The easiest and most popular way to measure sugar content of any liquid substances, sometimes also solids, is by refractometer method. Bunches of grapes could be sampled for average sugar content before harvesting. A way to ensure appropriate sugar content prior to wine processing. This also lessen the addition of commercial white sugar. In juice processing. Initial sugar contents could be easily measured by refractometer. Then computational adjustment could be done shortly.
Refractometer is simply a device to measure percentage sugar. And it is not limited to only one. It could be design to read salt and alcohol content and the basic > specific gravity.
Hydrometers which is being use to measure specific gravity could be use as cheap refractometer alternative. There are hydrometers that are specifically designed for sugar, alcohol and salt measurement. They require relative large amount of sample, 100 ml to 1 liter depending on instrument design and length. These are not suitable for grape sampling. Imagine how many grapes are needed to fill a graduated cylinder.
Refractive index. I won’t say it is a degree of refraction. Here is a more straightforward explanation. It is the solid horizontal line seen every time a liquid sample is placed on refractometer prism. Peep through the eyepiece and see it for yourself.
The °Brix. Refractometer scale is preconverted to °Brix. One measure is equivalent to 1 gram sugar per 100 grams sample or 1 percent. The measurement is weight over weight (w/w), not weight over volume (w/v).
Are sugar refractometers accurate?
Add a known amount of white sugar to measured volume of distilled water. Mix thoroughly and do a refractive measurement. You will get an accurate value if you did it right.
Refractometers measure not just sugars. It measure other dissolved solids too, like: minerals, vitamins, proteins, amino acids, hormones, etc… So when fruit °Brix measurement is 12. It means 12 percent of sugars and other stuff. The readings are usually expressed in term of Soluble Solids to cover up the discrepancy.