Abstract: A particulate and free water contamination measuring apparatus includes a slide tray adapted for carrying either particulate sample filter elements or free water sample filter elements. The slide tray carrying the filter elements is inserted in a reading chamber whereat, in the particulate mode, the apparatus reads the opacity of two sample filters and, in the free water mode, the apparatus reads fluorescence of the free water filter element. The voltage signals representative of particulates or free water are digitized by an analog to digital converter and read and stored by a digital computer. The digital computer monitors the location of the slide tray within the reading chamber by monitoring location detection switches operable in response to the location of the slide within the reading chamber.