Sanitary weld problems

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I have been tig welding a 3" pipeline on my dairy  with a .065 wall thickness.  I have been making excellent butt welds (sanitary) and have been welding 3/4 ID nipples on the 3" 304 stainless steel line.  I drill a 9/16 hole and  then pulling it out to 3/4 ID with a  tapered swedging tool.  Then I grind it flat, tack on my nipple and make the sanitary weld overhead so I can rotate 360 degrees. My amps on my Miller Maxstar 150 A STL is 45.  I have my purgeline set at 12 CFH.  I have my tig gun set at 15 CFH.  Today my argon bottle had only 250 lbs of pressure.  I could not not get my purge line pressure constant, and messed up my first weld.  The outside looked excellent but the inside did not penetrate.  My tungsten developed an arrowhead tip with the gas going over it looked like a bonnet.  What causes this?  Then inside weld looked chattered and pitted.  I have two regulators on a T on one argon bottle.  Do I need to put a flow meter on the purge regulator to be more precise?
Reply:That arrowhead tip is an indicator of the tungsten picking up contamination from spitting from the puddle, often if you weld where sugaring has taken place.  How did you set the purge line at 12cfh with no regulator?
Reply:It sounds like you aren't using enough gas, and/or didn't get a good purge in it.I ALWAYS run more gas flow than that.  I run at least 20cfh on the torch, and at least 15 for a purge.  I run more purge for larger pipe, or for things I want less color inside.
Reply:IF your argon pressure was 250 psi, and you're seeing fluctuation in the purge line pressure, try a new cylinder of Argon.To repeat an earlier question:  How did you set the flow rate on the purge line without a flow meter in-line?250psi is getting mighty low for a tank that starts out at ~2200psi.  Could be your regulator isn't accurate at that end of the pressure range.  You didn't mention if it's a 1 or 2 stage regulator.  In my experience, Single stage regulators are not very good at maintaining pressure/flow rate when the supply side pressure is that low.