A novel microsensor for determination of apparent diffusivity in sediments

Revsbech, Niels Peter, Lars Peter Nielsen, Niels Birger Ramsing

Limnol. Oceanogr., 43(5), 1998, 986-992 | DOI: 10.4319/lo.1998.43.5.0986

ABSTRACT: A microsensor for determination of apparent gas diffusivity in pore waters and biofilms is described. The measurement is based on diffusion from an internal gas reservoir, through an internal diffusion barrier positioned within the sensor tip and out into the surrounding medium. The concentration of the gas at the tip of the sensor is then a function of the apparent diffusivity in the medium. Hydrogen was used as a tracer gas in our test 140-µm thick sensor, but other less reactive gasses may also be used. A mathematical model, describing the signal as a function of diffusivity, is presented; it is shown that a calibration curve can be constructed from measurements in only two media with known diffusivity, such as stagnant water and a sediment composed of microscopic glass beads. The sensor-determined apparent diffusivities in a variety of media agree well with parallel determinations using a diffusion chamber. The signal from our 140-µm sensor depends on the diffusivity in a spherical volume within a radius of 0.5 mm from the tip. By use of the diffusivity sensor, it is possible to obtain a much higher spatial resolution of diffusivity in sediments and biofilms than by any other nonoptical method.

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