Two-factor authentication of data from an inexpensive water current meter

Session: Smart Lakes: Real-Time Monitoring, Networking, and Analytics Across the Great Lakes (3)

Thomas Hansen, University of Wisconsin - Milwaukee School of Freshwater Sci, [email protected]

Abstract

Tilt-based current meters are increasingly popular and inexpensive, with wide application. Unlike acoustic-based sensors, which by and large have no external moving parts, tilt-based meters rely on the free movement of a tethered subsurface float.  While giving the meter high sensitivity, at extremely low cost, it leaves the device susceptible to physical disruption by collision with vessels, entanglement in deployment lines, and so on.  Any contact with such foreign objects renders the accuracy of subsequent readings in doubt, potentially rendering a sensor’s data unusable.  Fortunately, we discovered a straightforward method to validate the accuracy of data obtained from a tilt-based current meter. Inspired in part by the much smaller industrial vortex-shedding flow meter, we performed spectral analysis on the high-frequency component of the motion data recorded by the sensor—normally filtered out—and were able to identify a portion of the signal as attributable to vortex shedding.  The linear relationship between vortex shedding frequency and current speed, although only useful in a limited range of speeds, was sufficient to validate the accuracy of the recorded data. We will also investigate whether more advanced spectral anaysis techniques, such as the Hilbert-Huang Transform, will allow the extraction of more meaningful information from data heretofore discarded as noise.