Abstract: A method and apparatus are described for continuously sampling a process stream, or reservoir of aqueous cellulosic pulp and detecting oversize contaminants in the sampled stream. This serves the overall objective of estimating the level of contamination in the process stream or reservoir during a given period of production; the method may include the following stages: 1) a continuous sample stream is drawn from the process or main stream, 2) the stream is passed through a centrifugal device which divides the sampled stream according to density, 3) the stream with the higher level of contaminants from the centrifugal device is passed to a screen which isolates the contaminants, and 4) the contaminants are flushed from the screen so that they can be quantified; these measurements are used to assess the purity of the material being produced. One useful application of this invention is to assess the presence of trace amounts of plastic contaminants in the production of the cellulosic pulp.