EaNX and PPO2 of 1.6

[Bismark]

I have noticed that a lot of divers prefer nitrox instead of trimix for certain depths and use nitrox whenever possible.

For 130 to 150 ft, EAN 25 is quite popular, although it is barely better than air.

For 100 to 130 ft, EAN 32 is quite popular, and for some the ideal mix.

For 50 ft to 100 ft, EAN 36 is a very popular mix, expecially with expert divers, who are not paranoid about a limit of 1.4 ATAs.

Nitrox is easier to get, and easier to mix, and faster to decompress from when you have only one gas. And for strictly NDL diving, nitrox gives you longer NDL times than does trimix of the same O2 levels.

Also with trimix, you need to have splendid buoyancy control. If you happen to shoot to the surface with trimix, it would likely be a fatal error. Helium is unforgiving of novice divers.

Here we go again.........I don't have the time to waste on this again........

 

[nereas]

Ok, after reading some more, I think the problem here is that we are saying the same thing, but you are saying it in more detail. PADI simply eliminates unnecesary information. According to sources other than PADI, CNS tox can be time dependant if single exposures to ppo2 exceeds whatever time. However these times are impossible to do as a recreational diver, which is why PADI discards them. For example you could be exposed to 1.4 for 150mins, which you cant do without doing deco, even at 40% nitrox. So, in effect, we are both right, but you are more right.

Stop quoting PADI. That is a common mistake made by novice DMs.

Most people view PADI as the definitive word on incompetence.

Learn the laws of physics, and the research data, and then develop your own views.

 

[tboxcar]

As a nitrox diver, I found this thread quite informative and interesting, good work guys!

 

[nereas]

As a nitrox diver, I found this thread quite informative and interesting, good work guys!

You also have the additional issue of high altitude, like the Swiss lakes.

Nitrox is invaluable for high altitude diving.

But in addition, a diver must make sure that the computer is set for high altitude, and that you have checked the NOAA tables for equivalent high altitude adjustments.

That is not an CNS nor pulmonary tox issue, of course. Just a regular N2 DCS issue. Therefore diving with richer oxygen in the mix is a major benefit with the altitude difference and added sensitivity to DCS from depths normally easy at sea level.

I am sure that you know all this already, since high altitude is your back yard. I just wanted to mention it for anyone else.

High altitude lakes are usually crystal clear and fun to dive in, just really cold waters!

 

[Charlie99]

the oxygen exposure table (which is the NOAA math) is only used by PADI to track pulminary tox. CNS tox is simply avoided by staying below 1.4.

Although this is apparently what you believe, and unfortunately what you are apparently teaching your students, the DSAT/PADI Oxygen Exposure Table has NOTHING to do with pulmonary toxicity. The PADI Oxygen Exposure Table is based upon CNS oxtox experimental data and is used to track CNS toxicity.

Look at the table and calculations of the references I linked to above. CNS oxtox limit at 1.6ata is about 45 minutes, while the 4% vital capacity (pulmonary oxtox) limit is around 360 minutes. Clearly, the PADI/DSAT table is CNS oxtox, not pulmonary.

CNS oxtox is a function of both time and ppO2. I have seen many posts on Scubaboard that show ignorance of that fact. It appears both you and many other instructors are incorrectly teaching their students.

Charlie Allen

 

[nereas]

...
CNS oxtox is a function of both time and ppO2. I have seen many posts on Scubaboard that show ignorance of that fact. ...

Charlie Allen

Exactly. It takes time to figure all this out, I think, and that is why there is so much bewilderment.

 

[jviehe]

Stop quoting PADI. That is a common mistake made by novice DMs.

Most people view PADI as the definitive word on incompetence.

Learn the laws of physics, and the research data, and then develop your own views.

Either they are right or wrong. Are you claiming that PADIs material is factualy incorrect?

 

[jviehe]

Although this is apparently what you believe, and unfortunately what you are apparently teaching your students, the DSAT/PADI Oxygen Exposure Table has NOTHING to do with pulmonary toxicity. The PADI Oxygen Exposure Table is based upon CNS oxtox experimental data and is used to track CNS toxicity.

Look at the table and calculations of the references I linked to above. CNS oxtox limit at 1.6ata is about 45 minutes, while the 4% vital capacity (pulmonary oxtox) limit is around 360 minutes. Clearly, the PADI/DSAT table is CNS oxtox, not pulmonary.

CNS oxtox is a function of both time and ppO2. I have seen many posts on Scubaboard that show ignorance of that fact. It appears both you and many other instructors are incorrectly teaching their students.

Charlie Allen

Then it appears that you think PADI is wrong. Is this your belief?

 

[HowardE]

The Table is called the "CNS Oxygen Exposure Table" and the times can be displayed in % CNS per minute.

OTU's are different.

CNS Oxygen Toxicity is a function of both exposure (time) and ATA's of O2 (ppo2).

A single breath at a PO2 of 2.0 ATA will (most likely) not cause a convulsion. However, prolonged exposure to high PO2's will (most likely) cause eventual convulsions.

From the TDI Advanced Nitrox Manual

Exen at the advanced nitrox level, pulmonary oxygen toxicity remains an unlikely condition to be suffered by the diver. Pulmonary oxygen toxicity is characterized by a long term, low dose rate of oxygen. Typical oxygen pressures must be at more than 0.5 ATA and less than 1.1-1.2 ATA. MANY HOURS of exposure to oxygen continuously are required before symptoms may manifest.

 

[rab]

Hey guys,

I'm going to be diving the Oriskany in a few weeks with a dive group and we will be diving nitrox. I've been certified to use it for over 2 years but have never found the opportunity to use it (air has worked fine for all my ocean dives since we'd do a deep dive then a shallow dive, shallow cavern diving etc.) . My computer can handle nitrox and I'm familiar with the rules etc. but I think maybe during my classes I misunderstood my teacher and just would like some clarification.

...

Now that I'm thinking about it. What if it's your first dive of the day on nitrox and you go straight to the bottom and hit PPO2 1.65 for say 1 minute and come back to a level where your PP02 is 1.3 or so. Is what my instructor was trying to say was "If you go below PP02 1.6 your O2 clock is used up much much faster so you consider it "instantly" using up 100% of your O2 clock because it only takes maybe 10 minutes to use it up at PP02 1.6?"

The reason I ask is we will be diving 29% O2 and at the beginning of the dive we're planning on touching the deck at 137'. This puts me at a PP02 of 1.58. We would only stay for a minute or two and then go hang out around 110' and shallower. Now I can have my computer beep at me if I were to hit PP02 1.6, but if something goes wrong (heaven forbid) and I get distracted for a second or take a second to become neutrally bouyant and accidentally go slightly over PP02 1.6 I'm not going to black out right?

Sorry if it's a stupid question, just like to be very prepared for things (taking my rescue diver classes this weekend, I guess it's rubbing off on me already ).

Thanks for your responses in advance!
Jim

FYI, I was one of the divers on the MBT/H₂OBelow trip to the USS Oriskany on 6-Sep-2008. It was their first trip out in a week due to Gustav. The Flight deck just forward of the "Island" measured 45.1m (148ft) on my Nitek Duo set to "sea" and 44.8m (147ft) on my Suunto Mosquito (sitting a couple inches higher on my forearm).

It will be interesting to know how the dive briefings change with respect to the flight deck. Our briefing called it 135' and the materials at MBT (More Bottom Time) Divers listed 137' -- a quick touch on the flight deck to say you'd "been there" was tacitly approved, but PPO₂ starts to be an issue for Nitrox above 25% with the new depths. (I noticed that Rich, the Dive Master for our trip, was using 24%)

IF you are planning to touch the deck, 29% has a 1.6 MOD of 149' -- not much of a safety margin. You're certainly not going to stay on the flight deck for "a minute or two" even if your profile is allowed by the time you make your trip.

Our viz was less than 25ft so you couldn't even see the flight deck before you were nearly on it, but that should improve by the time you're there. While I don't know if I'll ever make it back for another dive (it is rather expensive!), it would be great to have the 80ft viz that is more typical.

-Rob