An essential piece of test equipment used to determine the sugar content in a liquid, in this case your beer wort. The rugged exterior of metal, rubber and plastic protects the highly polished optical glass, mirrors and prisms that are contained within. Once the sample is in place underneath the daylight plate, the brewer can see the percentage Brix reading by looking through the monocular / eyepiece and reading the scale that is seen when the refractometer is held toward natural light.
