The Fast In situ Stratospheric Hygrometer (FISH), developed at the Forschungszentrum Jülich (Germany), is based on the Lyman-a photofragment fluorescence technique. Details of the instrument and the calibration procedure are described in Zöger et al. [1999]. FISH has been used in several campaigns both from balloon and aircraft and compared with a large number of other hygrometers [Kley et al., 2000].
FISH consists of a closed, vacuum-tight fluorescence cell, a Lyman-a radiation source, a PMT in photon-counting mode, detectors to monitor the VUV radiation output of the Lyman-a lamp, and a mirror drive that controls the measuring cycle (see diagram): determination of the fluorescence and background count rate and of the lamp intensity. With a measurement frequency of 1 Hz, the noise equivalent mixing ratio at 3 ppmv is 0.2-0.15 ppmv, and the detection limit is 0.18-0.13 ppmv.
FISH is calibrated between flights in the laboratory using a calibration bench under realistic conditions, that is varying the H2O mixing ratio of the test air from a few ppmv to several hundred ppmv and the pressure from 1000 to 10 hPa. A frost point hygrometer is used as a reference instrument. The overall accuracy of FISH measurements is 5-6 %.
