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The development of a laser diode absorption spectrometer that uses a strong water vapor absorption at 1393 nm is reported. Three spectroscopic techniques were compared in approximately 0.4 m of laboratory air, namely, frequency modulation, wavelength modulation, and two-tone frequency modulation spectroscopy. The first two techniques use a single-frequency modulation at 9.2 GHz and 1 kHz, respectively, generated either by a phase modulator operating at 9.2 GHz or injection current modulation at 1 kHz. The two-tone method requires modulation at two frequencies, in this case 9.19 and 9.21 GHz. It is shown that the two-tone method should provide the highest sensitivity for a trace moisture detection system.

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.

This paper suggests a novel method for determining the permeability of a membrane by spatially resolved measurements of the concentration profiles above and below the membrane. A compact diode laser absorption spectrometer is presented with a collimated measuring beam, allowing precise concentration measurements with a spatial resolution of 200 µm perpendicular to the laser beam. Measurements of the permeability of polyurethane coated polypropylene foils are presented, demonstrating the ability of fast and calibration-free measurements.

Diffusionsprozesse spielen in vielen verfahrenstechnischenBereichen eine wichtige Rolle, allerdings stehen geeignete Messverfahren zur lokalen Konzentrationsbestimmung nur in beschränktem Umfang zur Verfügung. Ein Diodenlaser-Absorptionsspektrometer wurde entwickelt, welches entlang dem kurzen, kollimierten Messstrahl integrierte WasserdampfKonzentrationsmessungen mit einer lateralen Auflösung von 200 µm ermöglicht. Es wurden Konzentrationsprofile in teilweise mit Wasser gefüllten Standardküvetten vermessen und daraus der Stoffstrom berechnet. Weiterhin werden Messungen der Feuchteprofile oberund unterhalb einer Polymerfolie vorgestellt, welche die direkte Bestimmung der Permeabilität der Folie erlauben.Diffusion processes play an important role in many fields of process engineering but only very few techniques for measuring local concentrations are available. A tunable diode laser absorption spectrometer has been developed enabling water vapour concentration measurements integrating along the short, collimated laser beam path with a spatial resolution of 200 µm perpendicular to the beam direction. Concentration profiles have been measured in standard cuvettes partially filled with water and the mass flux has been derived. Furthermore, measurements of concentration profiles above and below a foil are presented allowing the direct determination of the permeability of the foil.