Diagnosis of the DEF system requires measuring the percent concentration of the DEF, approximately 32.5%. GM-50422 is a refractometer that allows a direct reading of the DEF concentration. To determine the DEF concentration, a few drops of the DEF in question are placed on the sight glass, and the cover is closed. GM-50422 is held towards a light source, and the DEF casts a shadow, which is visible through the eye Piece. The edge of the shadow can be read on the scale in the viewer, and shows the concentration of the DEF.