Listen up, here come some Science bits.
The specific gravity or (SG) of a wine or beer is the density of the wine/beer in relation to the amount of dissolved sugar in it, the more sugar (less alcohol) the higher the SG and the less sugar (more alcohol) the lower the SG. Water typically has an SG value of 1.000. Specific Gravity is measured in degrees and the tool we use to measure it is called a Hydrometer. The Hydrometer is the single most important piece of kit in the amateur winemaker’s or brewer’s toolbox.
Regularly measuring the specific gravity of a wine will enable us to stop the fermentation at the correct time so that we can make a Sweet, Medium or Dry wine depending on the amount of sugar we decide leave unfermented in the wine. The specific gravity will also enable us to calculate the ABV (Alcohol by Volume) of the wine/beer.
Whether we’re making wine or beer, we always take a Specific Gravity reading of the Must (wine) or the Wort (beer) before we pitch the yeast. This is called the OG or Original Gravity. This is our starting marker. When fermentation has completely finished or if we decide to stop the fermentation at a specific SG reading, we call this value the FG or Final Gravity. Alcohol by Volume is calculated by the following formula:
ABV% = (OG-FG)/7.36
Hydrometers are calibrated to a specific temperature (usually 20 C) so in temperatures above 20 C, the recorded SG will be lower than it actually is and in temperatures below 20 C the recorded SG will be higher. This is partly why keeping a constant temperature of around 20/25 C for the entire fermentation process is very important. Varying temperatures will produce varying and inaccurate SG readings however if the temperature remains constant throughout, the SG readings, while slightly out will produce more accurate results over the fermentation period.
To take an SG reading of your wine/beer, syphon some of the wine/beer off into a trial jar and float the hydrometer in it. Spin it gently between your finger and thumb to remove any CO2 bubbles that may affix themselves to the hydrometer and produce inaccuracies and take the reading from the bottom of the meniscus.
Regular SG recordings are very important as they give a very good indication of what is happening in your wine/beer and can alert you to stuck fermentations, finished fermentations and the times to stop the fermentation.