Mind that welders aren't likely very clean (stability or output ripple), nor very well calibrated. At low frequencies they should be fairly stable/consistent at least (since that would affect usability if not!). Output V(I) characteristics likely not well controlled either, other than being somewhere resistive to CC for TIG/stick types, and lower resistance for MIG.
Kind of a similar thing with electrophoresis supplies, AFAIK they're quite noisy/nasty because it doesn't matter for just pulling charge through a solution.
Tim