Nitrox mixes above 40%?

[rjack321]

Is it in the advanced nitrox course as well that using 1.6 instead of 1.4 is taught? I understand that the 1.4 is merely a conservative safety factor on the 1.6. But does it become "common practice" to ignore the 1.4 after "Advanced Nitrox". Or is the 1.4 just a PADI limit?
Thanks!

Most technical divers use a maximum of 1.4 ppO2 on the bottom and a maximum of 1.6 ppO2 during resting decompression. Many people (myself included) use gases with a maximum of more like 1.2 ppO2 on the bottom. The deco 'penalty' between 1.4 and 1.2 is very slight.

Working hard at a ppO2 of 1.6 is a really bad idea.

Its all covered (or should be) in proper classes.

 

[rickthompson]

Or is the 1.4 just a PADI limit?

I don't think it is the limit, but PADI stresses considering planning with 1.4 to add that extra margin of safety in. If you plan your dive and dive your plan then pushing it to 1.6 is acceptable if you are properly trained and know what you are doing...

 

[NAM001]

As you go through the nitrox course you will cover things like cns and otu. These items are on one side of the scale and the ppo2 and narcosis are on the other side of the scale. As you digest the info and manipulate the mixtures you start to see the full reelationship of all the factors involved. For example,,,, by your question one would think tht the higher o2 the longer dive you can do. true to a point when you look at only the ndl issue. There is a price to pay for this,,,, and that is in cns and otu's of which there are limits. The advanced course will cover aspects using higher mixtures and thier uses. Above 40% the primary use is for enhancement of off gassing AKA deco. I hesitate to say this but from my beginning in diving i was taught that the normal person is dead at 3.0 ppo2 , is experiencing the effects (serious stuff) at 2.0 ppo2 and that the effects off ppo2 is lenear to about 1.6 then they grow exponentially past the 1.6 point. to say 1 in 1 million has a problem at 1.6 was not good enough,,,, so to make a buffer zone to account for the many bod factors that the individual may have it is set to 1.4. If you dont watch your depth ,you dont stray beyond the no-no line of 1.6 when you limit yourself to a depth that provides a ppo2 of 1.4. 1.6 can be handled short term by the body with little if any physiological pay back. the rec world lives with the limit of 1.4 max ppo2 and that is the limit your rec nitrox traiing will use. Advanced nitrox covers the > 40% mixes and the > 1.4 ppo2 world. Learn your basix nitrox over time and ask a lot of what ifs. When you run out of questions and it makes sence then look into the advanced stuff. High O2 mixes are not somthing to play with and the 1.4 rec ppo2 is a well deserved and established limit that is not to be toyed with.

You will find later that gas mixtures are chosen to control the ppo2 to a low level say 1.1 or so. You can find charts Learn To Scuba Dive | Scuba Diving Magazine that show how much dive time is being limited because of higher ppo2's. look at the chart and see that going from .8-1.5 cuts your time to about 1/4th now look at going from 1.5-1.6 it is cut to1/3 again. if you went from 1.6 to 1.7 yopu would be cut to 1/10th of 1.6 time. I say this to impress to you that this limit is not a nanny over protection thing to sell more gear, it is very real. Please be carefull when you use hgher o2 mixes. I am sure you already know the mod for 40% as opposed to the mod for 21%.

 

[boulderjohn]

Is it in the advanced nitrox course as well that using 1.6 instead of 1.4 is taught? I understand that the 1.4 is merely a conservative safety factor on the 1.6. But does it become "common practice" to ignore the 1.4 after "Advanced Nitrox". Or is the 1.4 just a PADI limit?

He does not say it directly, but this suggests the reason:

Working hard at a ppO2 of 1.6 is a really bad idea.

It is believed that the amount of effort the diver expends while breathing has a lot to do with the safety of the O2 partial pressure. During decompression, most agencies accept the higher 1.6 limit because the diver is typical hanging around making next to no effort whatsoever.

During the working part of the dive, however, the limit is lower. During the dive, different agencies suggest limits from 1.2-1.4.

 

[Centrals]

It seems most if not all agencies would set 40% as max for recreational diving. But some of the recreational rebreathers(CCR or SCR) will allow much higher % use eg. Dragger Dolphin SCR(60%). Does it mean you have to be tec certified before you can venture into rebreather?

 

[windapp]

PPO2 of 1.4, when diving within the time limits has a virtually zero risk of CNS toxicity. If you dive to 1.6, you have small, non-zero risk. However, if you go into CNS toxicity while submerged, you die. Your chances of being rescued even by the most proficient rescue diver are extremely low. 1.5 and 1.6 in recreational diving should be reserved for situations where somebody's life is in danger. There are a lot of things you can reasonably push the limits of in diving. PPO2 is not one of them.

As for the 40% limit, once you do the math with a PPO2 of 1.4, it becomes obvious that there is little benefit to exceeding this limit. You will run out of air before you run out of bottom time. Furthermore your regulators must be oxygen serviced prior to using more than 40% oxygen.

 

[snowmaker]

Is it in the advanced nitrox course as well that using 1.6 instead of 1.4 is taught? I understand that the 1.4 is merely a conservative safety factor on the 1.6. But does it become "common practice" to ignore the 1.4 after "Advanced Nitrox". Or is the 1.4 just a PADI limit?
Thanks!

1.6 is more a number for deco then the actual dive. I will plan my dives to stay below 1.4 for the dive itself. Lots of things effect O2 tox. so it's always better to play it safe. For the most part even 1.6 would be pretty safe for the dive but why take the chance.

 

[SailNaked]

Not certain however I know the Navy did a lot of tests on peoples functionality with higher O2, I would bet that almost no one had a problem with 1.4 and some has issues begining at 1.6. my memory sux but I think they got up to 3.x and some people were fine. the navy also uses 100% o2 at 30ft deco stop, Of course the Navy can do whatever they want, they have a chamber near by and guns.

 

[divengolf]

Not certain however I know the Navy did a lot of tests on peoples functionality with higher O2, I would bet that almost no one had a problem with 1.4 and some has issues begining at 1.6. my memory sux but I think they got up to 3.x and some people were fine. the navy also uses 100% o2 at 30ft deco stop, Of course the Navy can do whatever they want, they have a chamber near by and guns.

Many of the USN O2 tests at high PO2 were done in a chamber, not UW. So if you went into oxtox, you were not going to drown. They'd just reduce the O2 content of the breathing gas until you recovered.

 

[boulderjohn]

Many of the USN O2 tests at high PO2 were done in a chamber, not UW. So if you went into oxtox, you were not going to drown. They'd just reduce the O2 content of the breathing gas until you recovered.

Not only that, but they discovered that for reasons that are unknown, the PPO2 at which you tox in the chamber is much higher than when under water.