1.4 ....1.6...or ?????

[plclark196]

"dropping down to 1.6 for a few seconds is not a big deal"...???

The way I understand oxygen toxicity is that it does not always give us signs or symptoms....and it takes less than a few seconds to suffer a convulsion.

 

[LavaSurfer]

Personally? 1.4 is the limit
That gives me a margin of error and a nice buffer zone in case something were to happen like a down swell or momentary loss of buoyancy. 10 foot changes can happen quickly at depth. Having a buffer is crucial.

If I am @ 1.6, the margin of error is slim or missing and there is no buffer.
For me, that is risky behavior and exceeds the risks I intend on taking.

Each Diver must make their own decisions. My buddies mostly agree on this number and none wants to drag the other up unconscious.

Since I cannot yet dive Trimix, I prefer Air below 100 feet and use EAN32 from 60-100.
That is my choice, some will poo poo it and that is their choice!

 

[sphillips3]

The toxing incident was not at Ginnie, it was at "The Crack" and it was a deeper trimix dive.

That is not the incident I was referring to - In addition to the event at the Crack a diver also toxed at Ginnie in November, 2007. I don't have all the details right at hand but my understanding is that it was a nitrox dive in the 100 foot range. It was pretty conclusively a CNS tox event because there were witnesses that saw the diver convulsing. Even with experienced buddies following all known procedures for dealing with CNS tox they were unable to save him.

The diver was very well known in the technical diving community and this incident really got the attention of the tech diving instructors, instructor trainers and course directors I know. Several of them immediately reduced their max working PO2 to 1.2 for all dives, trimix and nitrox and started serious discussions about making changes to the training standards. Standards changes take a lot of time so it might be a while before you see changes but don't be supprised when they do happen.

 

[Blackwood]

All the nitrox training I have received (basic nitrox, advanced nitrox, trimix from three different instructors) has given guidance to plan for 1.4 for the working portion of your dive, 1.6 for contingency or deco. You use the "best mix" calcs to work out your bottom mix at 1.4, not 1.6. So while they can easily calculate a best mix at 1.6, I doubt they were trained to do so.

I've had guidance to plan to 1.4 and to 1.2, and to use standard gases.

While I haven't been told to plan the working portion to 1.6, I don't see it as unlikely that someone else may look at their options and select it even if they weren't explicitly told to do so.

Further, they appear to be using (or think to be using) some Navy standards.

 

[battles2a5]

"dropping down to 1.6 for a few seconds is not a big deal"...???

The way I understand oxygen toxicity is that it does not always give us signs or symptoms....and it takes less than a few seconds to suffer a convulsion.

1.6 is the generally accepted "contingency" PO2. If used for contingency purposes, it is not a big deal. Chances are, you won't spontaneously seize when you hit 1.6.

That being said, I mentioned previously, as have others, that there is no way to predict when or how a tox episode can occur and the best way to avoid it is to have a conservative plan, one with built in contingency (e.g. plan for 1.4, contingency 1.6). If you are not comfortable w/ 1.6 as a contingency PO2, then plan at 1.2 w/ a contingency of 1.4. However, if you are doing your working portion of the dive at 1.6, you don't have a continency plan within the accepted limits of recreational nitrox diving. That is the point I was trying to make.

 

[Meng_Tze]

That is not the incident I was referring to - In addition to the event at the Crack a diver also toxed at Ginnie in November, 2007. I don't have all the details right at hand but my understanding is that it was a nitrox dive in the 100 foot range. It was pretty conclusively a CNS tox event because there were witnesses that saw the diver convulsing. Even with experienced buddies following all known procedures for dealing with CNS tox they were unable to save him.

The diver was very well known in the technical diving community and this incident really got the attention of the tech diving instructors, instructor trainers and course directors I know. Several of them immediately reduced their max working PO2 to 1.2 for all dives, trimix and nitrox and started serious discussions about making changes to the training standards. Standards changes take a lot of time so it might be a while before you see changes but don't be supprised when they do happen.

I would like to see more about that one, could not find it on IUCRR

 

[battles2a5]

Sorry, I thought TSandM was talking the Crack, since she made reference to the female victim and the fact that her husband had posted all of the details in order order for others to be informed. That incident was the Crack unless I am mistaken. I do recall the incident you are referring to but I didn't think it was at Ginnie. It's not on IUCRR either. It's a shame, either way.

You are right about instructors taking notice. My instructor was banking 31% or 30% in January, as a result of these two incidents. Until then he had always used 32 for pretty much anything under 110.

That is not the incident I was referring to - In addition to the event at the Crack a diver also toxed at Ginnie in November, 2007. I don't have all the details right at hand but my understanding is that it was a nitrox dive in the 100 foot range. It was pretty conclusively a CNS tox event because there were witnesses that saw the diver convulsing. Even with experienced buddies following all known procedures for dealing with CNS tox they were unable to save him.

The diver was very well known in the technical diving community and this incident really got the attention of the tech diving instructors, instructor trainers and course directors I know. Several of them immediately reduced their max working PO2 to 1.2 for all dives, trimix and nitrox and started serious discussions about making changes to the training standards. Standards changes take a lot of time so it might be a while before you see changes but don't be supprised when they do happen.

 

[TSandM]

Well, honestly, I don't remember which cave it was; I just remember that the ppO2 was about 1.4, and that her buddies were unable to save her, despite doing about everything right.

There was also an incident shortly thereafter, which I thought was in the LA area, where a technical student seized at a low ppO2. His instructor was able to get him to the surface, and he survived.

I just try to keep a kind of perspective on the risk-benefit ratio of things (that's my medical training kicking in). There are risks involved in simply diving, but many of them can be mitigated by having good quality equipment and maintaining it, by developing good skills, and by being realistic about which dives are within your ability and which are not. And many, if not most problems that occur are salvageable by people who keep their wits about them and take proper action.

A seizure from high ppO2s is preventable (by avoiding them!) but is not treatable and is rarely salvageable. That's just too high a price to pay for a little more bottom time, for me.

 

[jviehe]

There was a death earlier this year at Ginnie Springs with a reported max P02 of about 1.3. Several tech diving instructors I know are reducing the max PO2 for the working portion of the dive to about 1.2. I'm not an expert, but I have heard that the Global Underwater Explorers (GUE) folks are specifying standard mixes to hold max PO2 to something close to 1.2.

There is a tech diving board - the Deco Stop - where you can find extensive discussions on this issue.

In fairness, that guy was switching from a rebreather to a open circuit at the end of a 4000ft dive solo, then back. When he got back his rebreather was flooded. Other craziness ensued. OxTox was not the problem here.

 

[String]

"dropping down to 1.6 for a few seconds is not a big deal"...???

The way I understand oxygen toxicity is that it does not always give us signs or symptoms....and it takes less than a few seconds to suffer a convulsion.

Acute exposures thats correct. 1.6 however is nowhere near that.

If you sucked in a 2.5 chances are youd have an issue near immediately. Below that and the clock is more of an issue.

Most technical divers use 1.6 for extended periods of time without any issue at all as a matter of routine.