Hi,
I am not an air expert but my experience has been if it is dry and clean enough to spray paint with, then it is good enough for plasma.
Dayton (Granger) makes a good, cheap air dryer.
Ask someone who sells automotive spray paint what to use, and you will be headed in the right direction.
The problem with Water or Oil in the air is it will shorten the electrode life drastically and the nozzle as well.
I dont know of anyone who measures the air quality.
If you are getting the expected life from your consumables then your air is OK.
If the time or pierces go down then look at the air.
Good Luck,
Alan Bradford
Plasma Technologies
Canaan NH
www.plasmatechnologies.com