Nitrox table vs computer question

[windapp]

Since we are getting all MOD has a period after it. I will tell you that the gas choice is dictated by your dive plans MOD. Now we come full circle back to basic skills like " peak bouyancy ". Can you trust yourself to dive a perfect mix for a planned MOD, even though the hard deck of 120fsw is too deep for the hotter shallow mix?
Eric

I don't . I always plan my MOD for the hard deck. Never know when narcosis is going to kick in.

 

[DiveNav]

I've researched the math side of it and will take the course, just trying to decide if I want to try to squeeze into this months hectic schedule.......

To get ready for the Nitrox course you might want to take a look at our Introduction To Nitrox online class

.......the DivePal program doesn't let me put in a mix - and when I try to upgrade to the full version to do so, I get a 404.

mmmhhh .... which version? Windows or iPad?
BTW, you can also purchase the divePAL Nitrox license directly from our online store, then when you login into divePAL it will upgrade automatically.

Alberto (aka eDiver)

 

[caseybird]

Not to mention that mixing a tank and measuring the mix is not as precise as it sounds when you're reading the book. Has the shop mixed the gas within .1 percent? Did you calibrate the tester perfectly? Every tank? Did you compensate correctly for humidity? How about your depth guage? If you're using an electronic digital one it's probably pretty accurate (within 2 feet), a rented mechanical one on a console, who knows?
Stuff like this makes it inadvisable to push the limits. I wouldn't dive 32 to 120 fsw.

 

[FoxHound]

I don't know about other training, but PADI doesn't really teach oxygen exposure anymore beyond "Do what your computer says and here is what happens if you don't". When I took the nitrox class, I had to go out and find the tables myself, just to be satisfied that I had a personal level of understanding I was comfortable with.

With SSI we were taught tables, plus O2 exposure percentages. The ssi tables include a card to figure out your 24 hour exposure.

Adv Nitrox pretty well just reinforced that.

 

[ianr33]

It's the CNS toxicity that kills you, due to causing convulsions, and that's purely a function of the PPO2, with no time element, and it can hit quite suddenly when it hits.

NO NO NO.

That is totally and dangerously wrong!!

If I spend 10 minutes on oxygen at 20 feet (pO2 1.6) I am (almost certainly) going to be just fine. Stay there for 2 hours and I would expect to Tox. Time DOES matter.

 

[FoxHound]

The time element is an issue for pulmonary oxygen toxicity, but that won't kill you underwater. That is a function of time.

It's the CNS toxicity that kills you, due to causing convulsions, and that's purely a function of the PPO2, with no time element, and it can hit quite suddenly when it hits.

That is completely wrong....it is a function of PP02 and time.....time is an extremely key factor!

Here is a link to the NOAA O2 exposure limits:
http://www.anaspides.net/documents/scuba_diving_documents/NOAA CNS Percentage Table.pdf

First line in the paragraph and I quote

"The NOAA CNS Oxygen Exposure Limits chart is used for avoiding CNS Oxygen
Toxicity problems while conducting multiple dives on one day."

 

[ianr33]

Diving recreationally (i.e no deco) the time factor of O2 toxicity can (almost) be ignored.

Some time ago I ran the numbers out of interest and found it is impossible to exceed O2 limits if :
1)You do not exceed 1.4 pO2
2)You do not use greater than 32% O2
3)You do not enter deco

If the above are followed then deco will always be the limiting factor.

Technical dives change the rules though. Many Florida caves are around 100 feet deep. 32% is pretty much a standard mix, but on really long dives (bottom time longer than,say,2 hours, ) many thinking divers prefer to use maybe 30% instead to reduce the O2 exposure.

Similarly many rebreather divers may use 1.4 pO2 for short dives but will scale it back for long dives.

 

[Pane]

The Risks of Oxygen at Increased Depth: Undercurrent 09/2010

 

[Lizard Leg]

Thanks guys for all of the info! Like, I said, this is just a math exercise at this point. The dive is planned to "about" 110fsw because the Instructor in the water with us will be the one giving me my soft bottom during dive planning. We will be on air, so other than the reduced bottom time, MOD is not really an issue on this dive, I was just curious as to the numbers.

---------- Post Merged at 09:18 AM ---------- Previous Post was at 08:16 AM ----------


Reading the above article...

"Yes, you have probably read conservative recommendations to keep your PO2 under 1.4, or even 1.3, but there have been no incidents of oxygen toxicity at 1.6 as long as the time limits are properly observed. The DAN Nitrox Workshop held in November 2000 (I was on the faculty along with other industry experts) universally concluded that a PO2 of 1.6 was an appropriate operational limit for sport divers, thus ending an ongoing controversy."

AAAARRRGGGGGGHHHHHHHHH!!! You read one thing, learn one thing, get told to do another - seriously, how the heck is a new diver supposed to know WHAT to do? I knwo the comment was made, take the course, learn the material and make the decision for myself (risk management). But how am I supposed to know what to actually plan my dives at?

Since my family seems to look to me for dive planning (wife and son are my normal dive buddies and have told both of them, they will start doing the planning) I will likely err to the side of caution and use 1.3-1.4 as a max until we garner some more experience and see where we go after that.

It just gets confusing when divers with 1,000's of dives say 1.3, other say 1.4, the books say 1.4 but according to the link above DAN says 1.6 but...

From DAN's website:

"For open-circuit scuba diving, consider the "green light" region any oxygen partial pressure of 1.4 ata or less (this is about 82 feet / 25 meters on a 40-percent oxygen mix.) As long as this level is never exceeded, other limitations of open-circuit scuba diving will limit the exposure time to lengths where CNS oxygen toxicity is unlikely to be encountered, even for exposures approaching four hours.
Proceeding With Caution
Between 1.4 ata and 1.6 ata (this is 99 feet / 30 meters on a 40-percent mix) is the "yellow light" region. The possibility of oxygen toxicity at 1.6 ata is low, but the margin of error is very slim compared to 1.4 ata. Individual variation, the likelihood of an unplanned depth excursion causing an increase in oxygen partial pressure, and the possibility of having to perform heavy exercise in an emergency put the possibility of oxygen toxicity at levels where caution should be exercised. Thus, levels of 1.5 to 1.6 ata should be reserved for conditions where the diver is completely at rest, such as during decompression. Again, as noted previously, the dive team must still be prepared for the possibility of an oxygen convulsion at these levels.
Stop!Above 1.6 ata is the "red light" area. Just don't do it. Yes, there is evidence that short exposures at higher levels of pO2 (oxygen partial pressure) are possible but so are convulsions. At these levels, oxygen exposure depth/time limits must be adhered to. Even mild exercise may put divers breathing high-density nitrox mixes at increased risk; and even open-circuit scuba divers can achieve durations likely to get them into trouble at these levels. Diving using these high partial pressures of oxygen should be left to the trained professionals who can weigh the risks and benefits and who have the necessary training and support structure in place, if an oxygen convulsion occurs."

Again, for the time being it just seems as if we will stay in the 1.3-1.4 range, dive conservatively and make some bubbles!

 

[beaverdivers]

Dick Rutkowski - "In the process of overcoming the negative hype surrounding oxygen-enriched air, he also gave us one of his other significant contributions to diving—a now famous quote—"Science Always Wins Over Bull****."