Abstract: A measuring device is provided that includes a camera, and an image-processing unit and a fractionating pipe. The receives a sample from recycled fibre pulp of a recycled fibre process and arranges the particles of the flowing sample in accordance with particle size. The measuring device processes the sample as at least two fractions according to particle size. The camera images at least one fraction. The image-processing unit receives images imaged by the camera and measures at least one parameter of an added substance in at least one imaged fraction.

Abstract: The invention relates to a method for measuring particles in suspension and a measuring instrument. In the solution the length of each fiber-like particle in suspension is one by one optically measured by a line camera in a thin equilateral capillary tube. The fiber-like particles in particular are imaged by two or more cameras in the capillary tube one or more times for measuring such properties as fiber thickness, wall-thickness and fibrillation.