Revision and uncertainty evaluation of a primary dewpoint generator

At NMi-VSL a humidity generator was built in the early 1990s. The generator is of the circulating single-temperature, single-pressure type. One pump recirculates air or nitrogen gas over two saturators, one of which can be selected, depending on the temperature range. Some components of the standard were found to operate not to the level that was anticipated during the design. The standard has been thoroughly refurbished. Experiments showed that the standard now operates from −60 °C to +70 °C, with much improved time constants and with lower uncertainties to within approximately 0.04 °C, depending on the temperature. Design considerations and requirements for this type of standard are given in the paper. The tests involved in evaluating the performance are discussed, and the results are presented with the resulting uncertainty budget.