In fruit juice, sugar concentration is measured by

The sweetness or sugar content of a liquid such as fruit juice is estimated using the Brix scale, which expresses the percentage of dissolved solids (predominantly sucrose) by weight. This is measured using refractometry: as light passes from air into a liquid, it bends (refracts) by an amount that depends on how much is dissolved in it — broadly, the more sugar present, the more the light bends. A refractometer measures this bending (the refractive index) and converts it into a calibrated Brix reading, the industry-standard estimate of sugar concentration.

In practice, a drop of fruit juice is placed on the refractometer's prism and the instrument reads off the refractive index as a Brix (percentage soluble-solids) value — this is exactly how sugar concentration in fruit juice is estimated, both in laboratories and with handheld field devices used by fruit growers and juice processors.

Comparing with the other instruments named here:

• potometer — measures the rate at which a cut plant shoot takes up water, used to study the rate of transpiration; it says nothing about how much sugar is dissolved in a liquid.

• chinometer — not an established instrument for measuring the dissolved-solid or sugar content of a liquid; the name does not correspond to any recognised measurement principle used for this purpose.

• tendrometer — measures the firmness or tenderness (a mechanical/textural property) of produce such as peas, not the amount of dissolved sugar in a liquid.

Because sugar concentration in a liquid is fundamentally assessed via an optical, refractive-index-based measurement, the refractometer is the instrument that fits — it is the basis of the Brix scale that the food and beverage industry uses as the standard estimate for grading fruit-juice sweetness.