Nitrox MOD and time

[SeaCobra]

Remember that computers make calculations, we make decisions.

 

[MrChen]

Did you mean MOD?

No, he was just talking about riding limits. I got close to my NDL limit 2 weekends ago and didn't even like being close to that, so I ascended to keep it in double digit minutes remaining. I'm purely a rec diver with limited experience; I'm not planning to exceed anything and was actually surprised when I saw 5 min left on my dive watch, but it was a busy weekend of diving. Simply ascending 5 feet over the reef increased my time remaining and I kept it that way. Soon after my buddy tapped my shoulder for an air check and she was close to calling the dive based on her air pressure anyways, so it all worked out.

 

[Superlyte27]

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?

I really don't think air breaks have anything to do with CNS Toxicity because I think that 5 minutes on air every 20 or 30 minutes isn't substantial enough to make a difference in regard to a tox, but I can tell you without air breaks, I can feel my lungs start to burn, almost like heart burn, but in the lungs.

To answer your question, I had a long dive at 1.2ppo2. But then my rebreather crapped out on me, and I ended up doing deco on pure o2 at 20' and 10'.

 

[stevensamler]

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?

wow...?

 

[Tigerman]

...
If you do dive below MOD, how do you determine when it is time to ascend to avoid problems with oxygen toxicity?

When your computer start going crazy and telling you that youre breaking your mod, its time to go back up..
Otherwise, if you know your mod and dive gauges, when you see youre breaking the mod, its time to go back up..
Also, did your course not say to consider your o2 clock to be at 100% if you at any time break your MOD? Which would mean its time to go back up..

 

[ianr33]

I really don't think air breaks have anything to do with CNS Toxicity because I think that 5 minutes on air every 20 or 30 minutes isn't substantial enough to make a difference in regard to a tox.

I think air breaks do help avoid CNS Toxicity.

If air breaks don't help,how can the WKPP etc do dives where the CNS % is through the roof. 1000+ % ??

I've only ever done a handful of dives where my CNS went over 100,but some people do it all the time. How ??

 

[PfcAJ]

Its not 5mins every 30.

12mins on o2, 6mins off (switching to the lowest ppo2 gas you have) is the current rotation for o2 times over about 25mins. The stop prior to the gas switch is done on back gas, too. I'd be long gone if this scheme didn't work.

 

[Superlyte27]

Not sure how. I know that I've spent 6-8 hours at 1.6 when working. I asked to have this explained to me more than a decade ago and the answer I got was, "it's different when you're surface supplied." To which I responded with, "Bullshiiit"

Ever done the math on an air break? Lets say you are 200% CNS, and you take a 5 minute break on back gas. Okay, now you're at 201% instead of 205%. I'm making up numbers, but a 5 minute air break doesn't drop your CNS enough to amount to a hill of beans. If you're at 200% and do an air break, you are still WAY WAY over.

What's the longest duration dive in WKPP? I'd love to see some real figures. Are they really at 1000%. Are they mitigating it by having very low PPO2. Is some of the issues mitigated by going very slow and taking it very easy and scootering everywhere? I don't know.

 

[PfcAJ]

The longest dive was the Q tunnel push. The entire dive was around 26hrs.

I think the o2 clock concept is baloney. Peter, like you said, take a 5min break and the "%" only drops a little. But the efficacy of the method is hard to write off.

 

[Steve_C]

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.

I agree that there is a lot of complacency but every time I have heard the bar or mins (psi or min in my case) it has been a situation where they controlled the SI and if I did the max number of dives (2 or 4) that day I could not use up all my NDL anyway. Where NDL becomes an issue we are advised to watch our NDLs. On these dives verybody has their own computer (or tables/timer) so they can monitor their own NDL.