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Its a hydrometer with a different scale other than specific gravity. Specific gravity is always relative to water. water has a specific gravity of 1 but ethanol has an SG of about .789. So if you mix water with alchohol , its density (the water's) goes down. The water would have a bouyant force of 1 gram per CC of water. If the meter is set in pure water , it should sink to the first line wich would have some sort of zero reading. The more achohol , the lower the specific gravity and therefore the lower boyant force. The meter will sink deeper into the liquid.
Now in your specific case , an instrument of that type can only be used with a single reading if you assume there is nothing else in your brew except water and alchohol. This is almost never the case in a brew but is approximately accurate for a liquid distilled from an achohol/water/low sugar mix. In this case a single test of the distillate can be used to closely approximate the alchohol content . The scale inside though must be adjusted to measure a value based on the assumed pure alchohol/pure water mixture with no unfermented sugar (or salt , etc) .
So anyway.
You numbers showing 15% alchohol are almost certainly in error if its a homebrew beer. . If you tell us your OG and FG reading its easy to figure out. If its a wine or mead brewed with a champagne yeast , you may have approached 15% alchohol. As a wine it will have little unfermented sugar (dextrines) and the Champagne yeast can attenuate to about a 14.8% alchohol by volume. As I said , I can calculate the almost exact alchohol by volume if you either tell me the OG and FG or give me a complete list of ingredients. If we know the OG and FG data , we can likely figure out the scale on your hygrometer and why it reads 20 in the finished brew..
No beer yeast will attenuate to that level of alchohol and all homebrews (in contrast to distillates) will have residual unfermented sugar in the form of dextrines.
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