Abstract: A compact portable instrument for determining the percentage concentration of a material in a solution such as the amount of concentrate in an output from an Aqueous Film Forming Foam generator used to extinguish fires. The instrument comprises a refractive index sensor with a reflective type refractometer prism having a measuring surface in contact with liquids placed in a sample holder formed by an opening through a retainer plate. The measuring surface of the prism covers that opening and presses the retainer plate towards an outer surface of the instrument, that outer surface having entrance and exit ports for sample liquids at that opening. The refractometer senses the refractive index of liquid in the sample holder and, when properly calibrated, provides an output to a digital display that directly indicates the percentage concentration of material in the liquid.