The Central Tools, Inc. DEF Refractometer, No. 3R201, measures the percentage of urea in the DEF. EPA regulations require all diesel engines manufactured on or after January 1, 2010 to meet lowered NOx emissions standards. These new engines require the periodic addition of Diesel Exhaust Fluid (DEF a urea solution) to enable the process. This kit includes the refractometer, dropper, lens wipe cloth, screwdriver (for calibration) and case.