At what percentage of nitrox does combustion really become a concern?

[thedragon]

Of course any percentage of enriched air will increase the burn rate of materials, but at what percentage should I be guarding that cylinder with my life, so to speak?

 

[kelemvor]

I believe the "cutoff" is supposed to be 40% O2. That said I treat all my tanks roughly the same. Deco bottles get stored with my nitrox bottles. I do have a decent storage area where the bottles are secure and out of the way. Even service is the same since my LDS blends and all my bottles get O2 cleaned.

 

[knotical]

Pulled out an old gas blender manual and it says:

Historically, the dive industry has treated up to 40 percent oxygen the same as air, and any concentration of oxygen higher than 40 percent as if it were pure oxygen.

and a bit later:

Even pure oxygen poses little risk when handled at a low pressure and temperature. Conversely, extreme damage has resulted at high pressure and temperature with just the oxygen content of air.

In other words, as with so many things, “it depends”.

 

[rongoodman]

There is some disagreement among agencies, but the cutoff for mixtures requiring O2 cleaning of tanks and regulators is usually 40%.

 

[Dr. Lecter]

40% is the general cutoff, but I've yet to hear of anyone having a fire or explosion from slamming a valve open or dropping a tank of anything less than 100%. Which is not to say it can't happen, or that O2 cleaning isn't important for anything into which you're decanting (much less boosting) pure O2...just that when it comes down to what mixes might kill you once they're filled, it's probably not your tank of 50%.

 

[Diver0001]

Of course any percentage of enriched air will increase the burn rate of materials, but at what percentage should I be guarding that cylinder with my life, so to speak?

It depends on the material and any catalysts present.

The only material I know of that has caused real problems in diving is Titanium. Some manufacturers make regulators from titanium and on the inside of the 1st stage it is possible to create conditions for spontaneous combustion. There have been a few cases of such fires over the years.

Aside from titanium regulators I've never heard of risks related to scuba diving that would make me raise my eyebrows. Perhaps people who blend fills have some horror stories but if they do I've never heard them.

In short. Don't buy a titanium regulator and you'll never need to think about this again.

R..

 

[antattack]

Note that unless your local dive shop has banked Nitrox, they will do partial pressure blending. This means that your tank and it's valve will be exposed to 100% oxygen during the fill.

100% O2 is always worth thinking about as a combustion hazard

 

[Diver0001]

Membrane systems fill at target O2 levels even without a buffer.

R..

 

[NorCalDM]

Oxygen itself will not burn it is an oxidizer and fuel needs an oxidizer to burn so the higher the oxygen percentage and pressure the more intense the burn. This is why in a weld shop or industrial facility fuel gases and oxidizers are stored a specific distance apart so unless you have a fuel source stored with your dive cylinders there should be no issue.

 

[Dr. Lecter]

Oxygen itself will not burn it is an oxidizer and fuel needs an oxidizer to burn so the higher the oxygen percentage and pressure the more intense the burn. This is why in a weld shop or industrial facility fuel gases and oxidizers are stored a specific distance apart so unless you have a fuel source stored with your dive cylinders there should be no issue.

Theoretically true, but practically irrelevant. The average non-O2 cleaned tank and valve have more than enough fuel sources in them to cause problems, in the form of oils deposited from less-than-clean compressed air and/or silicone grease being used in/on the valve. Often not nearly enough to cause your tank to blow up, of course, might just be enough to have a quick flash burn of the contaminants and leave your tank with a bit of CO byproduct.