Which instrument is used to measure grape must?

Grapes do not have "degrees" per se; what is measured is the sugar content in the must, which helps estimate probable alcohol content. We recommend two instruments:

• Handheld refractometer with 3 scales – Ref. 720.5150
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  Measures °Brix, Baumé and probable alcohol. Requires only 2–3 drops.
• Mustimeter (glass hydrometer) with 2 scales – Ref. 414.
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  Used with a 250 ml glass cylinder.
  Recommended cylinder

Both options are reliable, but the refractometer is more practical and faster.