Diving on nitrox and air with the same regs?

[Charlie99]

Is the transfer of gas to pressurize the hoses enough to raise the temperature of any part of the first stage high enough to cause a flash fire

I don't think it is related to how much gas transfer takes place to pressurizing the hoses. It's more a case of the ambient pressure gas already in the reg being rapidly compressed to 3000psi/200bar. "Rapid" in this context means rapid as compared to the dissipation of heat from the hot gas into the metal of the reg -- adiabatic is the fancy word. If the pressurization is fast enough that not much heat is transferred from the gas over into the metal, then the gas can get hot enough to ignite hydrocarbon contaminants left in the reg from previous fills.

Various reg techs have posted that they have noticed little burn spots inside regs. A little tiny flash fire of a small amount of hydrocarbon isn't a big deal unless it manages to ignite something else, such as o-rings or as in the link below, the titanium metal of the reg itself. (Contrary to the thread title, it was the reg that burned up, not the tank)

O2 tank explodes - San Diego 6-3-00 - rec.scuba | Google Groups

 

[charlesml3]

Right, but he was on something close to 80% O2. I'm not yet convinced that this translates over to the recreational 36% or less crowd.

It just seems to me that there's a lot of fear over Nitrox and these mythical "explosions" and no real incidents of it happening.

-Charles

 

[Charlie99]

Right, but he was on something close to 80% O2. I'm not yet convinced that this translates over to the recreational 36% or less crowd.

I'm not trying to "convince" anyone, including you. Indeed as I have posted above, I routinely swap back and forth between EAN32/36/40 and regular air in my brass Atomics B1 first stage with a titanium 2nd stage.
My post is in response to your question

Is the transfer of gas to pressurize the hoses enough to raise the temperature of any part of the first stage high enough to cause a flash fire?

The answer is YES, there can easily be a flash fire of the hydrocarbon contaminants. Indeed, they are very common. What happens, if anything, after that initial small and relatively harmless small flash is dependent upon the flammability of the material in the reg.

The flammability goes up with increasing FO2 and ppO2 (or to put it a different way -- the energy needed for ignition goes down with increasing FO2 and/or ppO2.) A titanium reg in EAN80 or pure O2 is an extreme case.

Charlie Allen