Development of Measurement System of Water Vapor Fluctuation using a Far Infrared Absorption Band in Building Canyon

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An eddy correlation method is most useful for estimating the latent heat transfer in the complex building canyons. In order to apply this method, it is necessary to measure the fluctuation of specific humidity. In the present study, an infrared hygrometer with optical fiber by using 6.3 mum absorption band of water vapor is newly designed. In the wavelength range, the measurements are insensitive to solar radiation and infrared radiation from the surroundings. The fluctuation of the averaged specific humidity in the measuring path is obtained from the transmittance measurement based on Beer's law. In order to minimize the disturbance of the hygrometer on the measurement, the hygrometer is divided into two parts called a light source block and a measurement block. These blocks are connected by an optical fiber cable.The static characteristics of the hygrometer were examined in a test chamber as a function of the specific humidity (5-20 g/kg), the air temperature (283-303 K) and the path length (150-250 nm). The resolution better than 0.1 g/kg was obtained comparing with an optical dew indicator. The dynamic responses were also checked by the intermittent supply of wet air in the dry test chamber. The dominant frequency obtained from the spectral analysis of the experimental results agreed with the controlled supply frequency less than 10 Hz. The hygrometer was finally changed for the field measurements. They will be performed in several types of building canyons.