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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.

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We report the first measurements of a quasi-spherical microwave resonator used as a dew-point hygrometer. In conventional dew-point hygrometers, the condensation of water from humid gas flowing over a mirror is detected optically, and the mirror surface is then temperature-controlled to yield a stable condensed layer. In our experiments we flowed moist air from a humidity generator through a quasi-spherical resonator and detected the onset of condensation by measuring the frequency ratio of selected microwave modes. We verified the basic operation of the device over the dew-point range 9.5–13.5 °C by comparison with calibrated chilled-mirror hygrometers. These tests indicate that the microwave method may allow a quantitative estimation of the volume and thickness of the water layer which is condensed on the inner surface of the resonator. The experiments reported here are preliminary due to the limited time available for the work, but show the potential of the method for detecting not only water but a variety of other liquid or solid condensates. The robust all-metal construction should make the device appropriate for use in industrial applications over a wide range of temperatures and pressures.

Laser absorption spectroscopy offers the potential for fast and precise trace moisture detection in gases at atmospheric pressure with a small cross sensitivity towards other molecules. We report on the development and calibration of a fibre-coupled tunable diode laser absorption spectrometer operating in the 1 - 100 ppm humidity range. The spectrometer was tested at three European humidity standards laboratories. The performance of the spectrometer was characterized by monitoring constant water vapour concentrations over several hours, yielding a good long-term stability, reproducibility and accuracy. The standard deviations of the measured water vapour concentrations for values above 5 ppm were below ±2%. Comparisons of the instrument with chilled mirror hygrometers demonstrated its fast response time. The dependences of the signal upon the flow rate and temperature are discussed.