Hydrometer alcoholmeter is used to find the alcohol percentage of alcohol by volume (abv) in a completely dry or distilled liquor. It does this by measuring the density of the liquid compared to that of alcohol; any residual sugar or other cause of change to the density will result in a false low reading.
Because there is often a minute amount of unfermented sugar following standard fermentation, along with dissolved carbon dioxide and spent yeast or solids remaining after fermentation, it is very difficult to obtain an accurate reading of the abv in such a liquid.
In addition, an alcoholmeter is increasingly difficult to read at lower alcohol percentages, compounding the problem. For this reason, it is suggested that you use an alcoholmeter only for the testing of distilled spirits.