Is a Frost Point Hygrometer developed by the Laboratoire de Meteorologie Dynamique (LMD) of France in the late 80's for make "in situ" measurements of the contents of water vapor in the stratosphere. The weak water vapor contents, and the surrounding atmospheric conditions make this kind of measure very difficult and also its necessary to take many precautions to avoid pollution from the balloon and from the payload.
ELHYSA works by the so called chilled mirror technique, in wich the air enters the instrument and circulates over a cooled mirror. The occurrence of dew or white frost is detected in an optical way and the temperature of the mirror is maintained at the temperature of condensation by a feedback loop controlling the cooling and the heating of the mirror. This temperature, the dew or frost point temperature is directly connected to the partial pressure of water vapor that characterizes the water vapor content of the atmosphere. The simultaneous measurement of the air pressure and air temperature allows the determination of the mixing ratio and the relative humidity.
In a previous version of the instrument used prior 1994, the control of the temperature of the mirror was provided by a Peltier thermoelectric module, but it has several limitations so it was replaced by a cryogenic cooling system. In this new configuration the mirror is stuck on a copper rode plunging in liquid nitrogen enclosed in a dewar container, especially adapted, which provides a permanent cooling. The control of the mirror temperature is made through the adjustment of the heating of the mirror which is provided by a resistive wire rolled up around the copper rode. The use of cryogenic cooling made it possible to decrease considerably the response time of the hygrometer, compared to the Peltier instrument. The optical system and the associated mechanical structure remain the same ones as for the Peltier instrument as well the mirror and the embedded thermistor.
Balloon launched on: 6/21/1994 at 8:40
Launch site: Aérodrome de Gap-Tallard, Haute Alpes, France
Balloon launched by: Centre National d'Etudes Spatiales (CNES)
Balloon manufacturer/size/composition: Zero Pressure Balloon model 35SF Zodiac - 35.000 m3
Balloon serial number: 35 SF Nº 57
End of flight (L for landing time, W for last contact, otherwise termination time): 6/21/1994 at ~ 18:30
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 9 h 52 m
Payload weight: 378 kgs
Gondola weight: 87 kgs
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