testing for free SO2

I have a Vinoferm acid test kit, and bought the reagent(?) to test levels of SO2, but the directions were translated from french, and not real clear.they say to add the reagent till your sample turns blue for ten seconds, then multiply the amount by 10 to give you the amount of so2 present.I used a bunch of the reagent,stirring as constantly as possible,and never really did see anything blue,other than a blue ring around the top of my test vessle.Understandibly testing reds is difficult when looking for a color change,but I've got to be missing something.The only test I felt confident I got correct was total alcohol,and ph. I'd like to keep the SO2 optimal for longevity purposes , without overdoing it. Thanks. (toad)