Humidity Sensor (ICH, part of Package1)

Instrument PI:     Herman Smit, Forschungszentrum Jülich GmbH (

The ICH consists of a capacitive relative humidity sensor (Humicap-H, Vaisala, Finland) plus a platinum resistance sensor (PT100) for the measurement of the temperature at the humidity sensing surface. The humidity and temperature signals are linearized by a microprocessor controlled transmitter unit (ICH-TB: HMT 333, Vaisala, Finland), which passes the relative humidity (RH) and temperature (T) signals to the data acquisition system of Package1.

The humidity sensing element, together with the PT100-thermistor, is mounted at the top of an axisymmetric body (ICH-SC), which is designed for installation in an appropriate housing (ICH-RS: Model 102 BX, Rosemount Inc., Aerospace Division, USA). The sensor housing is known to deliver accurate ambient air temperatures. The ICH is very similar to the humidity sensors flown in MOZAIC (Helten et al., 1998).

relative humidity (RH):
Time resolution:    1s at 300K to 120s at 200K
Precision:              ±1%
Accuracy:              ±6%

temperature (T):
Time resolution:    4s
Precision:              ±0.2 K
Accuracy:              ±0.5 K


The instrument is designed for autonomous deployment over periods of up to 2 months. Data retrieval is complicated by the fact that the ambient air is adiabatically compressed in the housing, leading to a significant temperature increase of the air sampled by the sensor (up to 30°C). The thermal recovery process at the sensing element is well defined and requires knowledge of the true air speed of the aircraft, which is obtained by the data acquisition system of Package1. Because of the strong temperature increase, the relative humidity RHD detected by the sensing element in the Rosemount housing is appreciably lower than the static relative humidity of the ambient air, RHS. The associated reduction in relative humidity by up to a factor of 12 requires a dedicated calibration of the capacitive sensor for the humidity range encountered. For details see Helten et al., 1998 and SOP.


The ICH is installed in all IAGOS-CORE aircraft based on EASA STC obtained in 2011 for Airbus A340 and in 2014 for Airbus A330 aircraft.


ICH was developed by Forschungszentrum Jülich GmbH in cooperation with enviscope GmbH and is manufactured by enviscope GmbH under licence agreement. Responsible Design Organisation (DO, EASA Part 21J): Sabena Technics BOD, Bordeaux.