Water can occur in jet fuel in three forms, dissolved (chemically in solution in the jet fuel), settled (as in free water at the bottom of the tank), or as finely dispersed undissolved water held in suspension. The Shell Water Detector is designed to detect undissolved water; it has been proven reliably for 30 years. This is water that a filter separator can remove, and is usually invisible to the eye in quantities below 40-60 ppm.