Help with Tig 175 arc cleaning failure

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Though I am new to Tig welding I have gone through 4 bottles of argon getting used to Tig welding aluminum. I have a Lincoln Tig 175 that I bought new about 2 months ago.I changed the bottle today (supplied by Praxair) and have found that I cannot weld aluminum. It seems that the arc cleaning has disappeared. The entire time I am welding it is like a skin is being formed and there is no evidence of arc cleaning on either side of the weld when I am done. This welder only has "auto balance" but it did used to leave an ion bombardment on either side of the weld. I am using the same settings and material as I was before. I read the manual and it talks about spark gap but I honestly don't know if that could be the problem. It always seemed to be a bit lacking in cleaning action.0.050" thick aluminum sheet1/16" dia pure tungsten1/16" 4043 rodabout 15 cfh argondial at just under 30Amp, pedal used as required.I have tried different tungsten sizes and cups but it was only to satisfy myself that was not the problem. Gas flow seems fine.The DC seems to work fine on cs and ss.The HF start does seem to work as good as it ever did.I have searched the forum but couldn't find anything directly related. I am pretty much convinced it is the machine.RickI can post a picture of what I was capable of and what it is doing now.Last edited by MechEng; 10-12-2012 at 09:15 PM.
Reply:Bad gas.Happens quite often. Take it back for a free exchange.Last edited by shovelon; 10-12-2012 at 10:08 PM.Weld like a "WELDOR", not a wel-"DERR" MillerDynasty700DX,Dynasty350DX4ea,Dynasty200DX,Li  ncolnSW200-2ea.,MillerMatic350P,MillerMatic200w/spoolgun,MKCobraMig260,Lincoln SP-170T,PlasmaCam/Hypertherm1250,HFProTig2ea,MigMax1ea.
Reply:Oxygen in the gas?I should note the valve on the (older) tank is brand spanking new. Could be an improperly purged bottle after the valve change I suppose.Thanks for replying. Being a mechanical guy I always jump to conclusions and want to blame the electrical guys. Ha ha.Why does this stuff only happen on Friday evening. Never Monday morning. Grrrr. Can't swap it until Monday.Last edited by MechEng; 10-12-2012 at 10:11 PM.
Reply:Bad gas happens so often, that is the first thing I look at. Good reason to have a backup bottle to check against.No other supplier near you open Saturday?Weld like a "WELDOR", not a wel-"DERR" MillerDynasty700DX,Dynasty350DX4ea,Dynasty200DX,Li  ncolnSW200-2ea.,MillerMatic350P,MillerMatic200w/spoolgun,MKCobraMig260,Lincoln SP-170T,PlasmaCam/Hypertherm1250,HFProTig2ea,MigMax1ea.
Reply:Praxair are supposed to be open Saturday from 9-12 so I drove the 10km and found out they were closed for inventory. Argh. A message on the answering machine would have been nice!I went back today and they had no problem exchanging it for a new bottle that actually had argon in it. The bad bottle was an old one but it had a brand spanking new valve on it. My guess is that it was contaminated with oxygen / not properly purged when the valve was changed.Also, the bottle was only at 2000psi when I first connected it. They are usually about 2100 psi. I think someone else had already taken it back and they pawned it off on me hoping I was just doing DC Mig or something where it may have been acceptable. With AC on aluminum it was impossible to weld with. Being new to Tig I thought it was me at first but even I am not that bad!I purged the lines good and presto, it works like it is supposed to.I am very much thankful for your input. I thought a follow-up would be good to this post. I see a lot of posts that are kind of open ended on what actually happened.A happy ending!
Reply:Yes. Many times I hear that they just pawn off bad bottles to the next buyer.Weld like a "WELDOR", not a wel-"DERR" MillerDynasty700DX,Dynasty350DX4ea,Dynasty200DX,Li  ncolnSW200-2ea.,MillerMatic350P,MillerMatic200w/spoolgun,MKCobraMig260,Lincoln SP-170T,PlasmaCam/Hypertherm1250,HFProTig2ea,MigMax1ea.