Hot air, both dry and moist, has been widely used for heating and/or a reaction atmosphere in a broad range of industrial fields including food processing, material drying, and other heat treatments. In a process that uses hot air and superheated steam, the gas flow temperature, pressure, and humidity inside the treatment chamber have a major impact on the quality of the finished product. However, there is no simple method of measuring humidity that can be used over an extended duration in applications that use superheated steam at temperatures above 100°C. In this paper, we introduce a device developed for measuring humidity, which consists of a wetted porous ceramic probe with thermocouples, a water retention control system, and computing devices. The measured humidity (steam mole fraction) is calculated from the measured wet- and dry-bulb temperatures using the adiabatic saturation line equation on which we previously reported. The equation was extended to enable its application to temperatures of up to 350°C and a humidity up to that of superheated steam. Accuracy measurements were conducted with a humidity generator and a gas flow temperature of 200°C, a gas flow velocity of 3 m/s, and steam mole fraction ranging from that of hot air to that of superheated steam. The cause of any errors was investigated.
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