O2 Question: Why is 40% O2 the threshold for the industry?

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Practicality:
1) 40% is a practical limit for recreational sport diving. Most regulators and cylinders can be used without having them oxygen cleaned. There is a consensus amongst the various agencies that gasses with more than 40% O2 should be treated and respected as if it was pure O2.
2) If you are a normal recreational sport diver diving within no-deco limits increasing O2 will merely limit your depth even more while the gas volume in your tank remains the same.
3) Diving with higher O2 percentages not only requires O2 clean equipment but also a much deeper understanding of the dangers and implications of high O2 mixes. Things like CNS and OTUs needs to be well understood.
4) There is also the question of diving ability and skill. A lot of recreational divers with a handful of logged dives and simple Nitrox courses has not necessarily mastered buoyancy and other essential skills. Nitrox 40 or less gives these divers a margin of safety. Unless they dive too deep. That is also one of the main reasons why you also have a max limit of 1.4ppO2. Technical divers will operate between 1.4 and 1.6 depending on the phase of the dive.
5) Nitrox 40 or less also gives you relatively low CNS percentages at the end of dives. That makes repetitive diving very simple from an O2 standpoint. When you increase the O2, planning becomes much more complicated.

Physiology:
1) O2 is a more limiting factor to dive duration and depth at high partial pressures than Nitrogen. Oxygen is also a lot more dangerous under water. When under narcosis you might feel euphoric and do stupid things but an Oxygen Toxicity CNS hit will drown you without much warning.

When you progress to courses like Adv Nitrox with deco you will start using richer mixes than 40%.
At that level of training you will be much more experienced as well as empowered with better equipment and more knowledge.
I highly recommend the Adv Nitrox course or equivalent Tec Rec certification if you are interested in taking your adventure to the next level.
 
The NASA event was thirty years before nitrox made it's way into recreational diving.

I'm not sure of this point. Are you suggesting that the results of NASA's research following this event were lost during those 30 years and were thus unavailable when decisions about the use of nitrox were being made?
 
Two people have now mentioned that when using cylinders with 40% or less they don't need to be O2 cleaned.. While I don't dispute the physics, this is still bad practice (IMHO). Partial pressure blending is still used where 100% O2 is added before the tank is topped off with air. Not having a O2 cleaned tank - well you all know the consequences.

i appreciate a lot of filling stations are via a bank or membrane compressor but still. As someone qualified to partial pressure blend I felt it was an point important to make.
 
Two people have now mentioned that when using cylinders with 40% or less they don't need to be O2 cleaned.. While I don't dispute the physics, this is still bad practice (IMHO). Partial pressure blending is still used where 100% O2 is added before the tank is topped off with air. Not having a O2 cleaned tank - well you all know the consequences.

i appreciate a lot of filling stations are via a bank or membrane compressor but still. As someone qualified to partial pressure blend I felt it was an point important to make.

If your tank will only see nitrox from a bank or membrane system and will never be exposed to O2 at greater than 40%, why go through the expense of O2 cleaning? But still....
 
I'm not sure of this point. Are you suggesting that the results of NASA's research following this event were lost during those 30 years and were thus unavailable when decisions about the use of nitrox were being made?

Not lost, just not as much on everyone's mind as it might appear in hindsight where decades of experience with nitrox I and II (32 and 36%) by NOAA and the desire to promote nitrox use recreationally by ANDI and IAND were colliding with other agencies who were claiming recreational divers were going to die from toxicity and CGA which (if you were to believe them) though tanks would explode if 24% were put into them. If <40% can go through an oil lubricated compressor it can sure go into a non-O2 cleaned tank.
 
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If your tank will only see nitrox from a bank or membrane system and will never be exposed to O2 at greater than 40%, why go through the expense of O2 cleaning? But still....

Plus it's only O2 clean until it gets filled with nitrox from a bank, stick or membrane system that's not O2 clean
 
If your tank will only see nitrox from a bank or membrane system and will never be exposed to O2 at greater than 40%, why go through the expense of O2 cleaning? But still....


I dont disagree with you one bit. We dive in different areas so have different needs. My point was that someone reading this could have presumed that their tanks was okay to fill having regularly got a EAN fill, then go somewhere and get rejected because that place does PP blending.

have no idea how much you pay for O2 clean. - my LDS includes it with yearly inspection. Different country different rules and culture.
 
i AGREE. tHE NANNY Justification IS THAT SOMEONE COULD MAKE 23.5% AT GREATER THAN 50 PSI USING PARTIAL BLENDING AND (oops) could have greater than 40% in it,,, in the process. Its the one rule fits all syndrome. If your tank is PP blended then you have in the process greater than 40%. The standard is O2 clean. Every one accepts that. This has been taken to the exstreem of 23.5% and >50psi requires O2 cleaning. BTW it started with all sytems no matter what % requires O2 cleaning. That included the fill station at the gas station that filled your tires. It was raised to exclude those systems to 23.5 and 50 psi. This ludicracy is a recommendation that was placed in the national regestry. A federal law exists that says things in this registry is to be considered law. Major buck passing scheme.


If your tank will only see nitrox from a bank or membrane system and will never be exposed to O2 at greater than 40%, why go through the expense of O2 cleaning? But still....


---------- Post added June 26th, 2015 at 05:06 PM ----------

The situation you cite could happen However the vis sticker says O2 clean or serviced to 40%. if 40% then no PP blending allowed. O2 cleaning was done to allow PP blending. Even if my LDS does bank fills i have the tanks O2 cleaned for the instance of getting a fill from a PP shop. It is very similar to the hyperfilter air requirements. Agai what the shops call for and the regs require if often diffferent.

I dont disagree with you one bit. We dive in different areas so have different needs. My point was that someone reading this could have presumed that their tanks was okay to fill having regularly got a EAN fill, then go somewhere and get rejected because that place does PP blending.

have no idea how much you pay for O2 clean. - my LDS includes it with yearly inspection. Different country different rules and culture.
 
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Plus it's only O2 clean until it gets filled with nitrox from a bank, stick or membrane system that's not O2 clean

Not necessarily. Some of us partial pressure blend and all gas is grade e modified clean too.


The situation you cite could happen However the vis sticker says O2 clean or serviced to 40%. if 40% then no PP blending allowed. O2 cleaning was done to allow PP blending. Even if my LDS does bank fills i have the tanks O2 cleaned for the instance of getting a fill from a PP shop. It is very similar to the hyperfilter air requirements. Agai what the shops call for and the regs require if often diffferent.

Most stickers around here say,
cleaned for 40%
OR
O2 clean

punch one or the other but not both. Its not all that complicated unless you start shop swapping like mad. Most people don't have 5 different shops with multiple fill options (we can only dream of such competition driving down prices and raising the service bar). They have one convenient shop.
 
It ois done because of a one rule fits all. There is a national registry that has a recommendation that says now something like 23.5 and greater than 50psi requires O2 cleaning. That is the legal reason why. There is no other reasonable reason why. PSI will say that if you did do a PP blend with an empty tank, put O2 in it and topped off with air to make 23.5 nitrox, you will have needed a sufficient amount of O2 at greater than 50psi. I think of it as a buck passing event. When the feds gave up legislating policy regarding gasses and made the blanket law that said all recommendations (of cga dot and other agencies) in the registry is to be treated as law. cga and such had to do something because if there were an incident they did not want to be the blame for it, so they lowered the limits below the threshold of risk. If speed limits were treated the same way interstates would be 20 mph.
If your tank will only see nitrox from a bank or membrane system and will never be exposed to O2 at greater than 40%, why go through the expense of O2 cleaning? But still....


---------- Post added June 30th, 2015 at 03:52 AM ----------

yEP THATS WHAT THE STICKERS SAY SO IF YOU PUNCH o2 SERVICE YOU ARE COVEREDC FOR EVERYTHING. oops


Not necessarily. Some of us partial pressure blend and all gas is grade e modified clean too.



Most stickers around here say,
cleaned for 40%
OR
O2 clean

punch one or the other but not both. Its not all that complicated unless you start shop swapping like mad. Most people don't have 5 different shops with multiple fill options (we can only dream of such competition driving down prices and raising the service bar). They have one convenient shop.
 
https://www.shearwater.com/products/teric/
http://cavediveflorida.com/Rum_House.htm

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