First thing to note is that Viton is NOT O2 "compatible" and nitrile is NOT unsafe for O2 applications. Viton is more durable but is very toxic if it does combust. Your concerns are misplaced in terms of the material of the O rings and other components. The main issue with O2 is hydrocarbon contamination and good cleaning should prevent this issue if coupled with clean air for filling. Heat builds up where gasses are forced through steep changes in direction and in that matter neither the piston nor the diaphragm type reg is ideal.
I am interested in the comment you made that Viton is not O2 compatible and that nitrile is not unsafe?
In everything I have read to date, including course notes, suggest different. "Viton is used in scuba diving because it has a lower probability of catching fire, even with the increased percentages of oxygen found in Nitrox. It is also less susceptible to decay under increased oxygen conditions" (I would assume compared to Nitrile O rings). Both will burn, however Nitrile has a lower burn point than Viton, thus Nitrile being less suitable.
From everything I have read to date it would suggest the following; the biggest risk is hydrocarbon contamination rather than components, or particle contamination followed by sudden high flows of oxygen. Components being less of a risk "unless" they are at the point where there is a significant high flow point and pressure difference (lets say a blown O ring), where high temperatures can be created thus having all the components for a fire "heat, fuel and oxygen".
As I said previously there is the potential for a fire when a seal blows out, as with the medical cylinder incident, being fitted with 2 face seals at the yoke. Opening the cylinder rapidly caused an Oxygen leak at the seal face with high flow due to significant pressure differences, a fire that under normal circumstances would never be an issue with sealing the yoke to cylinder. As Dog Diver said, when all the holes line up, a disaster in the making. I agree, O ring or component material is probably never an issue "unless" that component happens to be the failure point.
I figure if a manufacturer says their regulator set can be used with oxygen, they have built it to a spec where they are very comfortable with its design. What, however, is put into the set when serviced might be an issue. Hence my concern with the reg set I had cleaned and built. I have since found out that backup O rings are not made of Viton in my part of the world (or perhaps not stocked by anyone), thus the LDS appears to have used Nitrile. Perhaps the correct thing for them to do would have been to say "This reg is not suitable for oxygen use", and state the reasons. They were however apparently comfortable to build it the way they did, so I can only assume they were comfortable with the work they did (or perhaps were just plain lazy and incompetent).
So in essence, either the LDS are stupid and not acting professionally, or the risk of fire due to incorrect O ring/diaphragm components is not as high as alleged by some (who also use it as scare tactics to generate revenue from over servicing) and accepted by the majority in the industry as not an issue.
As Dog Diver stated, he has seen 2 incidents, and I have had related to me by my dive instructor 2 incidents from rebreathers, and I had read of the medical cylinder incident in a report. Clearly oxygen fires do happen, but at what frequency? As others have stated and I have too, there are too many not cared for welding kits about that have not burnt even though they are not well looked after, to suggest that unless you take the utmost care with oxygen gear, you will most certainly explode in flames would appear not as likely as some would suggest.
My personal outlook now after much reading, instruction and comments here as well; Yes make the effort to have gear prepared for Oxygen use by proper cleaning and the use of proper components and this reduces the chance of a fire, however if at a pinch one has to use a backup O ring or diaphragm not perhaps of the ideal material, as long as its cleaned as required, and the appropriate grease used and all loose material cleaned from the system, the chance of fire from other sources is minimised. Ensure in any regard that all replaceable components are in good shape or replace them thus reducing the risk of high flows at pinch points where O rings reside. Perhaps not technically correct but probably OK.
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