Air Grades

[GetWet]

Thanks guys for the info. I'm a strictly air breather so not acquainted with other mixes. It's been quite some time since I went to scuba school, so maybe sometime you can start a thread on what advantages there are to other air mixes.
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GetWet

 

[Warhammer]

So your saying they use a different compressor to pump the 21% air that they use in nitrox mixes? Now of course, I'm refering to a shop that does both air and nitrox. I was under the impression that the opposite was true. 1 compressor that pumps all the air, and they just add pure 02 to the tanks first when partial pressure blending, then pump 21% (from the same compressor used for ordinary fills) on top. I'm almost positive that's the case at the shop I deal with. So what I don't understand is why that same air would contaminate 1 tank and not the other.

 

[neil]

Warhammer,
They probably don't use different compressors. They either use a compressor that has the proper filters attached, or feed the air through a separate filter stack as needed when topping off for Nitrox fills. The filters are expensive so they most likely don't pump "ordinary" air fills through them, although some shops do. They would also need to have an O2 clean booster if they use one.

As I said before, if you put "dirty" air into an O2 clean tank, it is no longer O2 clean. If your shop is putting dirty air on top of O2, they are not following standard practices as I know them, and it's dangerous, IMO.

Neil

 

[Atomox]

Warhammer

In my case I use the same compressor for any fill. I also have filters that get the air to grade E, then I run it through a hyperpure filtration stack. I do this for all of my needs air, nitrox, trimix. If you are blending by entrainment than you do not need O2 cleaned tanks. As I said earlier if you are mixing by partial pressure or weight than it is wise to use O2 cleaned tanks. Some shops may be cutting corners on this stuff because it is time consuming and or costly to do this (or thay are just plain stupid).

 

[Warhammer]

I think that answers my questions then. So if I have an 02 clean tank, and that tank is filled via a clean system using the exact same filteration, procedures, and etc as used when blending nitrox except no pure 02 is added, then I still have a clean tank. Correct? Great.

 

[devjr]

I try to keep it simple, but that does not mean "Simple Green". That stuff is almost useless unless you jump through a series of hoops including tumbling and forced air drying. Personally, I use trichloroethane or "Ensolve". These chemicals will cut silicone grease or oil like nothing you've ever seen. Won't rust the tank and leaves no residue. A handful of greasy valve parts dumped into trichlor is O2 clean in 10 seconds. Same for O rings but be careful with plastics.

I've read all the posts and find them very well informed. The bit about distillation of gases was interesting but don't get hung up on variations of O2 content from compressors, doesn't happen. There are some concerns, mostly minor, concerning CO. Long ago, this was a problem with old war surplus two stage GE compressors creating a diesel cycle through overheating and combustion of the lubrication oil. Modern hp compressors using synthetic oils don't produce CO. There remains a problem, potentially, with portable compressors sucking exhaust gas. (position the exhaust downwind and use a long intake tube). The real source of CO, as one poster pointed out, is auto exhaust gas. I don't see how city people can get clean intake air and so a catalytic filter element should be installed on compressors located in metro areas. These are widely available. Doesn't hurt to ask the shop if his filter stack includes "hopcalite". If he knows what that is, you are probably OK. Some will display an air assay report.

Grade E air is theoretically not pure enough for use in an oxygen clean tank. Over a period of time and many fills, minute amounts of oil can accumulate inside the tank to create a problem should it be subsequently charged with O2. The compressor should have an extra "fine" filter to be safe.