CALIBRATION OF A PAR-TEC-200 LASER BACKSCATTER PROBE FOR IN-SITU SIZING OF FLUVIAL SUSPENDED SEDIMENT

The in situ or effective particle size distribution of fluvial suspend ed sediment may differ considerably from that of the chemically disper sed mineral fraction owing to flocculation. Obtaining a meaningful mea sure of the effective particle size distribution ideally requires that measurements should be made in situ. A rigorous assessment of the ass ociated degree of flocculation also requires that the same measurement technique is used subsequently to establish the absolute particle siz e composition of the suspended sediment by analysis of the chemically dispersed mineral fraction. While few in situ measurement devices curr ently exist, a Par-Tee 200 laser back-scatter probe has previously bee n shown to be capable of making both in situ and laboratory particle s ize measurements of fluvial sediment. The accuracy and precision of th is device is assessed in this paper. While able to distinguish relativ e size differences with a high degree of precision, the Par-Tee 200 pe rformed poorly in terms of accuracy when compared with measurements ma de using a laser diffraction device. A calibration algorithm has been devised for the Par-Tec 200 size data, using standard sediment samples sized by means of a laser diffraction device as the reference. Applic ation of the calibration to Par-Tee 200 measurements of heterogeneous sediment samples significantly improved the representativeness of the particle size distribution, both in terms of overall form, and the med ian particle size. (C) 1998 John Wiley & Sons, Ltd.