Microsensor for in-situ flow measurements in benthic boundary layers at sub-millimeter resolution with extremely slow flow

Andreas Brand, Beat Müller, Alfred Wüest, Christian Dinkel, Niels Peter Revsbech, Lars Peter Nielsen, Ole Pedersen, Lars Riis Damgaard, Lars Hauer Larsen, Bernhard Wehrli

Limnol. Oceanogr. Methods 5:185-191 (2007) | DOI: 10.4319/lom.2007.5.185

ABSTRACT: The currents in the lowest few mm of the bottom boundary layer of lakes are highly important for the dissipation of kinetic energy and for chemical processes like oxygen transfer into the sediment. So far, no high resolution flow velocity profiles close to the sediment water interface have been reported for such systems because a suitable flow meter was lacking. This paper introduces a novel sensor for the measurement of extremely low flow velocities. The sensor is based on a gas transducer which is surrounded by a gas reservoir. It measures the change in the partial pressure of a tracer gas on the outside of the reservoir tip due to advective transport. The sensor is suitable for measurements of velocities smaller than 1 mm s-1 with a spatial resolution of 100 to 250 µm. The flow measurements prove to be insensitive to temperature changes between 5 and 15 °C. The sensor is robust against relative pressure changes, and angular differences in the sensitivity can be calibrated. We present high resolution in situ measurements at the bottom of a pre-alpine lake with shear velocities as low as 0.13 ± 0.02 cm s-1. The velocity profile resolves nicely the transition zone between the viscous and the logarithmic boundary layer.