The detectors covered by this specification are intended for use at Naval shore activities and aboard ships. The combined contaminated fuel detector is comprised of two portable instruments used to quickly determine the quantity of solid contamination and free water present in middle distillate fuels. The detector processes a sample of a fuel on site in usually three to four minutes, and establishes the level of contamination in the fuel. The level of contamination is measured by using the principle of differences in light transmission through membrane filters. For greater accuracy and to eliminate any fuel color effect, the two membrane filters are used in series. The first filter traps the solid contaminant, plus any fuel color effect. The second filter is subjected to only fuel color effects. The light transmission through the two filters is measured by a light sensitive cell system. Determining the difference between the amount of light transmitted through the contaminated and clean filters establishes the level of solid contamination in the fuel. The established level is evaluated with the calibration curve supplied with each detector to ascertain if the fuel is contaminated beyond required limits. All components necessary for filtration and measuring of transmitted light are incorporated into this one serviceable instrument. Also incorporated is a free water detector conforming to the requirements of ASTM D3240 or MIL-S-81282. A fuel sample is passed through a uranine-coated free water detector pad which is inserted in the pad holder; under the lamp illumination, free water absorbed by the pad fluoresces, and yields the fuel free water reading in parts per million. This product is known to produce invalid results with +100 additized fuels and therefore should not be used to determine particulate or free water content of +100 additized fuel. Due to the natural fluorescence of some diesel fuels, procedures for testing diesel fuels in these units may differ from aviation (kerosene) fuels.