Application of humidity sensors and an interactive device

In many applications, sensors (e.g., temperature, humidity and chemical composition) are used downstream of passive interactive devices to determine their transient response. In this arrangement, the sensors alter the apparent response of the device. As a practical example, the transient characteristics of a humidity sensor are investigated during a step change in humidity with no change in temperature. This response of the humidity sensor is measured for a step change in the humidity and the data are correlated with exponential functions with two time constants of about 3 and 100–300 s. When the same humidity sensor is used downstream of an interactive passive moisture transfer device (e.g., a regenerative wheel for transferring water vapour), the measured humidity is found to correlate with exponential functions with longer time constants than the sensor itself. In this case, the measured response of the sensor includes the combined effects of the device and sensor. An analysis is presented to determine only the transient moisture transfer characteristics of the unknown device using the measured responses of the sensor alone and the response of sensor with the device upstream. The time constants of the interactive device may have errors as large as 20% when the transient response of the humidity sensor is neglected for the application studied in this paper.