What is the type of mixed gas for shielding are you using?  If it's Argon/ O2 or Co2 then you will have a problem.  If it's a Ar/ Helium your good to go.  It's most cost effective to make the sheild gas and your plasma gas argon.  We have the same type of set up on a robot and use it for all types of tubular joints, with and without filler.  Sometimes for speed reasons we use hot wire and a 85/15 Ar He mix and it gives us about 10% faster speeds.