In sciences involving soil and sediment, particle size distribution (PSD) has been one of the parameters given most attention over the past few decades. Formerly measured by sieving and sedimentation techniques, it is nowadays routinely characterized by the laser diffraction method (LDM). Many manufacturers develop particle size analyzers using LDM, but each device is characterized by specific parameters that can lead to different PSDs. At the Rhône Sediment Observatory, suspended particulate matter collected along the Rhône River is analyzed for PSD by four different LDM devices. Analyses were conducted on certified materials and sediment samples for each device. The tests highlighted the difficulty of accurately characterizing PSD, even in the case of certified materials. First, differences observed for a specific device were linked to the heterogeneity observed in the subsamples due to the presence of organic materials such as tree leaves. Second, the difference regarding the certified materials was linked to the laser diffraction method which leads in some cases to underestimating clay content and sand. Third, the main difference observed between the devices was linked to sonication. The results demonstrate that its power is rarely investigated and that it has a considerable impact when used. However, despite significant differences, the trend was similar for each device, with accurate characterizations of the main modal class in most cases. Thus, in the absence of exact knowledge of parameters such as sonication power and pump speed, it is recommended to compare only the trends of the results obtained from the different devices.