Brix is a measure of sugar content by weight in an aqueous solution. 1 degree Brix is equivalent to 1 gram of sucrose in 100 grams of water at a temperature of 20 degrees Celsius. We measure Brix in berries in the vineyard (to track their maturity) and in the must during fermentation (to track the conversion of sugars into alcohol).
We use a simple Optical Refractometer (picture) to measure the sugar content of grapes.. The instrument measures the optical refraction of sunlight as it passes through a thin layer of sugary water. The measurement is reasonably accurate under normal temperatures around 20 °C – more sophisticated digital instruments would able to compensate for deviations of temperature.
We use Hydrometers (picture) to measure the sugar content of must during fermentation. They measure the relative weight of the sugary alcoholic solution vs. a standard. We use two different hydrometers – one at the beginning for high sugar content, the other at the end of fermentation to get better accuracy at low sugar contents. Because the must contains suspended particles, the measurement is not immediate; the suspended particles have to be given time to settle out in a fridge before the measurement is taken.
Densitymeters and Spectrometers
A more sophisticated tool would be digital Density meters (they can measure brix and alcohol and compensate for temperature).Even more sophisticated tools would be Spectrometers to measure Brix levels near zero, they actually measure the existence of glucose & fructose molecules in the must.