Design, fabrication and preliminary uncertainty analysis of a primary humidity measurement standard

This thesis describes the design and implementation of a new primary humidity standard in the National Metrology Laboratory of Ireland. In this thesis the basic principles of humidity metrology and primary dew point generation are explained. A number of existing designs of primary humidity standards are discussed. Useful equations relating to humidity metrology and particularly humidity generation as a primary measurement standard are given and in some cases derived. Several new designs of the components of a primary humidity standard are proposed and the benefits of each design is compared based on theory, cost of fabrication and simulations using computational fluid dynamics. From this comparison the individual components are chosen, and the complete design is given. A new type of corrosion resistant plating that has previously not been used with other primary dew point generators was chosen and its merits are discussed. The design was fabricated, and a number of modifications were made in order to improve the performance of the generator based on initial testing of the system. Finally, a preliminary uncertainty budget was developed for the calibration of chilled mirror hygrometers using the new primary humidity standard. This uncertainty budget covers the range from -40 °C frost point (fp) to 90 °C dew point (dp) and gives an uncertainty of ±0.060 °C fp at -40 °C fp and ±0.038 °C dp at 90 °C dp both with a coverage interval of 95 %. A calibration of a chilled mirror hygrometer was performed, and the results are compared to the calibration results obtained at an external metrology laboratory. The results obtained at NML were found to agree with the external laboratory to within ±0.009 °C dew/frost point. Future work involving the generator is outlined