Degrees Brix (symbol °Bx) is the sugar content of an aqueous solution. One degree Brix is 1 gram of sucrose in 100 grams of solution and represents the strength of the solution as percentage by mass. If the solution contains dissolved solids other than pure sucrose, then the °Bx only approximates the dissolved solid content. The °Bx is traditionally used in the wine, sugar, carbonated beverage, fruit juice, and honey industries. (source: Wikipedia)
The refractive index of a liquid is related to its concentration and so a refractometer can display the sugar concentration in °Brix (sucrose) also known as Refractive Index / Brix (RI-BRIX).
YSI analyzers are used extensively in the Food/Beverage space for raw materials testing, in-process analysis, final product QC testing and effluent monitoring.