UV Absorption Hygrometer for Fast-Response Airborne Water Vapor Measurements

A next-generation vacuum-ultraviolet (Lyman-alpha) absorption hygrometer for high-rate research aircraft humidity measurements designed by the National Center for Atmospheric Research is described. It retains the high data rate, optical and mechanical simplicity, and low maintenance of previous Lyman-alpha hygrometers, while incorporating modern electronics and rugged, long-lived commercially available lamps and detectors. The mass of the sensing head is 2.0 kg in a volume of 3700 cm(3), while the power supply is 1.3 kg mass in a volume of 1100 cm(3). Power draw is 0.2 A at 120 V alternating current (AC). In bench and aircraft flight testing the prototype shows a bandwidth of 35 Hz and mixing ratio noise of +/- 0.5% over a water vapor mixing ratio range of 2-15 g kg(-1). This range can be scaled to lower mixing ratios by increasing the path-length. This performance enables measurements of water vapor concentration with high spatial resolution from research aircraft. The prototype instrument has flown over 380 h with minimal maintenance or repairs.