Hysteresis of Capacitance Type Relative Humidity Sensors

摘要

The hysteresis of several capacitance type relative humidity sensors has been investigated. These sensors were tested using the secondary standard apparatus which is used to conduct humidity calibrations at NMI. In this paper, a description of the method used to study the effects of hysteresis is reported, along with the uncertainties associated with these measurements. It was found that at 50 (percent)RH the relative humidity sensors tested exhibited hysteresis values of approximately 0.5 (percent)RH to 1 (percent)RH. It was also found that after 4 hours of stabilisation time, two relative humidity sensors had still not returned to their initial readings obtained during the first part of the hysteresis curve at the same setpoint. Further it was noted that extending the soak time at each setpoint from 30 minutes to 80 minutes had no appreciable effect on the hysteresis observed. The results obtained show that for typical uncertainties observed during relative humidity calibrations, the effects due to hysteresis are significant.