Accumulated 02 following a large number of repetitive Nitrox dives over 3 days.

[GJC]

OK, I know the OP is gone, but this is for anyone else that might be interested.

It is known that if you are exposed to more than 850 OTUs in 24 hrs, you start to show symptoms of pulmonary O2 tox. Additionally, all the cells in your body are affected by free radicals and have biochemical disruptions yada yada yada (I'm not that interested in this level of detail). It takes some time for your body to recover from the exposure after you go back to air at sea level.

Every day you are exposed to 302 OTUs from air at sea level. That leaves 548 for diving.

But you might get bent diving and need a chamber treatment. A table 6 treatment exposes you to 268 OTUs. So if you want to be able to go into a chamber after diving, you only have 300 OTUs for diving in 24 hrs.

But what if I'm too far away from a chamber, can I use those chamber OTUs for diving? Not if you want to breath 100% O2 on the boat if you get bent. 268 OTUs would be just over 4 hrs on 100% O2 at sea level.

Oceanic computers track OTUs in the last 24 hrs and alarm at 300.

Shearwater computers use 90 min half life of O2 exposure to reduce your OTUs.

It's pretty hard for recreational divers to get to 300 OTUs. So most of us don't even think about it. My NAUI Nitrox textbook only had a few sentences mentioning not to exceed your OTUs. But it is possible to get there diving very frequently like on a multiday liveaboard.

 

[GJC]

I didn't think any lines were crossed either.

And I am one of probably many who learnt a quite a bit as well.

But a few of the recent posts have confused me. My EAN course was from PADI in Dec 2010, let me declare that up front.

1. In post # 182 Tursiops says "AT PPO2 (not PO2) of ....". My question is not related to the meat of the post, just on the PPO2 vs PO2. I thought they were the same (?).

2. In post # 176 rjack321 says
"there are 3 parts to oxygen and nitrox use
the depth mod
the CNS has a 90min half-life while the oceanic uses a 24hrclock
the daily pulmonary limits do not have a half life - and you exceeded those"

a. I am under the impression that there is no half life with respect to CNS toxicity.
b. I am under the impresion that there may well be a half life with respect to pulmonary toxicity. Maybe 90 minutes, maybe 120 minutes. Some computers take this into account explicitly, some may explicitly not, and some may be silent as to what they do.

Could someone please advise?

3. With respect to what Manatee Diver said in post # 192

"Not only that but those numbers are for pulmonary not for CNS. Recreational computers don't even bother calculating the pulmonary numbers because it is very very hard for non-occupational divers to hit it."

and what GJC confirmed in post # 197

"Absolutely correct about pulmonary, but the exposure over time may also make you more susceptible to a CNS hit if you do another nitrox dive or have to take a chamber ride."

a. I am under the understanding that the so called "oxygen clock" tracks oxygen loading with respect to pulmonary toxicity. The NOAA tables refer to pulmonary toxicity, not CNS toxicity. Divers and various computers (to my regret, even Shearwater) are using a misnomer by referring to a "CNS clock". It is not primarily CNS related.

b. Just so that I clearly understand this, let me go out on a limb and say that CNS toxicity is related only to depth for a given EAN%. You can spend unlimited time within 1.4 ATA, and consider that safe diving practice with respect to CNS toxicity. (I understand about diver to diver and day to day variation, I am specifying "safe diving practice", acknowledging that "things happen".)

Could someone please advise?

High O2 affect all cells, not just pulmonary. The pulmonary symptoms just show up first. Your cells need time to recover from an exposure. Time and pressure affect your exposure. Recovery time is not an exact science at this point.

What is pretty clear is that short term exposure at high pressure causes seizures, thus the 1.4 limit.

That's the simple version.

We were typing at the same time. See my previous post for a little more detail.

 

[clownfishsydney]

Well, all that was an interesting read! I still cannot understand why someone posts about what happened and asks "why" but then refuses to accept any of the explanations given. Clearly some people are accidents waiting to happen, not understanding anything about the physics of diving on compressed air/nitrox.

Our dive club had a new member many years back and he just did not understand anything nor would he listen to more knowledgeable members. In the end all the boat owners decided that he could no longer come diving on our boats. We then learnt that he was trying to do a tech course and soon after, the dive shop owner refused to continue to teach him. This was amazing as this bloke would take money from anyone no matter their ability. Hopefully this bloke will give up diving rather than continue and suffer dire consequences.

 

[Saniflush]

Looks like I've wasted a lot of time in this thread!

Not at all! It made me go back and look harder at some things that I thought I knew but only had partially correct.

 

[doctormike]

But a few of the recent posts have confused me. My EAN course was from PADI in Dec 2010, let me declare that up front.

1. In post # 182 Tursiops says "AT PPO2 (not PO2) of ....". My question is not related to the meat of the post, just on the PPO2 vs PO2. I thought they were the same (?).

Yeah, that confused me as well... I have always used them interchangeably. Ambient pressure is the total pressure of all gasses, and it is made up of the sum of the partial pressures of each individual gas (Dalton's law). So by specifying PO2, PN2, etc... I thought that meant by definition a partial pressure.

2. In post # 176 rjack321 says
"there are 3 parts to oxygen and nitrox use
the depth mod
the CNS has a 90min half-life while the oceanic uses a 24hrclock
the daily pulmonary limits do not have a half life - and you exceeded those"

a. I am under the impression that there is no half life with respect to CNS toxicity.
b. I am under the impresion that there may well be a half life with respect to pulmonary toxicity. Maybe 90 minutes, maybe 120 minutes. Some computers take this into account explicitly, some may explicitly not, and some may be silent as to what they do.

That was the controversy discussed upthread. But the half life does refer to CNS toxicity. For example, from the Shearwater manual:

During a dive the CNS never decreases. When back at the surface, a half-life of elimination of 90 minutes is used. So for example, if at the end of the dive the CNS was 80%, then 90 minutes later it will be 40%. In 90 more minutes it will be 20%, etc. Typically after about 6 half-life times (9 hours), everything is back close to equilibrium (0%).

I don't remember reading about half life with respect to pulmonary toxicity, but time with a lower PO2 is definitely protective - this is one of the reasons for air breaks during chamber treatments and long decompressions on rich gas.

3. With respect to what Manatee Diver said in post # 192

"Not only that but those numbers are for pulmonary not for CNS. Recreational computers don't even bother calculating the pulmonary numbers because it is very very hard for non-occupational divers to hit it."

and what GJC confirmed in post # 197

"Absolutely correct about pulmonary, but the exposure over time may also make you more susceptible to a CNS hit if you do another nitrox dive or have to take a chamber ride."

a. I am under the understanding that the so called "oxygen clock" tracks oxygen loading with respect to pulmonary toxicity. The NOAA tables refer to pulmonary toxicity, not CNS toxicity. Divers and various computers (to my regret, even Shearwater) are using a misnomer by referring to a "CNS clock". It is not primarily CNS related.

The clock refers to CNS toxicity, pulmonary toxicity is tracked with OTUs. Check out that Dive Rite article, and the Shearwater blog post.

b. Just so that I clearly understand this, let me go out on a limb and say that CNS toxicity is related only to depth for a given EAN%. You can spend unlimited time within 1.4 ATA, and consider that safe diving practice with respect to CNS toxicity. (I understand about diver to diver and day to day variation, I am specifying "safe diving practice", acknowledging that "things happen".)

No. CNS toxicity is depth, mix and time. If you stay long enough at 1.4, you will go over your maximum single dive limits (150 minutes on the NOAA table). And, of course, there is a huge amount of individual variation.

 

[Bob DBF]

Well, all that was an interesting read! I still cannot understand why someone posts about what happened and asks "why" but then refuses to accept any of the explanations given.

Or it could just be a troll that is seeing how long it takes to get bounced out of the board again. May be put a poll togather, or a thread to enumerate the names used. The threads involved have been very informative, yet annoying.



Bob

 

[dmaziuk]

Interesting. I must admit I never really looked into the actual differences, I just assumed it would be enough to make it worth setting the P02 higher (and assuming the slight greater risk) and dive the slightly richer blend to maximize my allowed bottom time in regard to NDLs.

(Although OP is not here someone else might be reading this)

The formula, at least in Buhlmann's version, simply uses % inert gas. The difference between EAN 32 and EAN 36 is whooping 0.04. So if one wanted to optimize, they'd have to take the entire trip, or at least a whole day, and look at oxtox vs NDL, to find the mixes that minimize both.

You'd have to pick your model though: 90-minutes vs 24-hour window for O2 and whether you go for fully symmetrical inert-off-gassing a la Buhlmann or 36-hour window a la Wienke (if you knew how he does it, exactly) or 6-hour to clear (but take the 6th day off) a la DSAT.

 

[tursiops]

Yeah, that confused me as well... I have always used them interchangeably. Ambient pressure is the total pressure of all gasses, and it is made up of the sum of the partial pressures of each individual gas (Dalton's law). So by specifying PO2, PN2, etc... I thought that meant by definition a partial pressure.

PPO2 and PO2 are often used interchangably, but only PPO2 follows NOAA's definitions (see the NOAA Diving Manual section on Dalton's Law), which are good enough for me. The confusion comes from P being correctly written with the subscript O2, but subscripts don't come through well today in our text-based communications.
See this, from the 4th Edition of the Manual:

 

[doctormike]

PPO2 and PO2 are often used interchangably, but only PPO2 follows NOAA's definitions (see the NOAA Diving Manual section on Dalton's Law), which are good enough for me. The confusion comes from P being correctly written with the subscript O2, but subscripts don't come through well today in our text-based communications.
See this, from the 4th Edition of the Manual:
View attachment 543800

OK, that's fine, but your post implied that PO2 and PPO2 were separate things and mixing them up was causing confusion.

Maybe PO2 is lazy shorthand usage and the formal term is PPO2? Are you saying that there is some other meaning of PO2 which means that it shouldn't be used in place of PPO2? I just looked at the TDI CCR training material and they use PO2.

 

[GJC]

OK, that's fine, but your post implied that PO2 and PPO2 were separate things and mixing them up was causing confusion.

Maybe PO2 is lazy shorthand usage and the formal term is PPO2? Are you saying that there is some other meaning of PO2 which means that it shouldn't be used in place of PPO2? I just looked at the TDI CCR training material and they use PO2.

Yeah, when I see ppO2, I think partial pressure of O2 in a gas mix. The total pressure of the mixed gas can be anything (it would be the total of all the partial pressures of all the gasses)

When I see pO2, I think pressure of O2 in a tank with pure 100% O2. So the pressure is the total pressure of gas delivered.