Is an oil free compressor suitable for O2 clean tank?

[Baconated]

Hi all,

Apparently my LDS uses an oil free compressor but they do not do enriched air fills. If I had an O2 clean tank and wanted a normal air fill would an oil free compressor be sufficient to keep the tank O2 clean?

Thanks
Nathan

 

[skippy31]

Normally air used in an O2 clean tank should be Grade E. Grade E air is generated by using the proper filtration on the compressor for both oil/hydrocarbons and moisture.

An oil free compressor still needs filtration to remove the moisture from the compressed air.

I once knew a guy who had an oil free compressor and told his buddies the air was good. One friend found 1/4 cup of water in the bottom of his tank after several fills.

Not saying this shops air is bad or good, but they need some filters on the line to have good quality air.

Normally shops get their air tested periodically. That would be a good indication of the air quality. Beware that some shops test their air right after a filter change rather than right before a filter change. Testing right after a filter change does little to test if the filter is lasting as long as it should...

Mat.

 

[rakkis]

Actually "grade E" air is what you would normally get. It's regular air. "Modified" grade E air has been filtered further to reduce the hydrocarbons even more.

Here are some specs:

#3 Nitrox Scuba

Ask to see your shop's compressor's latest air quality results. They are required to provide them by law in the US.

 

[Dive-aholic]

rakkis is right. Grade E is not enough to keep your O2 clean tank O2 clean. You need modified grade E. If they have the compressor, why don't they do PPB? I'd avoid them with that tank.

 

[Baconated]

OK, thanks all. They aren't advertising as being O2 compatible, I was just curious if there was more to it. From what you have said filtration is also an issue. As to why they don't do PPB, I don't know. Probably cost vs demand or something.

Nathan

 

[skippy31]

Guys, modified grade E is a reduction in hydrocarbon over grade E. This is an oil free compressor. If the dew point is good the air should be fine for O2 use.

I would still check their test certificate, but if they have enough filtration to bring the dew point into line all the other numbers should be just fine with an oil free compressor all else being equal.

Mat.

 

[jimdiverman]

Guys, modified grade E is a reduction in hydrocarbon over grade E. This is an oil free compressor. If the dew point is good the air should be fine for O2 use.

I would still check their test certificate, but if they have enough filtration to bring the dew point into line all the other numbers should be just fine with an oil free compressor all else being equal.

Mat.

Dew Point reduction is caused by cooling the air. It is essentially dehumidification of the air. In a sense it is a filtration process, but not strictly speaking a filter like you would use to remove particulates. This is usually accomplished through a direct expansion cooling process.

According to the ANDI statistics I read in the referenced report, ANDI requires the Dew Point to drop to -50. If you look at a psychrometric chart, you will see that there is very little water possible left in the air at that temperature, and it is certainly very dry. It didn't say whether it was Celsius or Fahrenheit, but they are very close in that range anyway.

 

[skippy31]

Water removal in compressor systems is accomplished by two means a mechanical separator and a filter cartridge with a desiccant (among other things) in it. The separator removes oil and water, while the filter removes the last bit.

Occasionally you'll see an air dryer attached to the compressor at the intake, but this is an expensive option and normally reserved for climates with extreme humidity.

Have a look at this article.

Neptuno's World Articles - Understanding SCUBA Compressors and Filtration

Also keep in mind this is an OIL FREE compressor we're discussing here.

Mat.

 

[Dive-aholic]

Guys, modified grade E is a reduction in hydrocarbon over grade E. This is an oil free compressor. If the dew point is good the air should be fine for O2 use.

I would still check their test certificate, but if they have enough filtration to bring the dew point into line all the other numbers should be just fine with an oil free compressor all else being equal.

Mat.

True, I wasn't arguing that this compressor isn't pumping suitable air, just that grade E is not suitable for O2 clean tanks. But depending on where they keep their air intake, they may not have modified grade E even through an oil free compressor.

 

[pescador775]

Bacon, are you confused enough? An oil free compressor should be fine for OC air. About the humidity thing, we are hearing that ANDI requires -50F? Jim didn't say, really, he just said -50. Grade E requires -60F (24 ppm) and that is extremely dry. You don't need "modified" grade E because Grade E is good enough in practice if not in theory. Fact is that most compressors with modern filtration systems produce air which exceeds Grade E criteria. In the case of the oil free we are more concerned with moisture and I will discuss that. The modifed E recommendation for OC air is a product of NASA and sellers of filtration systems which is passed along to students by fellow travellers called "instruction or standards agencies". Regardless, users of small, oil free compressors often overlook the humidity requirement and also the need for enhanced filtration required with gasoline engine driven machines. So, there is a presumption that users of oil free compressors are a little naive or careless about the definition of good air. This behaviour is primarily due to propaganda put out by RIX, a manufacturer of these compressors who brags that their machines emit no oil and suggest that wet air is good which is a fundamental misunderstanding of diver requirements. (They charge extra for dry air filtration). However, a dive shop would be the subject of numerous complaints if they were at fault with respect to moisture content of air. Bottom line, go ahead and top off your OC tank at the shop.