Central Tools 3R201 UREA Refractometer 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. To ensure the proper operation of the system, operators must ensure the DEF in their vehicle consists of a 32.5% Urea / Water solution