Evaluation of Capacitive Humidity Sensor as Working Humidity Standard in the Humidity Calibrator

摘要

The performance of some capacitive humidity sensors has been significantly improved in both accuracy and long term stability. But, not all capacitive humidity sensors perform the same and a challenge still exists which capacitive sensor can be used in instrumentation that could be considered as a working humidity standard. A capacitive sensor was first used by an American company in a humidity calibrator and then improved on with special performance characteristics. This concept was accepted and followed by some European companies, so that now several manufacturers of humidity calibrators use control probes with capacitive humidity sensors. An evaluation was done on a new humidity calibrator that incorporated an RH sensor with special performance characteristics. This system was compared with the two-pressure humidity generator and precision dew- point hygrometer seven (7) times in the past year to evaluate the accuracy and stability over long term. Tests were done over a humidity range of 10-95%RH and a dew point temperature range of -15 to 20°C with chamber settings of 15～35°C. Results indicate that the measured differences show an uncertainty of <1.5%RH and clearly shows that this humidity calibrator using the capacitive RH sensor with special characteristics has an accuracy and stability which makes it suitable as a working standard for humidity measurements. The Certification by a NVLAP Accredited laboratory and the National Research Center for Certified Reference Materials (NCRRM, China) proved these results.