My experience in reading posts on Scubaboard (and other similar sites) is that the advice given by senior and more experienced divers usually tends to be cautionary and conservative, considering the experience range of the readers.
If a diver is considering tech, they are already in a fairly limited experience range, if they are not considering tech, it is a moot issue. Besides, if everyone keeps moving toward and erring on the side of excessive caution eventually no one will ever get in the water. And if we had always erred on the side of caution, we would never have started diving nitrox in the first place, as anyone diving in the 80s can tell you just exactly how evil nitrox was considered to be then.
It's also clear that my opinions are just that - opinions- and they are my own and do not reflect those of the owners of Scubaboard. Readers can apply whatever weight and credibility to them that they choose - it won't make them any more "right" or "wrong".
You are saying that you can use any reg (that has been originally O2 cleaned) on any cylinder (non-O2 cleaned) any number of hours, and then back on a 100% deco bottle.
No. You can take any reasonable example to an unreasonable extreme, as you just did. What I said was that using an O2 clean reg on a tank filled with non-O2 compatible air will not neccesarily make it any less clean than a reg used for several dives with only O2 compatible gasses. O2 clean is a matter degree and it stops being "o2 clean" once it leaves the clean room - if it was ever in a clean room to start with.
Obviously if the air source came from a compressor puking oil to the point you have visible mist, you have a serious issue and one tank would be more than enough to prompt a cleaning. On the other hand an awful lot of grade E air also has hydrocarbon content way below the limit allowed for grade E air and in fact much of it may even meet O2 compatible standards but not be labled that way. Check the most recent test report and make your own decisions.
In the same vein, if a shop's "O2 compatible" air is tested with new filters, it may not be "O2 compatible when you fill 3-6 months later. A test with new filters means absolutely nothing. So take it all with a grain of salt and put the effort in preventing O2 fires into smart operational practices designed to reduce the potential for creating an ignition source as in the end that is the only thing you really have control over.
I know the standard for Grade E air Hydrocarbon level is 5 mg/m3 and for Oxygen compatible air the standard is 0.1 mg/m3. That sounds like a big difference to me. If I use my deco reg say on a 50% mix with OCA for the number of hours I do deco (i.e. not so many), I realize it's not as clean as it started. However I can't see how it could be comparable to using it routinely with Grade E air and then switching it over. How much muck in the regulator is too much?
We are not talking about "using it routinely", we are talking about not getting hysterical about a reg being used on rare occasions with air and then "requiring" a new O2 cleaning.
How much muck is too much is exactly the point. If you don't provide an ignition source through thoughtful operation, you could coat the inside with 40 weight motor oil and still not have too much muck. On the other hand, if you slam the valve open on your O2 reg with 30 or so dives with only "o2 compatible" gasses, don't be surprised when it starts leaking due to a flash fire and charred o-rings.
That is the other point that gets missed. The vast majority of O2 fires inside a reg do not result in a detonation with reg parts, diver gore and goo flying everywhere - there is just not that much fuel and energy available. I have only heard of one reg that failed catastrophically due to an O2 fire and that resulted in a minor injury to a hand. If there are more they have not been well publicized.
What happens 99.9% of the time is a brief flash fire that chars the o-rings and causes some interesting leaks. A fire is usually not apparent until the reg is taken apart.
Excess hydrocarbon contaminants potentially cause two problems - they reduce the ignition temperature as the that temp is lower than the ignition temp for the o-rings (but again this process begins happening as soon as the reg leaves the clean room as 0.1 mg/m3e does not equal zero and is enough to introduce something that will ignite before the o-rings) and it provides slightly more fuel to feed the O2 fire for perhaps a small fraction of a second longer.
Again, the smart money on O2 fire prevention is on denying the reg, even an O2 clean one, an ignition source by pressurizing it as slowly as possible (turn the valve on very slowly and keep the purge depressed). That is basically the approach used with welding regs that by design pressurize slowly and do not result in compression heating, even though most seem to spend their lives in very greasy environments.
You'll note I do hold the line on O2 cleaning of tanks and that is an appropriately conservative view. Countless medical O2, aviator O2, and welding O2 tanks are in service that have never been O2 cleaned. Over time with various hydro tests and countless fills, hydrocarbon buildup (and rust in steel tanks) occurs even when filled only with O2 but they are never O2 cleaned and accident rates are very low. The critical difference is that the valves are designed to pressurize slowly with low flow rates and no hot spots. Scuba valves on the other hand are not O2 freindly and you need to keep the O2 clean tanks O2 clean to prevent contamination of the valves to reduce available fuel since it is more difficult to totally prevent an ignition source inside a scuba valve and manifold.
