Nitrox MOD and time

[Tigerman]

The 24-hour limit IS the limit for a single dive. The 1.6 is CONTINGENCY and not NOT for planning your dive. You will be dictated by NDL, not O2 clock regardless unless you intend to do decompression dives. Theres a REASON why they tend to do 1.4 for MOD calculations and 1.6 for CONTINGENCY ONLY - safety.
If you want to go test if its fine to dive to a pO2 of 1.6, well deeper than your MOD for 45 minutes, go ahead, youll probably be fine. Just dont blame me if youre not. The table also dont say that youll die if you do, it says you shouldnt do it.
Gas planning might say you will die though - even with 40% a pPO2 of 1.6 would be to a depth of 30 meters and my 0,5 cfm/min SAC says id need 90 cuft of air just for the bottomn time, for me alone, with no regard of the descent, ascent or any buddies..

 

[Foxfish]

The 24-hour limit IS the limit for a single dive. The 1.6 is CONTINGENCY and not NOT for planning your dive. You will be dictated by NDL, not O2 clock regardless unless you intend to do decompression dives. Theres a REASON why they tend to do 1.4 for MOD calculations and 1.6 for CONTINGENCY ONLY - safety.
If you want to go test if its fine to dive to a pO2 of 1.6, well deeper than your MOD for 45 minutes, go ahead, youll probably be fine. Just dont blame me if youre not. The table also dont say that youll die if you do, it says you shouldnt do it.
Gas planning might say you will die though - even with 40% a pPO2 of 1.6 would be to a depth of 30 meters and my 0,5 cfm/min SAC says id need 90 cuft of air just for the bottomn time, for me alone, with no regard of the descent, ascent or any buddies..

I have a copy of the NOAA Oxygen Exposure Limits for Working Divers in my course book. It gives allowable oxygen exposure values for a 24 hour period and for a single dive at various PPO2's including 1.6 b. I would presume that most courses would have something similar.

At 1.6 b the allowable exposure time for CNS toxicity is 45 minutes. The allowable time for a 24 hour period is 150 minutes ie. 2.5 hours. That ties in with the numbers I quoted previously as being diving limits that SDI state you should not exceed.

I dive with a PPO2 setting of 1.4 b so that I never get down to MOD's determined using a PPO2 of 1.6 b.

With MOD's determined using 1.4 b the table indicates that the maximum dive time is 150 minutes for a single dive and 180 minutes for a 24 hour period.

 

[Tigerman]

Are you certified as a Working Diver?

Not that its relevant anyways though for all above mentioned reasons.


EDIT: Having looked up the table in question it seems to be "un-specified" who its for and NOOA seem to just allow longer 24-hr exposures. Good for them I guess.
If you look at the other NOOA tables however (i.e. this http://www.anaspides.net/documents/scuba_diving_documents/NOAA CNS Percentage Table.pdf) they are specifying that a pO2 of 1.4 is the recommended maximum for the working portion of the dive as well. The working part being the "actual dive" and teh "non-working" being i.e. deco stop - which is irrelevant to rec diving..

 

[Foxfish]

A summary to date:

1. When the partial pressure of oxygen in your body increases to a certain level it becomes toxic.

2. Recreational diving agencies have set the allowable limit for the partial pressure of oxygen you breathe at 1.6 b (absolute pressure). The risk of oxygen toxicity becomes unacceptably high if you exceed that limit.

3. As your depth increases on a dive the partial pressure of the oxygen in the gas mix you breathe increases. At some depth the partial pressure of the oxygen (PO2) will exceed 1.6 b and become toxic.

4. The depth at which the PO2 reaches 1.6 b will vary depending on the percent of oxygen in the gas mix (FO2).

5. Divers must determine a maximum depth for the gas mix being used to avoid exceeding safe PO2 levels. This depth is determined using the FO2 of the mix and the PO2 they consider to be safe. A typical PO2 used for this purpose is 1.4 b (absolute). The calculated depth is called the maximum operating depth (MOD).

6. The maximum operating depth is calculated using the formula P = PO2/FO2 where P is the absolute pressure at the MOD. (Using SI units, P is changed to a depth by first changing it to a gauge pressure ie. subtracting the atmospheric pressure, and then dividing the water density and the gravitational constant.)

7. In addition to being toxic once the PO2 of oxygen exceeds 1.6 b, it can also be toxic at PO2 levels up 1.6 b where the exposure is over an extended period.

8. Exposure to oxygen exceeding the allowable limits can cause problems with a divers central nervous system and lungs. These are knows respectively as CNS and pulmonary oxygen toxicity. Pulmonary oxygen toxicity results from exposure over very long periods of time and is no real risk to recreational divers.

9. Tables such as the one provided by NOAA, state the allowable time exposure limits to oxygen at high PO2 levels before CNS oxygen toxicity becomes a problem. The table provides the allowable time of exposure for a range of PO2 levels for both single dives and a 24 hour period. Other tables are available that give the PO2 and MOD for a range of FO2 and the allowable times at those depths.

10. From the NOAA table, the allowable exposure limit to prevent CNS for oxygen toxicity for oxygen at PO2 1.6 b is 45 minutes for a single dive and 150 minutes for a 24 hour period. The SDI course notes state that in addition to not exceeding the MOD, these time limits should not be exceeded at the MOD. (I may have missed it, but from what I've seen the course notes don't indicate how these time limits were established even though the tables are provided. I think it is safe to infer these time limits come from the tables.)

12. The Suunto Cobra dive computer tracks your oxygen exposure and provides a bar graph on the left side of the computer that indicates how close you are to your oxygen time exposure limit. The graph is expressed as a percent of the oxygen fraction limit (OLF) and is a combined measure of both CNS and Pulmonary oxygen toxicity.

13. I'd infer that the SDI time limits at the MOD are a simple way of not exceeding oxygen time exposure limits without the need for a dive computer.

14. The time limits at the MOD I've quoted has caused some confusion. I'd surmise it is just another way of expressing a divers allowable exposure to oxygen. If so, by giving the time limit at the MOD it is effectively giving the exposure limit at a PO2 of 1.6 b and ignoring exposure accumulated at depths less the MOD. The allowable time at a given PO2 increases exponentially as the PO2 reduces, and hence as your depth reduces from the MOD. I'd surmise the reason SDI quoted the limits in this way is that at a recreational level, the contribution to oxygen exposure at these depths is considered negligible. If you are doing more advanced Nitrox diving for longer periods of time with higher percentages of oxygen the chance of CNS oxygen toxicity becomes more likely. Under those conditions I expect your exposure at all depths can be important and monitoring the computer exposure times would be critical.

15. In the OP I asked if it was okay to go beyond the MOD and base the dive on the computer indication of oxygen exposure times. The SDI course book says no. You must not exceed the MOD. Further it states that beyond the specified safe limits, the divers reaction to oxygen exposure becomes unpredictable. I'd interpret this to mean that what may be okay one day could kill you the next.

(These are my conclusions on the discussions to date. Feel free to disagree. The intention here was to restate and highlight some of the things taught during the Nitrox specialty course, not supersede them. Thanks for your contributions.)

 

[Tigerman]

1. yes
2. NO! Most agencies has set 1.4 and 1.6 is CONTINGENCY ONLY! Including the NOAA tables!
3. Yes and no. neither 1.4 nor 1.6 is an absolute. The only thing thats certain is that at SOME point your body will tox from increases pO2 and that pO2 can vary from person to person, from day to day!
4. Yes
5. Yes and as you state here 1.4 is whats used for rec divers so why do you keep going about 1.6 which give you a depth deeper than the MOD?
6. MOD = ((pO2/fO2)-1)*10 ( MOD = (P-1)*10 ) you shouldnt referr to partial formulas when you "calculate something by..."
7. Yes and No - although highly unlikely O2 could possibly be toxic at a P02 LOWER than 1.4 - and you keep referring to your CONTINGENCY P02. You do know the difference between max and contingency, no?
8. Yes, generally
9. Yes and No, some tables dont give other than 24-hr exposures i.e. the DSAT tables. (Which in effect means you have the same limit for single dive and 24-hr period)
10. The NOAA table use 1.4 MAX 1.6 CONTINGENCY (or technical divers doing deco). The MOD is calculated at 1.4 NOT 1.6 (http://www.anaspides.net/documents/scuba_diving_documents/NOAA CNS Percentage Table.pdf)
11. Where is 11?
12. Cool (although not of much use maybe?), mine only use CNS I think..
13. All tables are a simple way of not exceeding limits
14. you keep focusing on your CONTINGENCY pO2 for some reason.. And no, you dont ignore former exposure, you cant dive 25 minutes at a pO2 of 1.2 and then drop down to a pO2 of 1.6 for 45 minutes - thats not how the tables work. Doing this you have a total exposure of 70 minutes. That is IGNORING the fact that you also violate your MOD by diving to 1.6!
15. Yes and thats the big issue with both nitrox and decompression theory. We dont know exactly why and how everythings connected, we just have some guidelines as to what has shown to generally be safe..

 

[Foxfish]

Tigerman,

The allowable limit for PO2 is 1.6 bar. That is why the tables publish values like MOD for this value. That said, SDI recommends using a more conservative approach. You call it a contingency. I think we're arguing semantics here. But which is worse, setting your MOD with a PO2 of 1.6 b and always staying away from it or setting your MOD with a PO2 of 1.4 b and occasionally dipping below it?

The formula can be modified for changes in units and water density.

I'd surmise the reason that SDI give values for both a single dive is that a lot of the effects from oxygen toxicity will be cleared during the surface interval. I don't have too much trouble accepting the exposure time at a MOD for a single dive and ignoring the exposure at depths less than the MOD. Like you, I do question this approach for determining the exposure limit over a 24 hour period. I've offered an explanation with which I'm not entirely comfortable. I'd watch the computer oxygen time exposure graph if I was doing say four to five dives a day with Nitrox. Keep in mind this is recreational diving.

Absolute pressure is the pressure measured above an absolute vacuum and is often used when working with gas laws. (I'm supposed to know this stuff as a mechanical engineer!)

 

[Tigerman]

If you read exactly what the PDF I linked before says it does NOT "allow" a pO2 of 1.6 it specifically says that above 1.4 is included for contingency only or resting deco for tech divers. The max "allowed" pO2 of a dive is 1.4 according to the table you yourself refer to!

Yes, given youre making a qualified decision you can change those numbers as you yourself wish, but the agency standard is 1.4 MOD and 1.6 contingency.
If you wish to do so you can set your pO2 max for a dive to 2.0 giving you a MOD of 52,5 meters on 32% EAN. I wont recommend doing it, but you could and people DID back in the day.. (not neccesarilly on 32% but used pO2 2.0)

The reason you have that contingency is for the same reason you have "emergency deco" should you break your NDL or the tables goes to 42 meters, although youre only qualified to 30 or 40 - its there IN CASE SOMETHING HAPPENS, its NOT there for you to take it as advice to DO IT.