About Degrees Brix (°Bx) To Specific Gravity (SG)
Using a refractometer is a quick way to determine your present gravity (and efficiency) while in the process of brewing. Additionally, it uses only a few drops of wort as opposed to a much greater amount that is required for taking a gravity reading with a hydrometer and trial jar. Being that both refractometers and hydrometers are calibrated to take readings at room temperature, this is a great time savings as well since it’s much easier to lower the temperature of a few drops of wort as opposed to a full trial jar of wort.
Most refractometers are in units of Degrees Brix (oBx), which describes the amount of sugar in an aqueous solution. By definition, 1 gram of sucrose in 100 grams of solution is one degree Brix which represents the strength of the solution as percentage by mass. Degrees Brix is traditionally used by vintners during wine production. It is also used in the production of various foods in which sugar content is important (e.g. fruit, maple syrup, and honey).
Brewers work in either units of Specific Gravity (SG) or Degrees Plato (oP). Degrees Plato is very close to Degrees Brix, but not exactly the same. Some refractometers will also have scales alongside Degrees Brix in Degrees Plato or Specific Gravity. Specific Gravity in particular is not linear in relation to either Degrees Plato or Degrees Brix, so caution is advised when reading Specific Gravity from the scale of a refractometer.
Wort Correction Factor
Although wort contains sugar, it’s not primarily sucrose and there are other dissolved compounds besides sugar. Additionally, models of refractometers can be different, so you’ll need to determine a Wort Correction Factor. The most common Wort Correction Factor used is 1.04. Ideally, the Wort Correction Factor should be determined through sampling a variety of worts of known density. These measurements should be taken after the refractometer has been calibrated to zero with water and at the appropriate temperature.
Degrees Brix (°Bx) To Specific Gravity (SG) Once Fermentation Begins
Note that once the solution being measured contains alcohol, the refractometer readings become increasingly incorrect with greater alcohol content. Due to this, most brewers will use a hydrometer to take a direct density measurement once fermentation begins.