A HIGH-PERFORMANCE HYGROMETER FOR AIRCRAFT USE - DESCRIPTION, INSTALLATION, AND FLIGHT DATA

A new cryogenic hygrometer designed for airborne research has been dev eloped. It is based on the well-known chilled-mirror technique, but in stead of the usual Peltier elements, liquid nitrogen is used for mirro r cooling. The resultant greatly increased cooling capacity allows ope ration over a dewpoint and frost-point temperature range of -90-degree s to + 30-degrees-C, with response times of a few seconds. The instrum ent has operated aboard the DLR Falcon aircraft for about 2 years. Dur ing stratospheric flights frost points down to -80-degrees-C were meas ured, which at that altitude corresponded to mixing ratios below 10 pp mv. During its flights the Falcon encountered abrupt changes in humidi ty, data from which are shown. The hygrometer has also been flown in i ntercomparison flights with other aircraft carrying a variety of humid ity instrumentation. The agreement with humidity data from other instr uments aboard the Falcon in shown in detail. This paper describes the concept, technical details, and operating characteristics of the instr ument. Its installation on the DLR Falcon is also described, and the r esults of its operation in both test and research programs are present ed, along with flight data.