to quote national: "Do not replenish refrigerant solely because of bubbles in the sight glass. First find out the cause of the bubbles!" I'm thinking fractionization has an additional effect.
Humor me and explain this a bit further please..... I'm with jp, someone who knows what they are doing will know what to do with it. You can't make our industry idiot proof no matter how hard they try to. Everyone of us works with someone or knows someone that proves that point.
More specifics would be needed for your exact situation.
It's very possible that you need to leave the sight glass flashing & add a liquid/suction heat exchanger near the TEV.
That may not be such a good idea, a liquid to suction HX robs Pete to pay Paul and brings nothing to the big picture, why try to re-invent the wheel? having bubbles in the sigh glass is not a good thing, charge it till it clears and then some.