Porosity from a mostly Empty Argon Cylinder.

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On a recent welding project I MIG welded a fillet flare weld shown on picture 1 and everything was OK.  Then I flipped the part over to weld the flare groove butt weld shown on picture 2 and the weld was porous.  I hadn't changed the flow rate of the  75% AR 25% CO2 shielding gas between the two welds.  I was welding the back side of the same metal and I had ground off the mill scale before welding.  I was using ER70S-6 wire, which can tolerate some contamination.  I checked the tank pressure and it red 300 psi. I checked the flow rate and it was 15 SCFH, so I bumped it back up to 25 SCFH.  I made another weld and it was still porous.  It took me a while but eventually I figured it out.  I changed the Argon/CO2 tank to a new one and the problem went away.Here is my AssessmentIt turns out that that the zero on the tank pressure gage was off, so most likely the tank was below the minimum 50 psi that is required for the ball type flow meter (called a rotometer) to read correctly.  Rotometers measure volume flow not mass flow. so they are dependent on the pressure differential and the temperature. When the flow meter was reading lower then my initial setting it was most likely caused by the low inlet pressure as opposed to the setting on the needle valve on the flow meter changing.  If a rotometer doesn't have the required inlet pressure of 50 psi you really can't trust the reading. What faked me out was the false tank pressure reading of 300 psi when there was most likely less then 50 psi.  The fillet weld trapped more of the shielding gas compared to the more open butt weld.  I have gone through many tanks of shielding gas for both MIG and TIG.  For some reason I never experienced this phenomenon before.  I bring this to your attention for two reasons:1.So you don't have to scratch your head and waste time like I did figuring it out.2.To see if you have ever had the same experience.Don Attached ImagesSmith Oxyacetylene TorchMiller Dynasty 200DXLincoln SP-250 MIG WelderClausing/Coldchester 15" Lathe16" DuAll Saw15" Drill Press7" x 9" Swivel Head Horizontal Band Saw20 Ton Arbor Press BridgeportLincoln LE  31 MP & Lincoln 210 MP
Reply:That's a nice easy one to cut out. I had to plasma out some 3/4" plug welds once that looked like that. I had filled about 5 of them before I woke up and realized I was out of gas.SqWave 200Millermatic 190Airco 200 ACHypertherm PM45Boice-Crane Band SawVictor O/A
Reply:Have had that experience when tig welding. Sometimes even when the needle says a couple of hundred PSI, the needle is sticky and upon smacking it drops.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.