Historical Error Analysis for an EdgeTech Chilled Mirror Hygrometer

Chilled Mirror Hygrometers are primary standards and as such can be made traceable to the National Institute of Standards and Technology (NIST). A primary standard measures a fundamental property, in this case of water, e.g. the dew or frost point. Traceability is achieved by periodically comparing the readings of any given hygrometer over time with that of a standard humidity generator such as NIST's Two Pressure Humidity Generator, the Mark 2, or with the Hybrid Humidity Generator under test procedure NIST SP250-83, Calibration of Hygrometers with the Hybrid Humidity Generator. These generators have an uncertainty of 0.03oC.
The data in the enclosed graph was compiled from calibrations of one of EdgeTech's standard chilled mirror hygrometers, the DewMaster fitted with an S3 sensor, unit number S/N 3H2579MCR. The unit was calibrated at yearly intervals from 2008 to 2012 at NIST and at EdgeTech's calibration facility. This NIST data set is summarized in Figure 1, below. The data indicate that the DewMaster fitted with an S3 sensor is well within its published specification of 0.2oC compared to the NIST generator. This testifies to the excellent long term accuracy and stability of the DewMaster hygrometer.