The humidity was monitored using a chilled mirror hygrometer (FINEDEW, Yamatake Corp.). This instrument has a mirror cooled by a Peltier element; dew condensation on the mirror is optically detected and the dew point is determined. The measuring section is a cylindrical device with a diameter of 12 mm and a length of 60 mm and is connected to the data processing section by an optical fiber. A schematic diagram of the measuring section is shown in Fig. . The 90 % and 50 % response times for a stepwise humidity variation from RH = 30% to 60 % are 30 s and 7 s, respectively. The corresponding response times for a variation from RH = 60% to 30 % are 40 s and 5 s.