Intercomparison of in situ suspended matter (floc) size measurements

Simultaneous measurements of in situ particle (floc) size were performed with different instruments during several tidal cycles in the Elbe estuary in June 1993. Comparison of the size distributions (by volume) indicates that six instruments produced data that could be compared, while two instruments produced only estimates of mean, minimum and maximum size. The minimum size measured was related to the characteristics of the instrument, the maximum size to the concentration (by number) of the largest flocs, to the volume of the water measured, and to the degree of floc break-up during handling and/or measuring of the sample. The size distributions generally follow a log-probability curve so that the differences in size range measured do not strongly influence the median values. The variability of the results obtained with different instruments shows that measurements can best be made with only one instrument so that a maximum of consistency is obtained.