Hi rangerod
My advice is to introduce (if you have not already), restrictions to the purge process. You may be pulling O2 in if you do not have a sealed condition. IF the welded area is reasonably distant from your gas inlet and outlet, simply tape the area with masking tape and get a good seal on both ends, leave a very small diameter hole on the outlet for gas to escape. It will shorten your purge time greatly if you can configure the purge outlet to be higher then your inlet, make gravity your friend. Purge only the area necessary for welding, not the whole vessel. Provide enough CFM to bring your meter up into the .2 % range quickly, then taper off. I know this may seem high but it depends on the amount of space you are trying to purge. Personally I would run a very high flow until your meter reads over .2 and then cut it down and retest the welding. I have welded a lot of 100% only conditions and it can be very difficult to find the fine line, it will vary according to the structure, how fast you release it, and your flow vs. pressure. I agree with jf55, the meters can be dodgy and will not read correct, especially dependent on the flow past the sensor. I have seen them read wildly different dependent on which part of the chamber we had them in....even with what we know would be a full evac. I would expect to have very high percentages with a three hour purge, just off a gas bottle and hose, even on a good size vessel.
my little $.02
I hope that helps you
Tommy
rangerod,
With all the parameters you introduced us to, if your getting a dull blue you are there. That is not surgar, and unless you have some color ristriction you are fine. Remember when your dealing with stainless your goal is passive protection. Oxidation (black or surgar you don't have it), dull blue very little oxidation, silver no oxidation. Again the caveat is, what, if any color restrictions your under or what process will see the back side of that weld. Is it food service? then color matters. My last point is if you have access to the root side of the weld, even if it is sugared you can clean by light grinding to get rid of the sugar. Once surgar (oxidation) is removed then passivity is restored.
Jim