The refractometer is a portable precision optical instrument used for measuring the diesel exhaust fluid concentration When a liquid sample is placed on the prism the light passing through it is bent and the more concentrated the liquid the more the light will bend The refractometer 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 EPA regulations require all diesel engines manufactured on or after January 1 2010 to meet lowered NOx emissions standards Most heavy duty engine manufacturers have chosen to utilize SCR including Detroit Diesel DD13 DD15 DD16 Cummins ISX line PACCAR and Volvo-Mack