The NPL standard humidity generator: an analysis of uncertainty by validation of individual component performance

The NPL standard humidity generator-the reference for traceable humidity calibrations in the United Kingdom-has been validated by assessment of the performance of individual components of the generator system. The investigation has explored the effects of saturator efficiency, temperature conditioning of the gas, desorption, leaks, pressure variation and other factors that contribute to the uncertainty in dew-points generated by this instrument. The results of these experiments have led to some improvements in the design of the standard. Repeatability of overall performance has been monitored over a period of two years. The improvements in design, together with better characterization of the performance of the instrument, have enabled a smaller uncertainty in generated dew-point to be assigned. This uncertainty is now +or-0.05 degrees C dew-point in the middle of the operating range, rising to +or-0.08 degrees C and +or-0.15 degrees C dew-point at the upper and lower extremes of the range: +80 degrees C and -75 degrees C dew-point respectively.