The detectors covered by this specification are intended for use at Naval shore activities and aboard ships where fuel is dispensed to aircraft. The basic contaminated fuel detector is a portable instrument which is utilized to quickly determine the quantity of solid contamination present in aircraft fuels. The detector processes a sample of an aircraft 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.