Dew point instrument can directly measure the dew point temperature. Allow a mirror to cool in the humid air of the sample until the moment when dew drops (or ice crystals) appear on the mirror, the average temperature of the mirror is measured, which is the dew (frost) point temperature.
The resistance-capacitance method is a constantly improving method for measuring humidity. Using a high-purity aluminum rod, the surface is oxidized into an ultra-thin aluminum oxide film, which is coated with a hollow mesh gold film on the outside, and a capacitance is formed between the gold film and the aluminum rod. Due to the water absorption characteristics of the aluminum oxide film, The capacitance value changes with the amount of moisture in the sample gas, and the humidity of the sample gas can be obtained by measuring the capacitance value. The main advantage of this method is that the measurement range can be lower, even up to -100 ℃, another outstanding advantage is that the response speed is very fast, from dry to wet response can reach 90% in one minute, so it is mostly used in the field and rapid measurement occasions; disadvantages The accuracy is poor, and the uncertainty is mostly ± 2 ~ 3 ℃. Aging and drifting are serious, it must be calibrated after 3~6 months of use. With the continuous efforts of various dew point instrument manufacturers, this method is gradually being improved. For example, the stability of the sensor is greatly improved by changing the material and the process, and the saturation linearity is solved by the compensation of the sensor response curve, which solves the problem of automatic calibration.