Using A Refractometers to Measure Starting Gravity and Final Gravity
Many inexpensive refractometers are used for determining the amount of sugar in a solution. They can do this because the refractive index changes with the amount of sugar in solution just as the density of the solution changes. These refactometers often use a scale marked in degrees Brix. Degrees Brix is the measure of sugar in a solution. 1 degree Brix means that 1% of the solution is sugar. A refractometer can be scaled in degrees Brix using a one to one mapping between degrees Brix and the refractive index. Solutions used for producing alcoholic beverages have other components that also affect the refractive index reading so in these solutions the Brix reading is not entirely percent sugar but includes other dissolved solids.
However, the refractive index for solutions that have both sugar and alcohol is more complex. For any given percent sugar + other dissolved solids, we get a mapping from the refractive index to percentage of alcohol. So to make sense of the refractometer reading we need to know both the specific gravity and refractive index. We can use a hydrometer to measure the specific gravity and then use the refractometer to estimate the alcohol. The chart below shows how a mapping for the refractive index to %ABV for various specific gravities can be done.
%ABV by RI (in Brix Eq) given Specific Gravity
The equation for this is given by:
%ABV = 1.646 * RI – 2.703 * (145 – 145 / SG) – 1.794
Refractometers like the one in the picture above have Automatic Temperature Correction (ATC). They are used by placing a drop of sake on the plate and closing the cover. You then look through the eye piece and read off the value where the shade changes colors.
After measuring both the specific gravity with a hydrometer and the refractive index (given in Brix equivalents) with a refractometer, plug these values into the above equation and again, voila we get the %ABV for our sample.