The best advice I can offer is to purchase the following codes:
AWS D18.1 - Spec for Welding Austenitic Stainless Steel Tube & Pipe Systems in Sanitary Applications
AWS D18.2 - Guide to Weld Discoloration Levels on the Inside of Austenitic Stainless Steel Tube
ASME Bioprocessing Equipment - 2002.
These will cost approx $250 total. The first two contain color guides that show the decreasing quality of welds as a result of increasing contamination (primarily shielding gas). The second code listed is simply a laminated sheet with information and the color guide.
The ASME and AWS D18.1 contain a lot of description regarding visual acceptance criteria for welds. D18.1 & 2 explain the effects of varying the composition of shielding gas pretty well. The codes also touch on a very important aspect of this - iron out the acceptance criteria with your client before you get started. Otherwise, you know how that will turn out.
We have just gotten into this type of business also. There is no way to overestimate the level of quality expected.
Good luck,
Charles
While working for a company that made Ultra Clean Steam Generating Boilers for the Pharmaceutical industry, we fought a very similar battle.
We resolved the issues through the use of detailed step by step procedures and optimized our welding and purging equipment. This process was completed through the help of Mr. Chuck Carnes and team with Purity Systems Inc. They are a consulting firm that specializes in excatly what you are looking for. I highly recommend them.
Their contact info is:
Purity Systems Inc
303 S. Airport Road
Longmont, CO 80503
303-401-7049
1933 O'Toole Ave, Suite A104
San Jose, CA 95131
408-435-9119
Good Luck!
Tim