to all the stuff I give on this forum. Treat it all as suspect until you have verified it to be correct. If my comments prompt you to check and/or do the course then they have been some benefit!

 

[Tigerman]

But it is very much possible, and very much allowed. The old PADI nitrox tables allow much more than 45 minutes at the MOD for some of the depths and mixes. It is interesting that these conservative tables allow you to do something that we are being told here will certainly kill you, and they allow it without even a hint of warning.

Still speaking from a rec perspective as thats really what this thread was about in the first place..

Using my EAD/AIR/32/36 (Metric) tables;

MIX/EAD/MOD NDL/
30%/30m/35m 20 min
31%/29m/35m 20 min
32%/TBL/30m 30 min (Taken directly from 32% table)
33%/24m/30m 29 min
34%/23m/30m 29 min
35%/23m/30m 29 min
36%/TBL/28m 40 min (Taken directly from 36% table)
Theres only two things I can do with those tables to get more than 45 to be allowed without warning - use 37-40% (not very likely tbh) at an EAD of ~18m and a MOD of ~25m I get 56 minutes of NDL.
OR I could lower my accepted pO2 and as such get a shallower MOD giving more NDL. Changing the recommended limits kinda shouldnt be done without some understanding as to why and how theire set as they are though.

It wont kill you to be at the mod for more than 45 minutes, I havent really seen anyone but the OP claim that to be honest, but likely scenario?
45 minutes at 25 meters+ would require some badass SAC rates (think petite, experienced woman with gills) for a diver on a single AL80 (which is whats most likely for a rec scenario). Infact it would need a rather good SAC with bigger tanks as well.

---------- Post added September 1st, 2013 at 03:48 PM ----------

...
Using an oxygen partial pressure limit of 1.4 bar:

At EAN 32, MOD is 32.8 m, NDL is 27 mins (given @ 30 m).
At EAN 36, MOD is 27.9 m, NDL is 40 mins (given @ 27 m).
At EAN 40, MOD is 24.2 m, NDL is 62 mins (given @ 24 m).
...

Thats quite conservative - the math says 32% MOD = (14/0.32)-10 = 33.75m, 36% MOD = 28,8m and 40% MOD = 25m. (My computer say neither what yours does, nor what the math does btw..

EDIT: Sorry, my computer does say what the math does when its not set to freshwater

 

[Dan G]

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.

I am not confusing the two. Can anyone with a Suunto confirm whether or not the NDL number given includes the O2 calculation as well? While on my Oceanic display there are two different bars, one for N loading and the other for O2 loading, the big number in the center of the screen will be for O2 loading if it is my limiting factor before N loading.

 

[DiveNav]

...... Nitrox is not something that I use all the time and I found over time the different requirements becomes blurred or forgotten. .....

I suggest you play a bit with our divePAL simulator using different gas profiles and gas mixes and see how MOD and CNS correlates.

In the example in the image below, the diver surpassed MOD - by a lot (by accident or by choice?), the computer promptly warned the diver, and the CNS at the end of the dive was ~24%. The diver in example is still alive

​

A while ago someone that knows better than me suggested me something that I have been passing along to all our students:
"Try not too exceed your MOD, but if it happens accidentally ascend to a safer depth as soon as possible"

I suggest also you take a refresher on Nitrox theory

 

[OkByMe]

You need to come up just before you start twitching.

How do you know you are twitching???

Some people can tolerate higher PPO and nitrogen loading than others, and that can vary from day to day, dive to dive.

 

[Foxfish]

It wont kill you to be at the mod for more than 45 minutes, I havent really seen anyone but the OP claim that to be honest, but likely scenario?

Some questions Tigerman:

Have you done the Nitrox specialty course and if so, what agency did you use?

Did they provide a table that tells you for a given PPO2 how long you can stay at the MOD before CNS toxicity becomes a problem?

If so, can you check the table and advise how long you can stay at the MOD with a PPO2 of 1.6 b?

---------- Post added September 1st, 2013 at 12:07 PM ----------

I suggest you play a bit with our divePAL simulator using different gas profiles and gas mixes and see how MOD and CNS correlates.

DiveNav, are you an instructor in the Nitrox specialty?

I like the simulation by the way but I wouldn't infer from the results that it is okay to exceed your MOD by the specified amount.

 

[Tigerman]

pO2 1.6 is contingency, not max. For most(all?) rec training 1.4 is max and you should not be exposed to that for more than 150 minutes PER 24 HOURS. If you dive 40% you can do 56 of those 150 minutes (by table) at 25 meters in a single dive.

However, are you able to stay at 25 meters for 56 or even 45 minutes straight on a rec dive? Youll need a fairly significant ammount of gas or a seriously low SAC to do that (safely).

And yeah, I actually did the nitrox course while you still needed to be able to do the math and not learn to ride a computer..

 

[Foxfish]

pO2 1.6 is emergency, not max. For most(all?) rec training 1.4 is max and you should not be exposed to that for more than 150 minutes PER 24 HOURS. If you dive 40% you can do 56 of those 150 minutes (by table) at 25 meters in a single dive.

However, are you able to stay at 25 meters for 56 or even 45 minutes straight on a rec dive? You'll need a fairly significant ammount of gas or a seriously low SAC to do that (safely).

And yeah, I actually did the nitrox course while you still needed to be able to do the math and not learn to ride a computer..

Those numbers do not correlate with the numbers published in the SDI book at 1.4b. I would expect they should. Can you check them for 1.6 b for the purposes on this exercise?

What agency you did your course through?

Can you give the allowable CNS oxygen toxicity exposure time for a single dive at the MOD for a PPO2 of 1.6 b?

As you will know, giving a time limit at a certain PPO2 is the same as giving the time limit at the MOD for a given FFO2.