Oxtox and fatality at 1.4 PPO2

[Tangfish]

Wow. That is a powerful story, and I too can't imagine how the husband could've written that with so much clarity and composure. RIP.

[John Rawlings]

I went back through my email in box before I made that statement and had at least four people who've referenced the "maintain PPO2 at 1.4 and minimize decompression" as one of the reasons they are going CCR.

Ahhhh.....now it makes sense! People that are "going" CCR as opposed to divers that are already trained and are actually diving a CCR. People generally approach CCR diving with a lot of misconceptions and plans based on what they currently think that they"know".... I know that I did! I wouldn't be a bit surprised if your e-mail correspondents are diving at 1.3 PO2 or below like the rest of us after they are trained.

Just a quote from my 2-year old KISS CCR manual from TDI: "It is not recommended to ever run a dive with a set point higher than 1.3 atm PO2. This will likely lead to oxygen toxicity and other excessive oxygen syndromes." My CCR Trimix manual has similar statements. Clearly, the training agencies are not encouraging higher PO2 set-points.

....how has this practice evolved? Are CCR divers setting PPO2 lower than they used to?

The practice of diving with a set point at 1.3 or lower has evolved over time as more and more evidence has accumulated that higher constant PO2 levels bring along with them a host of problems that divers cannot afford to ignore. One such problem is "hyperoxia induced myopathy". This is a condition in which long exposures to high PO2 levels can induce nearsightedness! Dude! I'll choose a few extra minutes of deco ANY day over loss of eyesight....even if it IS a temporary condition!

Plus, as I mentioned earlier, CCR divers constantly battle with the Oxygen Clock. If, as you said your e-mail correspondents wrote, you had a set-point of 1.4 PO2 you will have a single dive limit of 150 minutes and a 24 hour limit of 180 minutes....not so bad if you are only doing one dive, but what if you want to do two or more? The corresponding times for 1.3 PO2 are 180 minutes for a single dive and 210 minutes for a 24 hour period, while for 1.2 PO2 it would be 210 minutes and 240 minutes, respectively. Gives it a different perspective, doesn't it?

So, to directly answer your question....yes, CCR divers in general are choosing to dive with lower PO2 set points than they have used in the past, both for personal safety AND for the convenience of multiple long dives. However....I am a FIRM believer that divers need to make their own informed personal choices regarding their diving methods, style and equipment.

- John

[dsteding]

So, to directly answer your question....yes, CCR divers in general are choosing to dive with lower PO2 set points than they have used in the past, both for personal safety AND for the convenience of multiple long dives. However....I am a FIRM believer that divers need to make their own informed personal choices regarding their diving methods, style and equipment.

- John

Good to know John. The agency I am training with (OC) generally preaches a maximum PPO2 of 1.2 for the working part of the dive, with 1.6 for deco. They've arrived at this from a similar analysis, namely that it isn't worth pushing your PPO2 limits during the working part of the dive to shave a few minutes of deco. I think it is interesting and informative that our two communities are arriving at a similar healthy level of respect for oxygen exposure.

[gcbryan]

Slightly off topic but I am curious...where does the term "flying" a rebreather come from. I hear people speaking of flying a scooter and flying a rebreather.

We use most other gear why does one fly a rebreather?

By the way, I know of at least one rebreather user who does multihour dives who keeps his setpoint no higher than 1.0.

[Curt McNamee]

Slightly off topic but I am curious...where does the term "flying" a rebreather come from. I hear people speaking of flying a scooter and flying a rebreather.

We use most other gear why does one fly a rebreather?

By the way, I know of at least one rebreather user who does multihour dives who keeps his setpoint no higher than 1.0.

"Flying a rebreather" means controlling your PO2 manually without the use of an electronic PO2 controller.

Some of us have a "Manual Rebreather" (MCCR)which the PO2 can only be controlled manually and some have an "Electronic Rebreather" (ECCR) which can be in automatic mode or can be "flown manually".

I hope that helps with the understanding.

Sincerely,

[John Rawlings]

Slightly off topic but I am curious...where does the term "flying" a rebreather come from. I hear people speaking of flying a scooter and flying a rebreather.

We use most other gear why does one fly a rebreather?

By the way, I know of at least one rebreather user who does multihour dives who keeps his setpoint no higher than 1.0.

I really don't know where that really started or why....my best guess (and that's ALL it is!), is that it is probably due to the constant monitoring of instrumentation, similar to flying.

Perhaps some of the more experienced CCR divers know the answer....

- John

[Tangfish]

This thread confused the heck out of me, b/c I thought the diver who died was diving a rebreather, due to the fact that this is in the rebreather forum and Doug's questions centered on rebreather diving. It wasn't till Nailer told me that they were diving OC that I realized the person wasn't diving a CCR.

[dsteding]

Well, my intent, which maybe wasn't well explained was to prompt a discussion of higher PPO2 diving-and I'd heard so much about 1.4 (from people not rebreather trained yet, as John pointed out) that I thought this would be directly relevant to rebreathers as well as OC divers.

The thought is that a rebreather is much more likely to push the limits of the O2 clock, if it can happen on OC with a ~20 minute exposure to 1.4, it could happen at higher exposures. But I'm an internet hack, and no expert in the field.

[Seth T.]

I'm a tad confused too. Is there a difference in oxygen partial pressure using a rebreather vs. using Trox with SCUBA? I'm pretty sure that physics dictates that regardless what equipment you're using, if you're at 1.4 OPP, it doesn't matter if your using a rebreather or SCUBA. Correct?

However, citing the article, am I understanding correctly that breathing Trimix (helium specifically) can exacerbate the OxTox effect of a "high" PPO compared to standard Trox? Can you tech guys help me out?

[CaptnJack]

N2 may have some CNS depressant effects, reducing the toxicty of O2 on the CNS system. Merely a working hypothesis for many divers since old skool deep air divers exceeded 2 ata ppO2 and never seemed to tox. Of course they were stupified at those depths and risked passing out.

You are correct a ppO2 of 1.4 is the same regardless of the method used to deliver it. Although the work of breathing probably varies among OC regs and rebreathers too.

[dsteding]

I'm a tad confused too. Is there a difference in oxygen partial pressure using a rebreather vs. using Trox with SCUBA? I'm pretty sure that physics dictates that regardless what equipment you're using, if you're at 1.4 OPP, it doesn't matter if your using a rebreather or SCUBA. Correct?

Yes and no. The number can be the same, but the cumulative oxygen exposures can be dramatically different. A RB will keep the PPO2 at a relatively constant rate, whereas a OC diver may top out at a certain level but have a lower average (and less O2 exposure) for the same dive. Then again, OC on deco will often start at high PPO2, and for O2 at 20 feet, stay there for quite some time.

[Seth T.]

Okay. I'm slightly more clear. So does helium affect a divers OxTox reaction or reaction time?

[CaptnJack]

The lack of nitrogen is postulated to enhance O2 tox susceptibility. Only because old skool deep air divers seem to be able to exceed 1.6 ppO2 and never tox. They might pass out and drown around 350+ft but not tox.

ppO2 of O2 on deco is managed by backgas breaks onto much lower O2 content backgas. 10-15 mins is the typical duration on O2 at 20ft before taking a break. Overall deco average is thus around 1.0 ppO2 despite being on O2 @ 20ft (ppO2 = 1.6) for a portion of that time.

[Seth T.]

Hey thanks Captn! That actually makes sense. So, if I'm diving Trox at 1.5 OPP and my theoretical buddy is diving Tri at 1.5 OPP, there is hypothetically a greater probability that he would tox out before me? Is that correct?

[CaptnJack]

Hey thanks Captn! That actually makes sense. So, if I'm diving Trox at 1.5 OPP and my theoretical buddy is diving Tri at 1.5 OPP, there is hypothetically a greater probability that he would tox out before me? Is that correct?

Ummm you mean
nitrox at 1.5 ppO2
vs.
Trimix at 1.5 ppO2
??

An unproven theory is that the nitrox diver may have a slightly depressed CNS and therefore be less susceptible to a siezure.

That's just an extrapolation from the massive ppO2s which historic deep air divers seemed to be able to tolerate (>2 ppO2 while working).

[Seth T.]

2 PPO?! That's just insane.

[Seth T.]

Ummm you mean
nitrox at 1.5 ppO2
vs.
Trimix at 1.5 ppO2
??

Yes

[CaptnJack]

2 PPO?! That's just insane.

Actually O2 tox apparently wasn't much of a problem on deep air. Read Exley's autobiography, its got gobs of good stories.

[Seth T.]

2 PPO?! That's just insane.

Actually O2 tox apparently wasn't much of a problem on deep air. Read Exley's autobiography, its got gobs of good stories.

I'll check it out.

[CaptnJack]

One of the best books about diving (ever). He's still an icon.
http://nssbookstore.org/index.php?mode= ... er=01-0328

[Seth T.]

One of the best books about diving (ever). He's still an icon.
http://nssbookstore.org/index.php?mode= ... er=01-0328

Sweet! :rr: