Hydrogen gas (H2) is a colorless, odorless, combustible gas. It has many industrial applications, mostly in the processing of fossil fuels, metal re–ning, and the production of hydrochloric acid, methanol, and ammonia. It has been used as the combustion material on the space shuttle solid rocket booster, and was used as a lifting gas until the now-infamous Hindenburg disaster occurred. Hydrogen poses several safety concerns. The upper and lower £ammability limits of hydrogen in air are 4% and 75% by volume. Furthermore, pure hydrogen £ame emits ultraviolet light; thus, it is nearly invisible to the naked eye. Finally, when stored in liquid form, it is a cryogen. Therefore, in an industrial process where hydrogen is used, a robust sensor is necessary.