What is a refractometer?
A refractometer measures how light bends through a fluid. Remember when you put a pencil in water and it looked “broken” as a kid? As it turns out, a bunch of smart folks a long time ago realized that the angle of the pencil’s break is related to the density of the fluid. In the automotive world, we can use that information to determine a given fluid’s concentration. The refractometer has a plate where a fluid can be deposited that allows light to pass through the liquid and a prism that has known characteristics.
Coupling the output of a given fluid with an appropriate scale, we in the service bay can determine the concentration of quite a few fluids—a very handy ability.
At the time of this writing, auto-specific refractometers can be had very inexpensively—$25 is about the going rate. You should know refractometers are used in determining the sugar concentration of jams and jellies, to determine the alcohol level (ABV) of wines and beers, and by apiarists to determine the moisture content of honey. All use different scales, so it’s important to look for an auto-specific model. Some will be calibrated for just one or two fluids, but it’s possible to obtain one with a series of carefully arranged scales like you can see in the image below. This arrangement allows a tech to test the concentration of battery acid, both types of coolant, DEF, and washer fluid—not shabby.