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In this paper, a new, automatic method of measuring air dew-point (DP) is presented. The system also realizes a relative-humidity sensing function, based on dew point and temperature measuring channels acquired data. The DP is detected on the surface of a lithium chloride-polyvinyl alcohol film fabricated on an optic fiber head using an optical system. The refractive index of the sensing film will be at its maximum value when the dew or frost starts to form at DP. A setup which includes a dew point sensor head and a measurement/control/data collection interface is assembled to enable automated measurements. Several programs are designed using the LabVIEW 8.6 environment to measure the dew point sensing properties of our sensor, such as response-recovery curves, temperature characteristic, and humidity calculation. Anti-interfered and good stability are its main advantages. The sensor can operate in the range of dew point temperature from 0 to 30 °C and relative humidity (RH) from 11% to 83% RH. The repeatability of the sensor was implemented with a standard deviation of 0.09 °C. The results showed that the measured dew point values and RH values were in very good agreement with reference values and with an accuracy of less than ±0.3 °C, 1% RH, respectively, over the whole temperature range. [ABSTRACT FROM AUTHOR]