What is a Refractometer and how do you use one?
A refractometer is a device which measures the sugar content of a solution by the refraction of light. It performs a very similar task to a simple hydrometer. Most refractometers give a reading in Brix, and some in specific gravity. Using a refractometer is simply a matter of placing several drops of the sample liquid on the angled prism and sealing the clear plate on top of it. Look through the eyepiece while pointing the refractometer at a source of direct light (do not look directly at the light with the naked eye).
One benefit to a refractometer is that most models have an automatic temperature conversion, so samples can be taken directly from a mash or boil kettle. The second benefit is that only a very small sample is needed. A couple drops of liquid will give an accurate reading. This can be helpful when all grain brewing or during fermentation when taking multiple hydrometer-sized samples is time-consuming and ends up wasting more beer.
The downsides to refractometers are that they’re more expensive, they require direct light to take a reading, and alcohol will skew the reading. The last of these is the biggest concern. Readings taken after fermentation has started will tend to read higher than they actually are, and some sort of calculator must be used to make a conversion. We have provided a calculator to convert final gravity refractometer readings and to convert from Brix to specific gravity: http://www.northernbrewer.com/refractometer-calculator