What instrument is used to test sugar content in grapes?

The refractometer is specifically designed to measure the sugar content in grapes, making it the correct choice for this question. It operates on the principle of refraction, which is the bending of light as it passes through different mediums. When light passes through a grape juice sample, the degree to which it bends is directly related to the concentration of sugars present. This allows viticulturists and winemakers to determine the potential sweetness of the grapes, which is critical for deciding the optimal harvest time and predicting the quality of wine that can be produced.

In contrast, while a hydrometer measures the specific gravity or density of liquid, which can also relate to sugar content, it requires a larger sample and is less directly focused on sugar concentration in the context of grape juice. A pH meter measures acidity and not sugar content, which is essential for assessing the balance and quality of grapes but does not provide any information regarding how sweet the grapes are. Lastly, a thermometer is used to measure temperature, which is unrelated to sugar content measurement altogether. Thus, the refractometer stands out as the most suitable and specific instrument for testing sugar levels in grapes.