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The method of measuring the dynamic characteristics of microhumidity sensors is becoming a bottleneck in developing fast-response humidity sensors. This paper introduces a method for measuring the dynamic characteristics of microhumidity sensors. In this method, two environments with different relative humidity (RH1 and RH2) are established using different saturated aqueous solutions of salt. There is independent air circulation for each environment. These loops of air circulation are connected to the test cell, where the tested sensor is located. Air coming from the RH1 environment always flows over the tested sensor until the valves controlling the air circulation of the RH2 environment operate. Then, the air humidity of the test cell is quickly changed from RH1 to RH2. By decreasing the time needed to change the humidity of the air in the test cell, the response time of the tested sensor can be obtained more precisely. A prototype based on this method is built, and it is tested by measuring the dynamic behavior of a parallel plate capacitance humidity sensor based on polyimide. The analytical results show that the response time of the prototype is less than 250 ms. The response time of the prototype can be improved greatly by enhancing the response time of the subassemblies used in the prototype.

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Polyimide (PI) film is widely used as the key component of the capacitive humidity sensor, whose diffusion coefficient has a significant impact on the sensor’s dynamic characteristics, but is rarely discussed. This paper provides a test method and processes for effective diffusion coefficients of water molecules in self-synthesis PI films. The films were formed by four ingredients (PMDA-ODA, BPDA-ODA and BPDA-BAPP, PMDA-BAPP) with PI acid concentrations of 23%, 20%, 17% and 15%, and tested in temperatures of 20 °C, 35 °C and 50 °C, respectively. The results indicated that BPDA-BAPP film was good as a moisture sensitive film, whose average effective diffusion coefficient was 2.709 × 10−14 m2/s. The temperature of the environment had a significant effect on the humidity-sensitive properties, but the PI acid concentration effect could be indirect.