Understanding proper gas usage in regulator

[epolice]

I recently bought a full regulator setup for the first time. Looking through all of the manuals/documentation, something stuck out to me that I need clarification on:

"Air used in SCUBA diving may contain minute amounts of flammable hydrocarbons that are considered safe to breathe. Over time these hydrocarbons may accumulate and represent a fire risk if used with Nitrox or oxygen. Specifically filtered air is available, called hyper-filtered air. Regulators to be used with Nitrox and oxygen must not be used with normal compressed air."

Does this mean I cannot switch between regular air and nitrox whenever I want? Does my regulator have to be one or the other forever? Please advise. I currently am not Nitrox certified but I imagine I will be in the near-future. Thanks for any help!

 

[Angelo Farina]

I do not know about Nitrox, but I can testify that when using pure Oxygen the risk of fire or explosion is very high.
A small amount of grease on your O-ring, or some dirt inside the metal filter, can ignite fire easily as you give 200 bar of pressure opening the valve of the Oxygen bottle.
It happened to me and the flame cut out through the metal of first stage. I was quick closing the valve, so apart destroying the first stage, nothing bad happened, apart my front hair being burned (not my face, luckily).
So you are advised: before being used with pure oxygen, a regulator must be serviced, cleaning it from any internal residual, and applying a new metal filter. No grease on O-rings, except for special grease which is certified for pure oxygen...

 

[Bob DBF]

That is correct, however with lower concentration Nitrox (32, 36, and basicly under 40%) this is not an issue. At higher concentrations it can get dangerous, as @Angelo Farina discussed above. As long as you use as clean air, not as big an issue as years ago, as you can and use low concentrations of Nitrox, there should not be a problem.

Although there is a reason to be careful, disclaimers usually take the Chicken Little approach.



Bob

 

[epolice]

to summarize, I can switch back and forth between normal air and Nitrox (up to 40%) without any problem?

 

[rsingler]

to summarize, I can switch back and forth between normal air and Nitrox (up to 40%) without any problem?

Yes and no. It depends upon how your Nitrox is mixed. If your Nitrox is banked, or is mixed with a mixing stick and compressed at the desired concentration, and if you trust the filling station (ask to see air analysis that should be done periodically to check compressor function for hydrocarbons), then you should be fine switching back and forth.
However, if you get "regular" compressed air, and then ask for a Nitrox fill, and if that Nitrox fill is done by partial pressure blending, then the fill station (not you) is at risk of accident from retained hydrocarbons. Partial pressure fill stations should be requesting oxygen clean cylinders from you.

At the user end, once the tank is filled, and at concentrations up to 40% and 3000psi, you should be fine. However, even this is argued from time to time, though I have not seen a Nitrox 40 regulator burn from impact ignition into a "dirty first stage", though some say they have...

The tank itself should be fine up to EAN40, once filled. But this too is argued, as scuba valves not designed for oxygen have sharp internal bends that are impact ignition points at high oxygen concentration.

Bottom line: everybody "says" up to EAN40 is okay going back and forth. The Compressed Gas Association might disagree depending upon factors like filling method and air quality during the "air" fills. Since my dive shop mixes with a mixing stick, and has "modified Grade E" air on the Air side, I go back and forth without worrying.

 

[scrane]

to summarize, I can switch back and forth between normal air and Nitrox (up to 40%) without any problem?

Yes. It is done ALL the time.

 

[Lorenzoid]

I have always assumed that when you ask for "air" at the fill station they fill your tank with the same filtered air that they use to make Nitrox. Is this not always so?

 

[KenGordon]

I have always assumed that when you ask for "air" at the fill station they fill your tank with the same filtered air that they use to make Nitrox. Is this not always so?

Not everywhere, some places are rubbish. Mostly I find it is as you expect but...

 

[rsingler]

I have always assumed that when you ask for "air" at the fill station they fill your tank with the same filtered air that they use to make Nitrox. Is this not always so?

Sad discovery for me at a previous dive shop: two completely different compressors and filter stacks for air and EAN. Air wasn't even close to OCA. They didn't want to over-use their precious new EAN compressor. Air compressor was an old POS. Or more important, since they weren't compressing oxygen, their filter stack was minimal.

 

[broncobowsher]

The real simple answer...
If the air fills come from the same compressor/filter/hoses the shop also uses for filling Nitrox (partial pressure filling, put pure O2 in first and top off with air), that air can be swapped for Nitrox and back without issue.

Regular air can be clean enough to not contaminate the O2 cleanleness of the regulator set. But if dirty air is used, it can contaminate the regulator set so it should be recleaned before using Nitrox. That is where going back to Nitrox after using air is to be avoided.

I know my local shops and the air I get is the same as used when they blend Nitrox. Since this is air being put into a pure oxygen environment to blend Nitrox, the biggest danger is the shop blending there own Nitrox, Using normal Nitrox (under 40%) there is less danger than the pure O2 the shop is playing with.