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The Transient Response of Capacitive Thin-Film Polymer Humidity Sensors

The transient response of capacitive thin-film polymer humidity sensors to step changes in humidity and temperature as a function of air velocity was investigated. Two duct-mounted humidity sensors (designated Candidate X and Candidate Y) from two manufacturers, each with thin-film capacitive humidity sensing elements and temperature output, were used in the study. The transient performance of each test sensor was characterized by measuring the time constants of the relative humidity (RH) and temperature outputs in response to step changes in RH and temperature over a range of air velocities. Duct air velocities of 0 (still air), 200, 500, 800, and 1100 ft/min (0, 1.0, 2.5, 4.1, and 5.6 m/s) were considered along with step changes in RH of 20%–40% and step changes in air temperature of 0°F (isothermal), 9°F, and 18°F (0°C, 5°C, and 10°C). The effects of a positive or negative RH step change direction (lower to higher RH and vice versa) were also considered. Results showed that the RH time constants of both sensors decreased asymptotically with increasing duct air velocity for all testing conditions. For the isothermal tests, the average RH time constants of the Candidate X and Y sensors were approximately 4 and 18 s, respectively. When a step change in temperature was introduced, the RH time constants increased by factors of 100 and 12 over the isothermal values, respectively. With the exception of the still-air tests, the time constant of the Candidate X sensor for negative RH step changes was, on average, about 65% less than compared to positive RH step changes. For the Candidate Y sensor, a time-constant dependency on RH step-change direction was not observed.