MOZAIC Capacitive Hygrometer
10 The compact airborne MCH consists of a capacitive sensor (Humicap-H, Vaisala, Finland)
whose capacitance depends on the relative humidity of the dielectric layer of the
condensor and a platinum resistance sensor (Pt100) for the direct measurement of the
temperature at the humidity sensing surface. The basic measurement process is based
on the diffusion-limited adsorption of the H2O-molecules by the dielectric membrane of
15 the sensor. Since diffusion is strongly temperature-dependent, the sensor response
slows down at lower temperatures. Figure 1 shows how both sensors are mounted in
the used air sampling housing (Model 102 BX, Rosemount Inc.; see Stickney et al.,
1990). The relative humidity and temperature signals are linearized by a microprocessor
controled transmitter unit (HMP230, Vaisala).
20 In its original MOZAIC mounting position aboard an Airbus A340-400 the sensor
housing is placed approx. 7 m downstream of the aircraft nose on the left side with
a 7 cm distance from the aircraft skin to avoid possible contaminating interferences with
the aircraft skin. Inside the Rosemount housing the air flow is separated into the main
flow, which traverses straight through the housing and the minor flow, which follows
25 a sharp right angle into a smaller channel where the sensors are placed. The housing
is equipped with small holes in the side wall to minimise internal boundary layer effects.
9806
MOZAIC Capacitive Hygrometer10 The compact airborne MCH consists of a capacitive sensor (Humicap-H, Vaisala, Finland)whose capacitance depends on the relative humidity of the dielectric layer of thecondensor and a platinum resistance sensor (Pt100) for the direct measurement of thetemperature at the humidity sensing surface. The basic measurement process is basedon the diffusion-limited adsorption of the H2O-molecules by the dielectric membrane of15 the sensor. Since diffusion is strongly temperature-dependent, the sensor responseslows down at lower temperatures. Figure 1 shows how both sensors are mounted inthe used air sampling housing (Model 102 BX, Rosemount Inc.; see Stickney et al.,1990). The relative humidity and temperature signals are linearized by a microprocessorcontroled transmitter unit (HMP230, Vaisala).20 In its original MOZAIC mounting position aboard an Airbus A340-400 the sensorhousing is placed approx. 7 m downstream of the aircraft nose on the left side witha 7 cm distance from the aircraft skin to avoid possible contaminating interferences withthe aircraft skin. Inside the Rosemount housing the air flow is separated into the mainflow, which traverses straight through the housing and the minor flow, which follows25 a sharp right angle into a smaller channel where the sensors are placed. The housingis equipped with small holes in the side wall to minimise internal boundary layer effects.9806
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