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Dear Dr. Vinny,

What’s the difference between a refractometer and a saccharometer? Can wine producers use them as substitutes for each other?

—Kreetika, India

Dear Kreetika,

OK, let me see if I can answer this so the kids at home can follow along. Both a saccharometer and a refractometer are tools that a winemaker can use to measure the sugar content of grape juice. This can help in the vineyard when deciding when to pick grapes, as well as while making decisions during the fermentation process, as the sugar converts to alcohol. It’s kind of like taking your wine’s sugar temperature.

So how are they different? To start with, they work differently. A saccharometer is a type of hydrometer, which is an instrument that measures the specific gravity or density of liquids. I used one in high school science class—it’s made up of two parts, a cylinder where you put the liquid you’re measuring, and a weighted stem that floats on the liquid. In the world of hydrometers there are various specializations, including a saccharometer, which is created specifically for determining the amount of sugar in a liquid. The higher the sugar content, the denser the grape juice, and the higher the bulb will float.

Meanwhile, a refractometer measures the sugar in grape juice by measuring its refractive index, which deals with how much a light is bent or refracted. A refractometer can look something like a miniature telescope that you put a drop of juice in and then hold up to a light source to get a reading.

Besides the science, what’s the functional difference between the two? I hear saccharometers can be a bit temperamental when it comes to temperatures; because they’re based on floating, if you have a lot of solids in your grape juice sample, it can mess with the results. Saccharometers also tend to be made of glass and therefore more breakable, and they also need a fair amount of liquid to pour into the cylinder. Refractometers can fit in your back pocket, and because they only need a couple of drops to test, the juice density isn’t so much of an issue in the measurement.

So, to answer your question about using one as a substitute for the other, the answer is basically yes, but it depends on the circumstances—a saccharometer is better suited for use in a laboratory or other indoor setting, while a refractometer might be preferable while working outdoors, in the field.

—Dr. Vinny

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