The hydrometer measures the specific gravity of a liquid and serves a number of functions in winemaking. Initially The typical hydrometer you would start with has a range of 1.150 to 0.900 and it usually includes a second scale for the brix. nitially you can ascertain potential alcohol content from the brix upon grape crush. As the fermentation commences you should see a gradual decline in the specific gravity but if this decline ceases prematurely you probably have a stuck fermentation and need to take action Assuming all is going well when the specific gravity drops by about one-third (i.e. one-third to 1.000) it will be time to add nutrients. Once fermentation nears completion the specific gravity will be near 1.000 but take heed because alcohol has a specific gravity less than water the finished dry wine will have specific gravity of less than 1.000. This is when you switch to a hydrometer that typically will read 1.010 to -.0.900 (brix 5 to -5). Because this scale is spread out more it is easier to see when the specific gravity actually stops lowering. Then switch over to Clinitest to see if your wine is actually dry.

The -5 to +5 hydrometer is used toward the end of fermentation. The final endpoint of a red fermentatiion is -1.5 to -2. The regular hydrometer is used to monitor the fermentation most of the time and is the most generally useful.

The brix. And depending on the brix you want to use the corresponding hydrometer.

It is primarily a question of range over which the particular hydrometer is calibrated. For determining the final stages of wine fermentation the 15 to 5 Brix hydrometer is extremely helpful.

Because in the fermentation process you are converting sugar to alcohol. Alcohol is less dense as the original juice/sugar mix. So as fermentation proceeds the buoyancy of each type of hydrometer also changes. So if you want to know exactly where your at in the process you would use a hydrometer that will be in the range of where you want your fermentation finish. Also the narrow range hydrometers have a built in thermometer that allows you to add any correction due to sample temperature. Hope this helps!!