The OTC DEF Refractometer, No. 5025, is a portable, precision, optical instrument used for measuring the DEF concentration. When a liquid sample is placed on the prism, the light passing through it is bent. The more concentrated the liquid, the more the light will bend. It contains a reticle, or scale, that is enlarged through the eyepiece to measure this light; and the values on the scale have been established to evaluate the DEF condition. The OTC 5025 kit includes the refractometer, a dropper, the lens wipe cloth, instructions and a quick reference guide. It also includes a screwdriver and a durable case.