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The 'Öchsle Scale' (or 'Oechsle scale') is a hydrometer scale measuring the density of grape must^[1], which is an indication of grape ripeness and sugar content used in wine-making. It is named for Ferdinand Öchsle and it is widely used in the German, Swiss and Luxemburgish wine-making industries. On the Öchsle scale, one 'degree Öchsle' (°Oe, or more seldom °Ö) corresponds to one gram of the difference between the mass of one litre of must at 20 °C and 1kg (the mass of 1 litre of water). For example, must with a mass of 1084 grams per litre has 84 °Oe.
The mass difference between equivalent volumes of must and water is almost entirely due to the dissolved sugar in the must. Since the alcohol in wine is produced by fermentation of the sugar, the Öchsle scale is used to predict the maximal possible alcohol content of the finished wine. This measure is commonly used to select when to harvest grapes. In the vineyard, the must density is usually measured by using a refractometer by crushing a few grapes between the fingers and letting the must drip onto the glass of the refractometer. In countries using the Öchsle scale, the refractometer will be calibrated in Öchsle degrees, but is an indirect measure, since the refractometer actually measures the refractive index of the grape must.
The Öchsle scale forms the basis of most of the German wine classification. In the highest quality category, Prädikatswein (formerly known as ''Qualitätswein mit Prädikat'', QmP), the wine is assigned a ''Prädikat'' based on the Öchsle reading of the must. The regulations set out 'minimum' Öchsle readings for each Prädikat, which depend on wine-growing regions and grape variety^[2]:
:Kabinett - 67-82 °Oe
:Spätlese - 76-90 °Oe
:Auslese - 83-100 °Oe
:Beerenauslese and Eiswein - 110-128 °Oe (Eiswein is made by late harvesting grapes after they have frozen on the vine and not necessarily affected by noble rot, botrytis, which is the case with Beerenauslese)
:Trockenbeerenauslese - 150-154 °Oe (affected by botrytis)
The sugar content indicated by the Öchsle scale only refers to the unfermented grape must, never to the finished wine.

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Other scales

References

Other scales

In Austria the 'Klosterneuburger Mostwaage' (KMW) scale is used. The scale is divided into 'Klosterneuburger Zuckergrade' (°KMW), and very similar to the Öchsle scale (1 °KMW =~ 5 °Ö). However, the KMW measures the exact sugar content of the must.
The Baumé scale is occasionally used in France and by U.S. brewers, and in the New World the Brix scale is used to describe the readings of a refractometer when measuring the sugar content of a given sample. All of these methods are similar and the differences are more cultural than significant, but all are equally valid ways to measure the density of grape must and other sugar based liquids.

References

1. By a slight abuse of physical terminology one says in German that the 'Mostgewicht' (must weight) is measured rather than the must's density.
2. Deutsches Weininstitut: Must weights