Hiya Firefysh, Wine ain't beer and if you sanitize all your equipment properly you can return any sample you have taken to measure the gravity. The risk of souring is tiny (you are fermenting fruit not grain so lactic acid is not an issue). OK, the device you are talking about is a refractometer but this is really designed to enable you to measure the sugar content of fruit. In the presence of alcohol the way light bends is very different so you need to apply complex formulas to allow the refraction of light in the presence of alcohol to be accurately indicated by this tool.