Refractometers measure how much light bends as it goes through certain liquids. By placing your coolant mix on the prism and putting the prism cover in place, you create a mini-prism of liquid. If the liquid is pure water, then the refractometer will read zero on its scale (in fact, the light does bend somewhat, but the refractometer is designed so the water is the baseline).
The scale on the unit is based on the Brix Scale, which was designed to measure sugars in water. However, this scale is still used to compare coolant concentrations. Many coolant compounders have done a good job of compounding their fluids to that of the Brix Scale, so your readings are a one-to-one correspondence. Check with your coolant supplier to see if your coolant measures 1:1 with the Brix scale. If not, ask them for the refractometer factor, which you will need to multiply the scale reading by for the actual concentration.
Browse our analog refractometers or our digital versions.