FloppyFoot:
If they dont, they are dead.
Deepest safe depth on air?
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If they dont, they are dead.
FloppyFoot:
They generally use a mixture of helium and air or nitrox called Trimix (tri = three : helium, oxygen and nitrogen). It's all calculated to keep the amount of nitrogen and oxygen in the mix to certain levels and the rest is helium. At 600ft you'd need less than 8% O2 in the mix for it to be safe. This is, of course deadly if you breathe it shallow, so they also carry other gasses in other tanks to allow them to get deep enough to switch over to their trimix etc. etc.
I've also heard stories of extremely deep dives using air but that's just plain old wacko and nobody does that anymore that I'm aware of.
R..
ChillyWilly
Contributor
wedivebc:
If you do some research you will find that each agency supports diferent levels and exposure time. Padi and SSi recomend a maximum of 1.4 but include a grey area up to 1.6 ATA. Dan research director proposed a limit of 1.2 on a Oxygen Exposure paper that he wrote. NOAA set a limit of 1.3 ATA for 3 hours of exposure but allows exposures up to 1.6 ATA.
In summary, only exposure up to 1.3 ATA will assure you that OXTOX won't be a problem. Over 1.6 ATA something most likely will happen and in between, there is always a posiblity. Why do you think that most agencies consider the 1.4 to 1.6 ATA range a grey area.
H2Andy
Contributor
ChillyWilly:
can you give us some authority for this statement?
ChillyWilly
Contributor
H2Andy:
http://www.diversalertnetwork.org/medical/articles/article.asp?articleid=35
With the advent of nitrox diving it is wise to consider these studies. Dr. Andrea Harabin, a scientist at the Naval Medical Research Institute (NMRI) in Bethesda, Md., analyzed the human oxygen exposures from the NEDU studies and used a mathematical model to predict the probability of CNS oxygen toxicity symptoms occurring. (See Reference 2, page 40 for details.)
When she considered all symptoms which resulted in the diver stopping his dive, she found that the model had a threshold at 1.3 ata; that is, the probability of a CNS symptom occurring at or below this level should be essentially zero. Some of the CNS symptoms that caused dives to be halted could have been due to many other reasons besides oxygen toxicity and were classified as "Probable." In contrast, with "Convulsions" and "Definite Symptoms" (see Table 3, page 37), there is usually no question that oxygen toxicity is the culprit. When Dr. Harabin considered just the convulsions and definite symptoms, she found the thresholds to be 1.7 ata. This analysis again reflects the large degree of uncertainty inherent in these types of human exposures.
Hope this helps,
Later,
H2Andy
Contributor
ChillyWilly:
PADI, the Professional Association of Diving Instructors, has proposed a limit of 1.4 ata for open-circuit nitrox scuba diving. Because open-circuit scuba diving would not expose divers to this level continuously, in practice it should be as safe, or safer, than the 1.3 ata U.S. Navy limit for continuous exposures. (See sidebar "Continuous vs. Intermittent Exposures," page 40.) In fact, the shallow exposure times in the 1.3- to 1.4-ata range are mainly to avoid lung oxygen toxicity; the likelihood of CNS toxicity at these levels is very low and probably not much different over this range.
the 1.3 ata you are talking about is for continued exposure, using close-circuit,
up to 8-hr dives.
1.4 is as safe (or safer) for open-circuit (i.e. scuba)
ChillyWilly
Contributor
H2Andy:
Yes I am talking about constant exposure, worst case scenario. All I'm saying is that there are many variables to be aware of and many things can happen. This article is actually very good and will explain the diferences so divers can be aware. Also the article says that above 1.3 ATA most symptoms were " Probable" not definate. So that is why the 1.3 Limit is important and is a no worry limit were nothing can happen.
Good discussion !!
Anyway I hope everybody reads the article and gets some good info out of it.
wardric:
Your study stated narcosis CAN have effects as shallow as 30. He might not have had problems at 87 because he had no problems and he might not have had problems because it wasn't effecting him.
I occasionally dive to the 125fsw range and have no problems. I have helped people that did have problems though. Narcosis affects everybody differently and at different depths. I have not personally had any problems that I had to handle with myself, but I have been diving with people that developed problems that I had to handle for them.
I do know what being narced feels like also, as the only time that I felt it was when I dove to about 105fsw once. The water was cooler than normal (about 72 F versus 77-82 F) and I didn't feel 100% that day either. I handled it though and ascended slightly to relieve the effects. I then went back down and had no problems.
Diver Dennis
Contributor
ChillyWilly:
I have always used the 1.4 limit, you are saying symptoms are "probable" over 1.3? Probable to me is more than a 50% chance.
xiong
New
This is the subject of much debate in both Tech and non-Tech circles, with some in DIR becoming absolutely livid on the subject. 
As someone with one foot in the Tech world, I'd say worry about narcosis first, before worrying about your PPO, because, most likely, you will feel narced long before your PPO becomes a problem....BUT: don't ignore the PPO factor either.
No sane diver/instructor would recommend pushing PPO limits either.
Wanna play it safe? Stay within Recreational Limits, ie: 130 ft.
Will going to 150 once in a while mean certain death? Probably not......
Rule #1: in diving: 'Know the limits of your training and stay within them.'
As someone with one foot in the Tech world, I'd say worry about narcosis first, before worrying about your PPO, because, most likely, you will feel narced long before your PPO becomes a problem....BUT: don't ignore the PPO factor either.
No sane diver/instructor would recommend pushing PPO limits either.
Wanna play it safe? Stay within Recreational Limits, ie: 130 ft.
Will going to 150 once in a while mean certain death? Probably not......
Rule #1: in diving: 'Know the limits of your training and stay within them.'
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