Air Grades

[turnerjd]

Originally posted by neil
Jon,
Thanks, I guess I should avoid reading spec charts! The point about the blending I was trying to make is that most people assume the air coming from their compressor is 21% O2. If you were blending and your mix came out a full 2% higher than you wanted, it'd be a pain in the butt. And you'd have to do more MATH! Thanks for the info!
Neil

Neil,

For all diving applications where the air is being filled directly from a compressor there will be absolutely no change in the air composition from bog standard air.

The reason (I suspect) for the allowable differences comes down to large industrial operations. If you want to get gasses such as helium and argon you have to distill LOTS of air, some of this you can then sell as bottled N2, O2 etc.. but for the quantities of Ar, Ne, He, etc.. distilled on the industrial scale (the total for all of these is less than 0.5% of the air) I am sure will result in vast quantities of unwanted compressed gasses such as N2 and O2.

Large gas companies often then sell off compressed air in cylinders for very little money as it is in essence a by-product, and often only has had 1% max of the gasses distilled out of it. Hence the range of specs for compressed air. The compressed O2 and N2 that they can't sell as compressed air is then just released back into the atmosphere.

(to distill a gass you have to cool and compress it untill it becomes a liquid, then you gradually let it expand and warm, and the gasses distill off like normal distillation)

HTH

Jon T

 

[Atomox]

In Florida they have addressed this issue. See...
http://www9.myflorida.com/Environment/facility/Air/

It is my experience that the helium I get is from the distillation of natural gas reserves that have a high He content. These reserves are in Amarillo TX.

The amount of oil and CO and other impurities that are removed by a filter stack are a function of the surface area of the media, type of media, and the time of exposure (flow rate). Not a function of size per say. A higher output volume compressor will need an appropriately sized filter stack(s).

The majority of impurities can be removed by an efficient mechanical filter (usually 80%, and up to 95%). To get breathing grade air this must be supplemented with a filter stack(s). An oil lubricated compressor will not only leave an oil mist in the air stream but will also generate CO (from combustion of the oil). To remove CO a catalyst is used to convert the CO to CO2 and then the CO2 is removed.

What I use is a high efficiency mechanical filter to remove the oil and water, followed by a grade E filter stack, and than a high purity filter stack from Lawrence-Factor. I use this setup when I continuously blend nitrox and trimix. The O2 and/or He is added upstream to a gas preblender and than run through the compressor. The output is than put into a storage bank and the scuba tanks.

The beauty of this is it reduces the risk associated with partial pressure blending with O2, I can use all of a "T" of the gas and I can dial in the exact mix that I want. (See.... http://www.diverite.com/news/atomox/index.htm)

Brad

 

[joewr]

Gentlemen,

In facilities that compress air for scuba and do not use that air for Nitrox, Trimix, etc., I have not seen them use an O2 analyzer regularly for that compressed air. I have not been by my local FD lately, so I do not know whether they compress their own air. In our labs, we had Scott Air Packs and a contractor filled them. Our FD may do the same. BUT, my point is: does your FD have an O2 analyzer? Does your LDS (sounds like it is not too L) use an O2 analyzer for their "vanilla" scuba air?(Obviously, they use one for the "exotics".)

And is the O2 content in ambient air that different within 100 miles? On most of this planet the air is backmixed pretty thoroughly over a 100 mile horizontal stretch. That is a long-winded way of saying O2 should not be an issue and it will be in the 20-22% range because that is where the O2 concentration is for most sea level environments.

CO, CO2 and oil are a different matter. That is because they can be generated different ways, depending on how the air is compressed. But note, that the specs for E and L are the same in the 20-22% region.

As atomox says, the CO (always present in combustion gas mixes to some extent) is removed by catalytic conversion to CO2. The CO2 is removed by adsorption--molecular sieves of the kind used in rebreathers are typical adsorbants. Oil, particles, and water are also removed by adsorbants which can vary from fairly low-tech activated charcoal or silica gel to the higher tech molecular sieves.

Now, Senor Warhammer, if your FD is making its own Grade L, it will have a compressor/catalytic/filter/adsorber system very similar to the one at the LDS because the spec on CO, CO2, oil, H2O are identical for the the compressed air you breath in scuba an scba (20-22%). And you can ask both for a tour and see for yourself. Then it comes down to maintainance--got to keep all those filters, adsorbants, etc. functioning!

But--and this is so American!!--your FD does not carry the same liability for customers that your LDS does when it comes to compressed air. So, your post-calamity law suit, if it ever comes to that, will probably be more complicated!

One more thing--odor--your nose is one of the most sensative chemical analyzers on this planet. If that air does not smell "right", do not use it. Those sensors can detect halogenated compounds, hydrocarbons that have odors, sulfur compounds, etc. at extremely low levels. But, compounds like methane (a hydrocarbon and the major component in natural gas), CO, CO2, etc. are odorless. Equally important is the fact that your olifactory sensors (fancy word for smell detectors in your nose) can stop detecting if you do not react and stop breathing the stuff in; i.e., if the air smells bad, stop using it--the smell could "go away" because your detectors "become accustomed" to it. But the bad guys are still there and they are doing their thing--just like politicians!

And, again, welcome back from Hawaii--where the air is at least 10 times better than CA right now--compressed or not!

Joewr

 

[Warhammer]

Originally posted by joewr

And, again, welcome back from Hawaii--where the air is at least 10 times better than CA right now--compressed or not!

Joewr

LOL!! Tell me about it, until landing at LAX, I guess I had never been exposed to smog. I was absolutely amazed by it. Don't see how Californians stand it.

I know my local FD compresses there one air. What I'm not sure about is the grade of that air. There are several local divers that have been obtaining their air from them for years without incident. But that doesn't mean they haven't killed a few brain cells along the way. I was talking to one of those divers the other day about an upcoming dive that he and I are planning, he suggested that I just get the fire chief to fill my tanks instead of the hassle I'd been going through. Until then, I just had never thought about the FD being able to do that. He said they were set up to do either or (SCUBA/SCBA), could also fill DIN or Yoke, and that the air was good. As you've probably guessed, several of the local firefighters are also active divers, so that may have something to do with them being able to do SCUBA fills. And the local dive rescue team has to get their air from somewhere and since several of those guys are also firefighters, it just makes since that they probably obtain their air from the FD as well (no place of their own per say). But I'll definately go see it for myself before breathing any of it.

The chief is a long-time friend of mine, so sueing would be a non-issue for me, regardless. Things are different in a small town as this one, the sue happy society hasn't really taken a hold here YET, and I stress the YET.

Thanks guys for all the helpfull info.

 

[GetWet]

Warhammer:
At the risk of sounding stupid, what is an O2 cleaned tank?
------------
GetWet

 

[Atomox]

When you are blending Nitrox or Trimix in the tank(s) by partial pressure blending or by weight you will be adding 100% oxygen at some point. 100% O2 with heat and fuel (the fire triangle) will start a great bonfire.

O2 cleaning involves removing all possible flammable material in the tank such as oil particles (the most common). The common method is to remove the valve and inspect the inside of the tank for corrosion. If corrosion is found the tank is tumbled to remove it. The tank is then filled with a hot solution of simple green (or similar) and water and then tumbled. NOAA recommends the use of simple green for their diving operations. Then the tank is rinsed numerous times.

The valve also needs to be inspected and cleaned for use with O2 as well.

For some good info on this go to:
http://www.globalseas.org/table_handling-o2.html

 

[Warhammer]

Glad you brought that subject up, Atomox. In my left ear, I here Simple Greening a tank is crazy, in the right ear, I here paying for 02 cleaning is crazy for the average do-it-yourselfer. Which is it?

Left-Ear's argument is that it's not the "approved" method set forth by the Compressed Gas Association of America. They supposely require tumbling regardless of the tank's cleanliness. A clean one gets tumbled with glass and some commercial chemical (not Simple Green or 409), while a dirty one gets tumbled with ceramic in addtion to the above. Then of course there is the valve that gets cleaned and new o-rings.

Right-Ear's argument is that Left Ear is full of *hit. Simple green, very hot water, and alot of rinsing, and a complete drying, is all that is required for both the tank and valve. Course the o-rings are replaced everytime with o2 compatible 0-rings for insurance.

Which ear has bumped it's head?

 

[Atomox]

Warhammer

To me O2 cleaning makes sense. Because:
It is very easy to generate heat in the filling or transfer of gases into a tank. If a short section of hose is rapidly pressurized to 3,000 psi, temperatures of 1500 F or more are reached. With the heat and O2 all that is needed is fuel for combustion.

A minor amount of oil in a tank when filling can result in a internal fire that goes unnoticed by the filler. The combustion will produce CO, not a good gas to be breathing at any time. Or a large fire could result resulting in the loss of equipment or more.

This should not be an issue if the compressor used for air is well maintained and has proper filtration.

Swift compression also creates supersonic gas velocities within a hose. With a supersonic oxygen stream, any hydrocarbon or other noncompatible substance is likely to be partially vaporized or ignited. Again, not good to breath.

So I clean my tanks. Cleaning them is relatively easy. It assures me that there are no unknowns in them. The use of simple green is because it is biodegradable and is non- toxic. Also it is recommended by a few sources that have been doing this longer than I have.

I too have heard the claim that O2 cleaning is silly. The argument used is welders and their lack of handling precautions. Well they are not filling tanks.

I would not pay some shop to do this because they charge an arm and a leg for the cleaning. As I said earlier it is very simple to do yourself.

In my mind it is one of the small details that can be done as a measure of prevention that could make the difference between surviving or not.

Brad

 

[Warhammer]

I wasn't questioning the need for 02 cleaning tanks being used for partial pressure blending nitrox. What I was questioning is the procedures for obtaining that clean. I can see you believe in doing your own. Do you tumble your tanks with glass as well as the simple green? Or just do it with simple green unless the tank has some corrosion?

What confuses me about all this is the where the regular air (21%) used for blending comes from. If it comes from the same compressor that is used for ordinary fills, then why does that air contaminate the 02 cleaning of a tank if filled with nothing but regular air? When it doesn't contaminate the 02 cleaning of a tank when used to top off a nitrox mixture? In other words, why is the air 02 combatible in one use/tank and not in the other, since it's pumped from the same compressor using the same filters?

 

[neil]

Warhammer,
I think I see where you're confused, let me try to explain.

Any time you use pure O2, you need O2 clean tanks. The air you top off with must also be "O2 clean", or hydrocarbon free. The air from a "regular" compressor is not clean enough for this, you need to filter it even further with hydrocarbon filters.

If you blend using other methods such as membranes or air entrainment, you don't need O2 clean tanks for mixes up to 40%.

If you top off an O2 clean tank with "regular" air, it is no longer O2 clean and must be re-cleaned.

So, if effect, there is "dirty" and "clean" air. I think that's the answer you were looking for.
Neil