For water vapor measurements three different instruments are used: a commercial aircraft dew point hygrometer (GE 1011B, General Eastern), a slightly modified capacitive sensor (Humicap-H ,Vaisala) and a Lyman-alpha absorption instrument (Buck Research, Boulder).The General Eastern 1011B is a dew/frost point hygrometer developed for aircraft application. A special reverse flow inlet is used for particle separation. The principal of operation is the detection and control of a thin layer of dew/frost on a mirror surface by optical methods and simultaneous temperature measurement of the mirror surface. The mirror temperature is controlled by a 2 stage peltier element.Due to its slow time response and limited range the dew/frost point hygrometer is mainly used for consistency check during takeoff and landing.The Vaisala HMP230 is a capacitive humidity sensor. The principal of operation is the measurement of the humidity dependent capacity of a polymer coated capacitor.The HMP230 shows excellent long term stability and good time response in the lower troposphere. Time response degrades with decreasing temperatures.The sensor is placed in a pressure tight flow channel together with the Lyman-alpha instrument. A modified Rosemeount BW102 Total Air Temperature housing is used as inlet. Since the HMP230 sensor measures relative humidity an adiabatic compression correction has to be applied similar to the Total Air Temperature sensor.The Lyman-alpha absorption hygrometer is a fast (100 Hz) humidity sensor. The principal of operation is the measurement of absorption of water vapor molecules at 121 nm wavelength.Due to its fast response time of a few milliseconds and wide sensitivity range the data of the Lyman-alpha instrument are used whenever possible. The absorption signal has to be corrected for O2 interference and long term lamp intensity fluctuation. Therefore intensive laboratory and in-flight calibration is performed.The sensor is placed in a pressure tight flow channel together with the Vaisala sensor. A modified Rosemeount BW102 Total Air Temperature housing is used as inlet.Error Analysis
The error analysis is somewhat complicated due to the fact that the accuracy of the humidity measurement depends on the flight parameters (height, speed), the humidity unit and the instrument. The calibration standard is accurate to 2-3% of the indicated value for mixing ratio, absolute humidity and relative humidity over the whole atmospheric range which can be accessed by the Falcon. However, the standard humidity sensors of the Falcon were designed for the lower troposphere (Additional instrumentation for the measurement of low humidity at high altitudes is available on request). Besides the decreasing sensitivity at higher altitudes additional errors like temperature dependencies start to degrade the sensor accuracy. The accuracy of the humidity data is estimated to be about 5% of the measured value in the lower third of the atmosphere. A more precise error analysis for specific flight conditions is available on request.Humidity sensors are calibrated by the DLR flight department using sophisticated calibration equipment developed within the division. Each calibration procedure is traceable to the national standard and includes an extensive error analysis.