SSI Nitrox Course

[Genesis]

is actually the BEST material to make high-FO2 gas-handling materials out of.

It will not burn in 100% O2 up to 10,000 psi (yes, you read that right.)

Monel is good, but not that good. The other alloy that's a good choice is Inconel, but you don't want to know how expensive a reg made out of that would be.

There are two issues - the first is the combustability of the material itself, and the second is its tendency (or lack thereof) to spark if small particles of it impinge on other things at high velocities (which has a funny habit of happening with high pressure gas systems.)

Both of those events will ruin your day right quick.

With that said, what ALMOST ALWAYS combusts is the seat material. Rarely, an O-ring ignites. The reason the seat material combusts is not due to contamination but because compression heating just happens to take place where there are sharp bends and constrictions in gas flow are, and that just happens to be right where the seats are.

On the LP side (second stages) its pretty much a non-issue, but it is a very real issue on the first stages and in valves.

You can cut the risk significantly by always SLOWLY opening valves. O2 cleaning is a darn good idea, but you can PP fill on top of a pool of oil and not get an ignition event if you do it slowly enough!

You need heat, fuel and oxygen to get a fire. Take any one of the three away and there is no fire (or explosion.)

BTW for anyone who thinks that Viton or other "oxygen compatable materials" won't burn (with the exception of brass), you're wrong. They will in fact burn like hell in a pure oxygen atmosphere, even at one atmosphere. At 200 bar they'll burn explosively. The whole trick is not to let them get lit. Even STEEL will burn in the presence of high FO2s (how do you think it is cut with a torch?)

Aluminum-body regs should not be used with high FO2 gas at all, because aluminum is VERY easy to get lit in a high-FO2 environment. Monel is kinda ok, but brass is the gold standard and should be preferred for any mix over 40%.

BTW, any mix 50% and over may as well be pure O2 from a standpoint of fire risk. Between 40-50% is kind of a gray area.

NASA did a number of studies on this (they play with high pressure O2 all the time) and INTENTIONALLY tried to get regulators to ignite in various FO2 atmospheres and pressures. They were unable to get them to do so at 40%, which is why most people consider up to 40% safe without special cleaning. They also found that at FO2s above 50%, there was no statistically-significant difference in ignition risk irrespective of the FO2 of the mix. Their methods including using very-high-speed solenoid valves to introduce a "pulse" of 3,000 psi gas into the system (!), which is an absolute worst case, of course, and impossible to replicate with a hand on a valve (you can't open it that fast.)

I'm perfectly comfortable using up to 40% mixes with no special handling. Above that, I clean them, but I'm not anal about the Buna/Viton thing. I will note that aircraft high pressure O2 systems typically use Buna O-rings, and that since Viton, EPR, etc WILL burn in pure O2 at even one atmosphere, any claimed "immunity" is bunk - the issue is wear resistance, and since you tear a reg down annually anyway and replace the O-rings, it simply should not be an issue.

 

[NJDiveGirl]

Epinephelus once bubbled...
SSI doesn't require dives - but plenty of SSI instructors do. SSI leaves it up to the individual instructor - 'round here most of them require the dives.
E.

I was certified SSI Nitrox last year - I had recently moved and the LDS happened to be SSI. I think its more important to understand the theory behind nitrox and how it works, all about ox tox, MOD and no deco limits. Too me, the dives are useless - its not like you are breathing the air any different.

 

[Warren_L]

Agreed, it is not the dive itself that is different, but how you plan the dive.

 

[Rick Murchison]

I require the dives be planned.
I require the bottles be analysed.
I require custody be maintained.
I require the dives be done.
I require the plan be followed.
I require good recordkeeping.
When my Nitrox students get their nitrox cards, they know what they're doing. I haven't ever had one tell me the dives were not worth the time and effort, nor have I ever had one say they failed to learn something useful from the experience.
A Nitrox diver isn't a nitrox diver until he/she has made nitrox dives.
Rick (SSI)

 

[ScubaRon]

Genesis once bubbled...
It will not burn in 100% O2 up to 10,000 psi (yes, you read that right.)

True, but corrosion increases and structural integrity weakens, both of which could lead to combustion events, which is why many manufacturers switch to stainless steel at higher pressures.
It's not too relevant for scuba though.

Thanks for pointing out the NASA study. It is the first facts-based argument I have seen that supports omitting oxygen cleaning when switching from standard air to EAN40.

I'm not convinced yet and even NASA continues to propagate oxygen cleaning, but these are the kind of information we need.
ScubaRon

 

[oversea]

Here is a good one, I am ssi ow and did padi nitrox, did one dive and got a naui card!!!!!!!!!!!!!! go figure.

 

[100days-a-year]

I'm still waiting for some documentation of some these now "abundant" incidents in the early days of nitrox.I was diving it before there was even a cert for it and I've never heard of one nor have any of my similarly experienced buddies,shopowners or instructors.Gen,it's called abdiatic .That is the heat that isgenerated when hi pressure gas it constricted or it's flow is otherwise interrupted.When using a blending station mixing gases you can feel more heat in bends,fittings and constrictions.

 

[Genesis]

I know what its called, but this guy seems to be a rather, well, newbie Don't wanna confuse...

BTW Ron, NASA of course will promote cleaning. Most of THEIR use for O2 is in a 100% environment.

Finally, dont' be so sure about SS. There are problems with SS, the most important of which is that as a ferrous metal it can create sparks due to frangible pieces coming "loose" and impacting upon it. This is one of the major ignition risks in a high FO2 atmosphere, perhaps even more important than adiabatic heating in most environments.

SS is used at pressures beyond 3000 psi or so mostly out of a "throw up hands as there is no better alternative" problem, rather than an actual preference. At those pressures things get veerrrrrrryyyy dicey with O2. One of the major issues with brass is that because brass is a relatively soft metal you can get some interesting failures at high enough pressures irrespective of the gas carried. SS avoids this but comes with its own risks.