A High-Performance Hygrometer for Aircraft Use: Description, Installation, and Flight Data

Abstract

A new cryogenic hygrometer designed for airborne research has been developed. It is based on the well-known chilled-mirror technique, but instead of the usual Peltier elements, liquid nitrogen is used for mirror cooling. The resultant greatly increased cooling capacity allows operation over a dewpoint and frost-point temperature range of ?90° to +30°C, with response times of a few seconds. The instrument has operated aboard the DLR Falcon aircraft for about 2 years. During stratospheric flights frost points down to ?80°C were measured, which at that altitude corresponded to mixing ratios below 10 ppmv. During its flights the Falcon encountered abrupt changes in humidity, data from which are shown. The hygrometer has also been flown in intercomparison flights with other aircraft carrying a variety of humidity instrumentation. The agreement with humidity data from other instruments aboard the Falcon in shown in detail. This paper describes the concept, technical details, and operating characteristics of the instrument. Its installation on the DLP Falcon is also described, and the results of its operation in both test and research programs are presented, along with flight data.