Miniaturized Two-Pressure Generator for Relative Humidity

The concept and design of a miniaturized two-pressure humidity generator are presented. The generator is suitable for achieving relative humidity ranging from 10% to 95% with uncertainties of under 1% at ambient temperature and can be used for the calibration of relative humidity instruments in the laboratory and on site. By virtue of the concept, the relative humidity achieved is traced to only two pressure measurements. Further references are not necessary. A special adjustment algorithm enables standard industrial pressure sensors to be used. The two pressure sensors are synchronized by aligning their readings at ambient pressure. The resulting correlation of the uncertainties leads to very small systematic errors in the humidity display. Each humidity generator can still be individually adjusted through comparison with a chilled-mirror hygrometer, the total uncertainty of the generator being largely determined by the uncertainty of this hygrometer. Any drift of the pressure sensors that occurs can be compensated at any time by performing an alignment at ambient pressure without changing the individual adjustment of the generator. It can be demonstrated that the uncertainty of the displayed humidity remains practically unchanged over the course of a year by virtue of this alignment process.