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An Instrument for Measuring Small Bottom Currents in Lakes

U. Lemmin, M. Schurter, D. M. Imboden and Th. Joller
Limnology and Oceanography
Vol. 30, No. 5 (Sep., 1985), pp. 1116-1122
Stable URL: http://www.jstor.org/stable/2836597
Page Count: 7
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An Instrument for Measuring Small Bottom Currents in Lakes
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Abstract

An instrument capable of recording bottom currents between 0.3 and $3 cm s^-1$ consists of an inverse pendulum with a slightly buoyant sphere and a light thread. At predetermined intervals a super-8 movie camera operating in single-frame exposure records the position of the sphere with reference to a fixed point. Illumination is provided by an electronic flash. The sphere excursions are digitized by a computerized digitizing table upon which each frame is projected. Calibration in a tow tank yields a power law between excursion and current speed. Deviation from the theoretical curve demonstrates than the drag forces on the thread cannot be disregarded. Overall accuracy of the instrument is $\pm 5%$ for sped, $\pm 15^\circ$ for direction. The instrument has been successfully deployed in several Swiss lakes.

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