ELHYSA acronym of Etude de L'Hygromérie Stratosphérique 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. Also its necessary to take many precautions to avoid pollution from the balloon and from the payload itself.
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.
The second payload on the balloon was an instrument called DIRAC which stands for Determination In situ by Rapid Analytical Chromatography. It is a portable, lightweight gas chromatograph aimed to perform fast analysis of a single tracer to maximize the vertical resolution of ascent/descent profiles. It was developed by the University of Cambridge based on the techniques used in another instrument called DESCARTES. The sample in DIRAC is collected on a Carboxen trap by passing through it a known volume of air. The use of the carbon based Carboxen adsorbent enables a wide range of halocarbons to be pre-concentrated prior to analysis, thus extending the potential range of measured species. DIRAC has a maximum autonomy of 10 hours and works automatically using a PC-104 embedded computer. Data is stored on board and can also be transmitted to ground via a telemetry link. It's overall weight is 21 kg.
Balloon launched on: 5/5/1999 at 17:43
Launch site: Centre de Lancement de Ballons CLBA, Aire Sur L'Adour, Landes, 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 90
End of flight (L for landing time, W for last contact, otherwise termination time): 5/5/1999 at ~ 20:45
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 3 h 2 m
Payload weight: 377 kgs
Gondola weight: 146 kgs
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