Free Sulfur Dioxide (FS02) â€“ Sulfur dioxide (S02) is used in winemaking as a preservative against oxidation and microbial spoilage. The free form, or Free S02, is used to measure the level of protection in wine. FS02 is expressed as a concentration in milligrams per liter of juice (mg/L) which is also considered parts per million (PPM), therefore, 1 mg/L = 1 ppm. It is recommended that a free S02 level of at least 20 mg/L should be maintained up until bottling (although it should be lower at the start of malolactic fermentation). For our Tier 1 test we us a Hanna 902 titrator.