Water is normally present in small amounts in liquid fuel, either dissolved in the fuel, in a separate liquid phase known as “free water”, or in a fuel-water emulsion. The existence of free water or a fuel-water emulsion can be dangerous, especially in the aerospace industry where frozen water poses a significant risk to aircraft, passengers and crew. It is therefore desirable to monitor the amount of water dissolved in fuel—to measure how close the fuel is to saturation—and to indicate the risk of free water or fuel-water emulsion formation.

A variety of sensing systems are currently used to measure existing free water or fuel-water emulsions, by measuring the resistive properties of free water, the dielectric properties of the emulsion, or differences in the refractive index of light propagating through multiple sensors. Other sensing systems measure changes in the refractive index of special optical fibre coatings to detect the amount of water dissolved in fuel. Each of these sensing systems tends to be highly complex and expensive.

Researchers at Aston University have invented a rapid and highly sensitive device for detecting minute levels of water dissolved in liquid fuel. The device uses an uncoated optical fibre probe fabricated from a permeable material that is highly sensitive to water. The presence of water therefore directly affects the optical properties of optical fibre in the probe. The device is sufficiently sensitive to monitor very small amounts of water dissolved in fuel (better than 10 ppm).