A question about O2 cleaning.

[jon]

Hey all,

I recently got my tanks O2 cleaned so partial pressure blending would not be a problem, as that is how almost everyone around here produces their Nitrox. This weekend I'm heading to Dutch Springs, and their Nitrox is pre-mix. Someone once told me that if I get a pre-mix fill I will void the O2 cleaning. Is this true? I certainly don't want to have to get them cleaned again so soon.

Thanks.
-Jon

 

[Rick Murchison]

It depends on the grade of gas in the premix. Because premixes below 40% are generally considered as "no special handling required," many premix stations use "regular Scuba air" standards, which do not meet "oxygen compatible" standards and will indeed void your oxygen service rating on your tanks.
Call the place you're going and ask 'em if their premix is tested and certified "oxygen compatible." If the answer is "no" then getting a fill there will void your oxygen service rating.
Rick

 

[greg somers]

Who does it and to what mixes? Everything here is mixed in the shops themselves.
Cheers,
Gasman

 

[turnerjd]

I haven't come across pre-mixed mitrox either!
Neither in france nor the UK

Jon T

PS - is pre-mix bought in as large industrial cylinders of pre-mix, or is it blended into a 'bank' and then dispensed?

PPS - If it is blended into a bank on site, then surely there isn't a problem, as blending it into your tank, and blending it into their bank are roughly equivalent.

 

[Warhammer]

Originally posted by turnerjd
I haven't come across pre-mixed mitrox either!
Neither in france nor the UK

Jon T

PS - is pre-mix bought in as large industrial cylinders of pre-mix, or is it blended into a 'bank' and then dispensed?

PPS - If it is blended into a bank on site, then surely there isn't a problem, as blending it into your tank, and blending it into their bank are roughly equivalent.

Here in the US, the answer is both ways. You can buy it from an industrial supplier and some shops mix their own. My LDS use to pre-mix their nitrox. They mixed 40% in a bank and then dialed it down as needed. They have since stopped doing this and now blend it all partial pressure method. They said it was more trouble than it was worth and just too costly.

 

[Thomas Ivey]

Originally posted by Rick Murchison
It depends on the grade of gas in the premix. Because premixes below 40% are generally considered as "no special handling required," many premix stations use "regular Scuba air" standards, which do not meet "oxygen compatible" standards and will indeed void your oxygen service rating on your tanks.
Call the place you're going and ask 'em if their premix is tested and certified "oxygen compatible." If the answer is "no" then getting a fill there will void your oxygen service rating.
Rick

Who would do the testing and certyifing?

PST makes medical and avaition oxygen cylinders. In the regulations that I have read, I cannot find anywhere where it states a criteria for a level of use at 40%. Who has stated that at 40% is where 02 cleaning starts. Any infomation regarding this would be very helpful.

ti



ti

 

[Rick Murchison]

The 40% rule is a "generally accepted standard" based on experience. Not everyone accepts it - ANDI (the last time I checked) demanded O2 clean/service rated equipment for any and all mixes above 21% O2. As far as I know all the other Nitrox certifying agencies (in the States) go by the 40% rule.
Pre-mix refers to any mix that is mixed somewhere outside your Scuba cylinder - there are several methods common in the States, and pre-mix is the norm. Pre-mix, by the way, is far safer than in-tank mixing, as far fewer parts come in contact with pure O2 - and in some systems - DNAx, for example - nothing ever has to contact pure O2. Again, back to your original question, the answer depends on whether the premix is made with oxygen compatible air, and the system keeps the gas "oxygen compatible" - and you won't know if you don't ask. As for "who certifies" there are labs all over the country able to do so, and Scuba shops know who the local folks are. Ask.
Rick

 

[Thomas Ivey]

Originally posted by Rick Murchison
The 40% rule is a "generally accepted standard" based on experience. Not everyone accepts it - ANDI (the last time I checked) demanded O2 clean/service rated equipment for any and all mixes above 21% O2. As far as I know all the other Nitrox certifying agencies (in the States) go by the 40% rule.
Pre-mix refers to any mix that is mixed somewhere outside your Scuba cylinder - there are several methods common in the States, and pre-mix is the norm. Pre-mix, by the way, is far safer than in-tank mixing, as far fewer parts come in contact with pure O2 - and in some systems - DNAx, for example - nothing ever has to contact pure O2. Again, back to your original question, the answer depends on whether the premix is made with oxygen compatible air, and the system keeps the gas "oxygen compatible" - and you won't know if you don't ask. As for "who certifies" there are labs all over the country able to do so, and Scuba shops know who the local folks are. Ask.
Rick

Thanks-

You used the word "standard" this means there is supporting documentation. I think you mean, "practice. I have been looking for the underlying reason why 40% was picked, there must have been some reason.

ti

 

[100days-a-year]

According to the info I've accumulated 40% is arbitrary but it is where o2 starts to pose an oxidation danger.

 

[turnerjd]

AFAIK the 40% is exactly the same as the ppO2 = 1.6

Both are best guesses. The ppO2 used by the USN for 100% O2 decompression for a long time was 2.0 and then gradually has been lowered to 1.6 as the 'recreational' market has taken to nitrox (rec vs professional) as there is some margin of safety in this number.

From anecdotal evidence, 100% O2 is definately dangerous if nothing is cleaned, and as safety concerns have come in, manufactures (eg Apeks) have decided that 40% is a reasonably safe figure for kit like regs. It wouldn't be surprised if people like Apeks have some evidence to base this figure on.

Jon T