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.