How conservative is the PPO2 Limit

[Foxfish]

Do you have any idea who Doppler is? Are you seriously questioning his understanding of the subject?

I'm still a bit hazy on some of the questions I've raised in the past. Doppler seems like he has given the matter some careful consideration so I'm interested to hear his thoughts.

 

[Doppler]

I'm still a bit hazy on some of the questions I've raised in the past. Doppler seems like he has given the matter some careful consideration so I'm interested to hear his thoughts.

I am packing to leave for a dive/teaching trip so this will have to be brief and I am too lazy to include links but Google is your friend. There are a couple of folks in Australia with whom you should try to connect or attended lectures by if the opportunity arises... Simon Mitchell (a Kiwi/Aussie) is a member here and is a very approachable bloke, and in my opinion you couldn't slide a piece of paper between him and an expert on the topic of gas toxicity. But don't tell him I said that because he might buy me a beer the next time I see him... and I'd hate for that to happen.


Before you contact anyone though, I'd also suggest some remedial reading of the NOAA diving manual and various articles available onLine from Bill Hamilton and Dick Rutkowski. Both Rutkowski and Hamilton worked with NOAA when the CNS tables were developed. ALL RECREATIONAL agencies base their suggestions to divers on these tables... so reading the thoughts of this little triumvirate is going to get you farther along the road to a complete understanding than listening to me rattle on or from reading the couple of posts on my personal blog through which I've tried to explain these issues.

However, that said, there are a couple of basic concepts you do not seem to grasp... and that is nothing to be ashamed of, because the myths perpetuated by, and misunderstanding rampant in a surprising number of the folks "teaching" nitrox confuses lots of people. Unfortunately, too few of them are as persistent as you seem to be in searching out the facts. (And by the way, the thread you cited in your earlier post if FILLED with bull****... sorry, but ignore most of what you read. Some of it is real information but it also contains the myths and misunderstandings. LOL. ANYHOW...)

#1... Divers are told to consider two forms of oxygen toxicity. (There's one more form that rebreather divers are taught about, but let's forgo that complication and stick with the two most commonly cited and sometimes confused.)

One is Central Nervous System toxicity (CNS). It is highly dangerous because once one exceeds the limits set out in NOAA's tables, the likelihood of a tonic-clonic episode in the water increases to greater than acceptable rates. For a graphic illustration of the usual outcome of this type of event when underwater, search for threads about a recent incident in Ginnie Springs, Florida.

CNS toxicity is a function of oxygen depth AND time. The time is dependent on the PARTIAL PRESSURE of OXYGEN being delivered by the diver's gas. That partial pressure is a function of the fraction of oxygen in the mix and the depth at which it is breathed.

FORGET YOUR PADI TABLES, and look at the NOAA ones. These are the GOLD STANDARD and the original. (We will not get into the whole embarrassing history for DEMA and PADI and nitrox but you may want to look that up sometime too.)

Anyway, look at the tables. I posted the limits in my previous posting. the TIME limits decrease as the oxygen depth increases. In other words, the oxygen time for 1.3 bar is 180 minutes, for 1.4 bar is 150 minutes, for 1.5 bar it is 120 minutes, and for 1.6 bar it is 45 minutes. Check out the tables.

You will also notice that as well as a SINGLE exposure time limit for each oxygen depth, there is a DAILY LIMIT or 24-hour limit. These time limits vary too. MANY, MANY instructors and a couple of agencies DO NOT TEACH divers about these limits and this is a HUGE mistake in my opinion.

THESE 24-hour limits have NOTHING TO DO with Oxygen Toxicity Units. These are used to track...


#2... Pulmonary or Whole Body toxicity. This is the second type of issue related to breathing elevated levels of oxygen. Pulmonary tox is NOT an issue for divers doing recreational diving... and I include most technical divers in that caveat.

It's results are not life-threatening... well, diving with advanced pulmonary edema would be but to all intents, this form of oxygen poisoning is the result of longer-term oxygen exposures than all but the most extreme divers will subject themselves to.

So, I have to go.

Do some research. You will find the answers. And if you dive nitrox on a liveaboard or on multi-dive, multi-day trips, PLEASE learn about and TRACK 24-hour CNS limits.

 

[Foxfish]

Appreciate you taking the time to respond Doppler. You've said the two NOAA limits are for tracking exposure to CNS oxygen toxicity on a single dive and pulmonary oxygen toxicity for a 24 hour dive. What limits set for tracking CNS toxicity over a 24 hour period?

 

[Doppler]

Appreciate you taking the time to respond Doppler. You've said the two NOAA limits are for tracking exposure to CNS oxygen toxicity on a single dive and pulmonary oxygen toxicity for a 24 hour dive. What limits set for tracking CNS toxicity over a 24 hour period?

NO... I MAY NOT HAVE BEEN CLEAR... NOAA limits for single dive AND 24-hour exposure are both for CNS that's all you need to really bother with. Pulmonary toxicity is tracked with OTUs and is a totally different thing and NOT to be confused

 

[Foxfish]

I am aware the NOAA table gives a single and daily exposure limits listed on them. I take it the following comment was made with reference to the daily limit on the table?

You will also notice that as well as a SINGLE exposure time limit for each oxygen depth, there is a DAILY LIMIT or 24-hour limit. These time limits vary too. MANY, MANY instructors and a couple of agencies DO NOT TEACH divers about these limits and this is a HUGE mistake in my opinion.

THESE 24-hour limits have NOTHING TO DO with Oxygen Toxicity Units. These are used to track...
#2... Pulmonary or Whole Body toxicity. This is the second type of issue related to breathing elevated levels of oxygen. Pulmonary tox is NOT an issue for divers doing recreational diving... and I include most technical divers in that caveat.

It appears this statement refers to pulmonary toxicity which clearly contradicts your last post. Please clarify.

 

[Nirvana]

I am aware the NOAA table gives a single and daily exposure limits listed on them. I take it the following comment was made with reference to the daily limit on the table?



It appears this statement refers to pulmonary toxicity which clearly contradicts your last post. Please clarify.

The 24-hour limits were established in NOAA tables and refer to CNS toxicity. The Oxygen Toxicity Units (OTUs) are used to track pulmonary or whole body toxicity. For most divers, technical or recreational, tracking OTUs is unnecessary. They should, however, track CNS toxicity in terms of single exposure and 24-hour exposure.

 

[Foxfish]

The 24-hour limits were established in NOAA tables and refer to CNS toxicity. The Oxygen Toxicity Units (OTUs) are used to track pulmonary or whole body toxicity. For most divers, technical or recreational, tracking OTUs is unnecessary. They should, however, track CNS toxicity in terms of single exposure and 24-hour exposure.

That is my understanding. May have just been the way that #1 trailed off and #2 started in Dopplers post that created the confusion.

 

[mathauck0814]

The 24-hour limits were established in NOAA tables and refer to CNS toxicity. The Oxygen Toxicity Units (OTUs) are used to track pulmonary or whole body toxicity. For most divers, technical or recreational, tracking OTUs is unnecessary. They should, however, track CNS toxicity in terms of single exposure and 24-hour exposure.

OTUs are also time based. You get 1440/day.

 

[Dr. Lecter]

I am aware the NOAA table gives a single and daily exposure limits listed on them. I take it the following comment was made with reference to the daily limit on the table?

You will also notice that as well as a SINGLE exposure time limit for each oxygen depth, there is a DAILY LIMIT or 24-hour limit. These time limits vary too. MANY, MANY instructors and a couple of agencies DO NOT TEACH divers about these limits and this is a HUGE mistake in my opinion.

THESE 24-hour limits have NOTHING TO DO with Oxygen Toxicity Units. These are used to track...


#2... Pulmonary or Whole Body toxicity. This is the second type of issue related to breathing elevated levels of oxygen. Pulmonary tox is NOT an issue for divers doing recreational diving... and I include most technical divers in that caveat.

It appears this statement refers to pulmonary toxicity which clearly contradicts your last post. Please clarify.

I'll try to rephrase the ambiguity, Doppler can correct on the off chance I've totally misread this:

You will also notice that as well as a SINGLE exposure time limit for each oxygen depth, there is a DAILY LIMIT or 24-hour limit. These time limits vary too. MANY, MANY instructors and a couple of agencies DO NOT TEACH divers about these limits and this is a HUGE mistake in my opinion.

THESE 24-hour limits have NOTHING TO DO with Oxygen Toxicity Units. [-]These[/-] Oxygen Toxicity Units are used to track...


#2... Pulmonary or Whole Body toxicity. This is the second type of issue related to breathing elevated levels of oxygen. Pulmonary tox is NOT an issue for divers doing recreational diving... and I include most technical divers in that caveat.

 

[Crass3000]

I haven't read any of the feedback you have got yet but it's generally best to keep the ppO2 at 1.4 or less (not like I'm an angel and always do but it's dumb not too). If you dive a lot like on vacation it's probably better to dive even air as 21% nitrox so your computer takes all your 02 in mind. Many people dive higher pp02 for short periods but nobody knows how much exposure is going to give you oxygen toxicity. I'm don't follow plenty of rules but oxygen toxicity is one not to screw with.