When you remove the daily post from your mailbox, how often do you find a catalog chock full of shiny new homebrewing toys? Anywhere from four to six of these arrive at my house per month, and although the products remain mostly the same from one issue to the next, I never fail to browse the whole thing cover to cover every time. The siren’s song of new gadgets is simply too beautiful to ignore.
I managed to ignore the refractometer for a long time. After all, a hydrometer is much cheaper and measures wort specific gravity just as well. But refractometers do offer some nice advantages. Only a small wort sample is needed, a couple of drops rather than a test tube’s worth. A refractometer won’t roll off the counter and onto the floor, shattering into countless shards of glass. Also they look cool.
The refractometer operates on the optical principle that light passing through a liquid refracts at different angles according to the density of that liquid. Rephrasing that statement, light passing through a small sample of wort refracts at different angles according to the concentration of dissolved sugar in that wort. The refracted light illuminates a scale, upon which is printed a range of wort densities. You peek through an eyepiece and read the wort density from the scale.
The refractive index of a liquid doesn’t just change with density; it also changes with temperature. Most good refractometers feature automatic temperature compensation (ATC) and will report the same value over a wide range of practical temperatures, usually between 55 and 85°F (13 and 29°C). ATC is a convenience well worth seeking out because a couple of drops of even boiling wort will quickly cool to within this range. This is of particular advantage to all-grain brewers who want to monitor the gravity of runoff, which is typically around 170°F (77°C).
Units of Measurement
Most refractometers available to homebrewers these days are dual-scale instruments that report results in both Brix and specific gravity units of measurement. Brewers usually prefer to express density in either specific gravity or Plato units, while winemakers often prefer Brix. In fact, Brix and Plato are closely related, and the two units may be considered equal to within 5 percent.
If you are accustomed to working in specific gravity (SG) units, then take a second look before pulling the trigger on a too-good-to-be-true refractometer bargain to make sure it includes a specific gravity scale. Working in Brix isn’t terribly inconvenient, but you don’t want to be surprised on brew day.
Final Gravity Adjustment
One area where the hydrometer remains superior to the refractometer is in measuring final gravity (FG). Unlike reading a hydrometer, which simply floats in the mixture of water, sugar, and ethanol that we call beer, using a refractometer to get FG is a little tricky. To wit, you’ll need to know the gravity of the original wort, and then you have to perform a conversion. You can calculate it by hand (not recommended), or numerous tools such as BeerSmith include refractometer calculators that will do it for you. This shouldn’t make or break your decision to purchase one of these optical wonders, but keep it in mind so that you have realistic expectations.
Ultimately, the refractometer is simply another tool for your homebrewing arsenal. It won’t replace the trusty hydrometer, but in the hands of an educated brewer, it can be a valuable investment.