- Availability: Specialty
A coffee refractometer (the VST and Atago models are most common) reads the TDS percentage of a brewed sample by measuring how light bends through it. Combined with the brew ratio, that number gives the extraction yield: the percentage of soluble coffee mass that ended up in the cup. Specialty bars use refractometers to dial in batch brew, validate espresso changes, and train baristas on what a "balanced" extraction actually measures.