Spektrometer für experimentelle Untersuchungen der Wasserdampfdiffusion in Gasen (Spectrometer for Experimental Investigation of Water Vapour Diffusion in Gases)

Abstract

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.

A new method for the determination of membrane permeability by spatially resolved concentration measurements

“…The absorption signal at the lock-in amplifier resembled the second derivative of the pressure-broadened absorption line profile. The amplitude of the absorption feature S was evaluated by signal processing routines described in [1,2]. The signal was also normalized to the broadband overall transmission, was obtained separately from the detector DC signal [3].…”

“…The overall interference background could be reduced to below the absorption signal caused by the water vapour background for a residual relative humidity of 5%. Further details about the optical layout can be found in [2]. All optical components were mounted on a small optical bench, which was vertically traversable (y-direction) by a precision stepping motor (Micos MT-65) with a resolution of 0.1 µm and a range of 25 mm.…”

“…This procedure requires instrumentation that allows spatially resolved concentration measurements. The authors have recently developed a diode laser absorption spectrometer for water vapour concentration measurements with high spatial resolution, high sensitivity and a fast response time, and have demonstrated its suitability for the investigation of diffusion phenomena on a sub-millimetre scale [1,2]. The system has also been applied for the investigation of droplet evaporation; the measurements of concentration profiles in the vicinity of levitated droplets are described in a separate publication [3].…”