If you had to choose, 80% or 100% for deco gas and why.

[danvolker]

In the Jane Orenstein tech student death in the 90's, the student Jane switched from bottom mix from 280 foot dive on the Rb Johnson, supposedly to her travel mix....her buddy and instructor was supposed to be watching, yet did not notice she went to her pure O2 bottle at 100 feet.....she stayed on this through stops to 30 feet, where she signaled her instructor she was out of gas in this bottle, and was going to switch and ascend to the 20 foot stop....where she toxed as she ascended away from him....and then on blacking out, began dropping to the depths, with the instructor, Derrick McNulty, watching and afraid he did not have enough back gas to go after her.....and she was left to drown.....At the time, I created a murder thread on this, as many of you who know me might expect

In any event, Jane breathed 100 at way beyond the "limits", for a very long time, prior to passing out from the tox event.

 

[kr2y5]

Hence my argument that it's a poor tool. It doesn't work on the low end or the high end. The failure of it working on the low end is the biggest issue, I think. People can think they're in the clear because its "less than 100%", but they aren't.

I would imagine that the weather channel is not one of your personal all-time favorites.

 

[PfcAJ]

I would imagine that the weather channel is not one of your personal all-time favorites.

Its ok, people don't generally drown if the forecast is wrong. Assume your o2 exposure is something that its not and you're going to have a bad time.

 

[100days-a-year]

CNS clock is more like a carcinogen,we know it has a cumulative negative effect but as individual tolerances vary widely even within a day there is no real quantatative statement that can be made beyond"Uh,too much oxygen exposure is bad M'kay"

 

[Dr. Lecter]

Hence my argument that it's a poor tool. It doesn't work on the low end or the high end. The failure of it working on the low end is the biggest issue, I think. People can think they're in the clear because its "less than 100%", but they aren't.

This is true of MOD and deco profiles, too, however. Why are you taking them so much more seriously?

 

[PfcAJ]

Exceeding MODs WILL drown you. A 3.0 while working underwater will cook your goose. A CNS of 200% may or may not drown you, depending on how you go about it.

Case in point: Jim Miller. 50% at 220 has a ppo2 of 3.84. His CNS% was roughly 200% when he had a seizure.

But a BUNCH of people routinely go past 200% at more reasonable PPO2s (1.6 and less) by using gas breaks. There are posters in this thread who manage CNS%s in excess of 600%-6000%. You've even done 400%, which is WAY past the "limit".

A high ppo2 will get you. A moderate ppo2+gas breaks over a log period of time doesn't. Going over MOD HAS been shown to kill divers. Going past "CNS%" with gas breaks doesn't kill divers.

I think comparing o2 toxicity to DCS is difficult. The faster you ascend, the closer your chances of experiencing DCS come to "1". O2 toxicity doesn't work like that. Apparently you can decompress from an effective saturation dive (thats MAX deco) using the 1.2 or less on the bottom, 1.6 or less through deco, incorporate gas breaks method and not experience o2 toxicity.

Riddle me this, why would YOU (Dr Lecter) exceed the limit if you think its a valid thing?

---------- Post added July 18th, 2014 at 04:51 PM ----------

I think the CNS% thing is a lot like overfilling steel tanks, actually.

The book says 2640 (100%). But we know that you can go to 3600psi (above 100%) by keeping the tanks in VIP and Hydro (kinda like doing gas breaks) and nothing happens. Don't do the VIP/Hydro (gas breaks) and you might get exploded (seizure).

Overfilling an al80 is a lot like going over MOD. Its less forgiving and you might get exploded.

People argue all the time that overfilling steels = super dangerous. But its routinely done without incident. Same same with exceeding CNS% by using gas breaks.

 

[kr2y5]

I think comparing o2 toxicity to DCS is difficult. The faster you ascend, the closer your chances of experiencing DCS come to "1".

And yet, even this does not meet your steep criteria of usefulness... an event that has probability 1.0 is not necessarily guaranteed to happen, just as an event with probability 0 is not impossible to occur. Indeed, as Heraclitus pointed out a while back, our entire life is composed of probability 0 events...

 

[PfcAJ]

1.0 means it happens 100% of the time and 0 means it happens 0% of the time. But I think you know this as your profile lists you as a research scientist. I think you're trying to be obtuse in a flimsy attempt to make a point, when you really don't have one.

 

[Dr. Lecter]

Exceeding MODs WILL drown you. A 3.0 while working underwater will cook your goose. A CNS of 200% may or may not drown you, depending on how you go about it.

Case in point: Jim Miller. 50% at 220 has a ppo2 of 3.84. His CNS% was roughly 200% when he had a seizure.

But a BUNCH of people routinely go past 200% at more reasonable PPO2s (1.6 and less) by using gas breaks. There are posters in this thread who manage CNS%s in excess of 600%-6000%. You've even done 400%, which is WAY past the "limit".

A high ppo2 will get you. A moderate ppo2+gas breaks over a log period of time doesn't. Going over MOD HAS been shown to kill divers. Going past "CNS%" with gas breaks doesn't kill divers.

I think comparing o2 toxicity to DCS is difficult. The faster you ascend, the closer your chances of experiencing DCS come to "1". O2 toxicity doesn't work like that. Apparently you can decompress from an effective saturation dive (thats MAX deco) using the 1.2 or less on the bottom, 1.6 or less through deco, incorporate gas breaks method and not experience o2 toxicity.

Riddle me this, why would YOU (Dr Lecter) exceed the limit if you think its a valid thing?

---------- Post added July 18th, 2014 at 04:51 PM ----------

I think the CNS% thing is a lot like overfilling steel tanks, actually.

The book says 2640 (100%). But we know that you can go to 3600psi (above 100%) by keeping the tanks in VIP and Hydro (kinda like doing gas breaks) and nothing happens. Don't do the VIP/Hydro (gas breaks) and you might get exploded (seizure).

Overfilling an al80 is a lot like going over MOD. Its less forgiving and you might get exploded.

People argue all the time that overfilling steels = super dangerous. But its routinely done without incident. Same same with exceeding CNS% by using gas breaks.

I'm obviously not convinced it's a gold standard, but I'm also not going to declare it to be some uniquely useless "bunk" based on my own experience -- especially as it pertains to repeated exposures rather than on a single dive basis. 3.0 while working is rather different from running a 1.7 on air...but I see people in hysterics over that.

I'm not claiming there's anything super-accurate about the CNS clock. I am claiming that it's something worth considering, and I often choose to spend a few more minutes on deco rather than run pure O2 in my loop from 20' on up even if I'm way past 100% either way.

My needling you about this is about 1 parts annoyance at seeing vocal certainity where it's not supported (either way, to be sure), and 2 parts finding it funny that someone who's so wedded to dogmatically following some made up conservative rules (1.2-1.4 bottom/1.6 deco, low END, deco schedules) can be so dismissive of an equally made up conservative rule.

At the end of the day, I think we're dismissive of this because there's currently no damn way around high CNS for our diving. You can, if you want, avoid PO2s above those commonly called safe; you can follow deco schedules; you can use helium to avoid narcotic impairment...but if you want to do a deep dive with 3+ hours of deco, you're going to be breathing a ton of O2 and there's just no way around it.

 

[boulderjohn]

According to my tech instructor, there is a significant safety advantage in terms of fire with 80% when compared to 100%, especially when a tank is overfilled. .

I would like to see some documentation on this. The information I had, which I believe came from Vance Harlow's Oxygen Hacker's guide, is that in examining the Gus Grissom tragedy, NASA determined that anything over 50% had a reasonably similar chance of making things go bad.

We know what really happens but I was just stating the facts. That is why you have to have a booster to pump O2 at higher pressures.

100% is readily accessible. 80% is a headache to obtain.

This can be an important factor for people (like me) who frequently dive in places where we don't have access to shops who make these kinds of fills regularly. I have done many, many dives where any nitrox mix, up to including 100%, was created by partial pressure filling from whatever we had on hand from an industrial O2 bottle, with no booster within hundreds of miles. That means you can't create a full 3,000 PSI of O2 even if you are starting with a full O2 bottle for your fills. When making fills, you have to plan the order in which you fill tanks very carefully so that you will be able to make as many 100% O2 fills at reasonable volumes as you can before doing the 50s and other mixes. As your supply bottles dwindle down, it becomes very tempting to top off the 100% bottles with some air for the sake of volume.

First of all, I agree with the preference for 100% O2 for the reasons already stated by many. On the other hand, I have used 80%. When I got my advanced trimix training, we did all the training dives on 80%. It was not just my instructor's preference, it was clearly what most people on the boats we used were using. The fill shop we used had 80% banked. It was the popular choice, and I was not going to make an ass of myself insisting on using 100% and being the only one doing it. I am able to make ass of myself in too many other ways to add this topic to my repertoire.

I did not think it made all that much difference. We made the final switch at 30 feet instead of 20. Fine. I asked the others why they preferred 80%, and they never gave any of the reasons you hear about. They did not talk about holding stops in swells, and their buoyancy control holding stops was plenty good. I got the sense that it was just something they had always done. I gave my instructor a copy of the Bakers Dozen for his amusement. He had never seen it before. He frankly thought it was silly.

That was a few years ago. I still go to that part of the country and do deco dives each year. It seems to me that 100% O2 is more the norm now than it was then. Still, if I ended up diving with a group that wanted to do 80%, and I was joining them, I would just shrug and go with the flow.