Brix (°Bx) is a graduated scale, used on a hydrometer, which indicates the weight of sugar per volume of solution at a given temperature.
Baumé(Bé°) is a hydrometer scale used to measure the specific gravity of liquids. It’s convenient because it gives winemakers an estimate of finished alcohol levels. Both Baumé (Bé°) and Brix (°Bx) scales give us a measure of soluble solids in grape juice.
Plato (°P) is a scale that expresses the density as the percentage of sucrose by weight. It’s primarily used in brewing industry to measure density of beer wort in terms of percentage of extract by weight.
Oechsle (° Oe) scale is measuring the density of grape must, which is an indication of grape ripeness and sugar content used in winemaking. This measure is commonly used by winemakers to decide when to harvest grapes and to predict the maximal possible alcohol level of the finished wine. The Oechsle scale forms the basis of most of the German wine classification.