Laboratory and field assessment of an infrared turbidity probe and its response to particle size and variation in suspended sediment concentration

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Abstract
Using a combination of laboratory and field experiments, the performance of a Partech Instruments Ltd IR40-C active head suspended solids sensor has been tested with respect to changing particle size distribution and rapid variation in suspended sediment transport rate. The sensor, which utilizes light attenuation in the infrared waveband, has several advantages both over visible light optical designs and over nuclear or optical back-scatter (OBS) devices. Three sets of laboratory tests are reported: calibration experiments using estuarine, pro-glacial and control media; flume simulations of pulsed sediment supply at a variety of frequencies, concentrations and ambient flow velocities; and simple tests for the effect of air bubbles within the sensor light path. The sensor was also deployed in the field, together with an electromagnetic current meter, to monitor flow and suspended sediment transport fluctuations in the turbulent frequency range. Although the sensor performed well with respect to drift, noise and stability of calibration, significant variation occurred with respect to suspensions of differing particle sizes. A simple correction procedure involving the weighting of sensor output by specific particle surface area may enable comparison of turbidity data from different locations and times, and with standard calibration solutions. At high frequencies, the sensor is capable of resolving fluctuations in the transport rate to c. 0.5 s, which appears to correspond well to field observations of significant velocity fluctuation and transport ‘events’ and is comparable with results obtained using OBS devices. However, the resolution of transport fluctuations is again dependent upon particle characteristics, and the presence of air bubbles in the flow whose size approximates the path width of the sensor may mimic a fluctuating transport process. Further interpretation of high frequency measurement awaits research into the fundamentals of rapidly varying flow and sediment transport characteristics.

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