The Brix scale was developed in the 1800's by several people, including Adolph Brix, Karl Balling, and Fritz Plato. One degree of Brix measures 1 gram of sucrose in 100 grams of an aqueous solution, as a percentage of the weight of the sample solution.
Originally, the degrees Brix scale was a measurement of specific gravity using a hydrometer, and it was mostly used by vintners and brewers. Many beverage producers still use hydrometers to measure “sugar.” Over time a prism-based refractometer was developed. This type of refractometer uses a prism to bend the light that has passed through the sample, toward a scale that is calibrated in percentages. There is a dark-light boundary falling onto the scale when viewing through the eyepiece. That boundry line will fall somewhere on a set of lines and percentage numbers.
Different refractometers are available that are calibrated according to the specific use, such as wine-making, brewing, sugar manufacturing, and checking the sugar content of fruits and vegetables.
The refractometer which I use is calibrated from 0% to 30%. That is more than enough for any fruit or vegetable you might grow and test. My unit has automatic temperature compensation (ATC), which eliminates conversion tables. The unit can be calibrated by turning an adjustment screw and a using a test solution.
Why use Brix testing? Degrees Brix shows the sugar, or sucrose content of your plant. Why is that important? Plants use sugar in many different ways to build structure, like polysaccharides such as starch for cellulose, hemicellulose, and lignin. Food and cell-matter is created for the plant by using photosynthesis and nutrients for making carbohydrate polymers called monosaccharides, such as glucose and fructose. The sugar content of a healthy plant directly correlates to nutrient quality and availability, temperature, pH, and moisture availability.
Some growers are avid (almost rabid) supporters of the use of degrees Brix refractometers for this reason. They are very vocal about store-bought, distance-shipped fruits and vegetables versus locally-grown, natural and on-plant ripened fruits and vegetables. There are a number of websites with refractometer comparisons of both. Locally grown natural stuff almost always wins the sugar-content tests. This makes some sense: fruits and vegetables that are handled and shipped are genetically modified to be more thick-skinned and hardy, and ripen during shipping, while local produce can be thinner-skinned, ripened in the field, and usually will contain more “pulp and juice.” More “pulp and juice” generally means better taste and “mouth feel.”
More scientific studies by agricultural universities are coming online about this correlation and more tables of percentages are showing up. There are charts available for virtually any fruit and vegetable.
I've been using my refractometer for awhile. It is not the “do-all, end-all” tool that some might want you to think. It is however, an important tool to measure the final growth cycle of your plant. It is not particularly useful during the other stages of plant growth, although I did use it during my watermelon experiment with some crushed stems and leaves from the plant. I averaged 3% of the total crushed samples. That's not too much lower that the fruit of some store-bought fruits and vegetables! If I had not pinched the main stem of the watermelon, which severely restricted water and nutrient uptake, I believe I would have grown a large, sweet watermelon out of a milk jug! See my video here.
In conclusion, it's best to use a degrees Brix refractometer in conjunction with other tools, such as a pH meter, or a “multimeter,” which measures pH, Total Dissolved Solids, and Electrical Conductivity (see my blog here). The multimeter gives on-going, “real-time” data and the refractometer will give you end-data to compare with previous rounds of growing the same plant. The combination will give you information for improving the growth of future crops.