Tig Misbehaving

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I was just finishing off some coffee tables and entertainment units when my Tig played up.Its a PT 185 3phase 415v, had it for over 5 years it hasn't skipped a beat.I was welding mild steel 30mm shs ,At 185 amps .. pedal down about half100 % argon as I normally only do ally or SS , have a mix bottle but rarely change over for one or two non  critical welds.Bottle  fairly new at least 80% full , Gas was fine 5 seconds ago (not a "bad" batch)Gas going through torch.Flow meter fine.Earth fineConnections to Earth , Gun and Power connections  to machine fine.All of a sudden when ever I tried to weld it would make a "farty " type noise and blow a hole in the tube.Ate the tungsten also.Changed to AC checked on a piece of Ally ... same story.Cannot see anything obvious.Any one have any ideas I'm home now but any thoughts appreciated.Brett ThompsonA good guess is better than a bad measurement
Reply:Do you have a way to bypass the pedal such as a finger controlled on/off button? You may have a foot pedal that is sticking on higher amps. If you have a digital readout you could weld and see what was your last amp reading to verify the foot pedal is working properly.Is the torch a water cooled or air cooled torch? Could it be as simple as contaminating your tungsten? If air cooled you may want to double check the gas flow through the torch especially if you have a valve on the torch itself. I have accidently not turned the valve fully on and burned up a cup and tungsten.
Reply:Random thoughts:The current is spiking very high.  I've read of this problem with the Miller Econo Tig.The inert shielding is momentarily lost, solenoid valve acting up?   Does the cremated tungsten and tube weld show extreme oxidation?  That would be a clue of a gas problem.Did you happen to pick up a stick of pure tungsten by accident, it can cause severe arc problems with inverter welders.
Reply:Huh, I have always used pure tungsten on my inverter for alum.  If there is a foot pedal problem and it is getting stuck wide open, you should be able to just turn the max amperage down and see if it holds it there.  Can you hear the gas flowing when it happens?  Check for leaks.  I once rested the the like on a hot piece of metal on accident while I was welding and burnt a small hole in the line.  Acted similar to what you are describing.
Reply:Well, got to work the next day , wound the amps right down, changed to tungsten and ... no problems ??Tried a few more times gradually bumping up the amps and its going fine.Thanks for all the replies even though I could hear and feel gas it just seems as if it was flat out without gas, mmm maybe the welder wasn't reading the pedal or visa versa?Oh well thanks anyway and I guess the  "If it aint broke...dont try to fix it"  rule should apply and I wont play with it BrettA good guess is better than a bad measurement