High precision trace humidity measurements with a fibre-coupled diode laser absorption spectrometer at atmospheric pressure

Laser absorption spectroscopy offers the potential for fast and precise tra ce moisture detection in gases at atmospheric pressure with a small cross s ensitivity towards other molecules. We report on the development and calibr ation of a fibre-coupled tunable diode laser absorption spectrometer operat ing in the 1-100 ppm humidity range. The spectrometer was tested at three E uropean humidity standards laboratories. The performance of the spectromete r was characterized by monitoring constant water vapour concentrations over several hours, yielding a good long-term stability, reproducibility and ac curacy. The standard deviations of the measured water vapour concentrations for values above 5 ppm were below +/-2%. Comparisons of the instrument wit h chilled mirror hygrometers demonstrated its fast response time. The depen dences of the signal upon the flow rate and temperature are discussed.