Reliability of Freeness Measurement - Errors in the Analysis Procedure, 2000 Papermakers Conference Proceedings

Ari Ämmälä, Heli Rautjärvi, Anne Seppänen and Jouko Niinimäki--The freeness test is a commonly used routine method for characterizing pulp, as measurements are easy and quick to perform. It is usually applicable to prediction of the strength and optical properties of mechanical pulps, because these properties, like freeness, correlate well with the amount of fines in the pulp. The freeness test is dependent not only on pulp properties but also on the conditions under which the test is carried out, such as sample preparation, fiber concentration, temperature, water quality and the human factor involved in the use of the tester. All of these steps cause errors, which may be cumulative, so that the total error may be substantial. We examine here the procedure adopted for freeness measurement (Canadian standard method). Systematic and stochastic effects in the critical steps of the procedure are elucidated and the results used to estimate the presumable error in these steps and total error in the whole analysis chain. The results show that if the freeness test is performed with care in every single step, an individual laboratory technician can attain reasonable and repeatable freeness values. Moreover, the extraction of the sample from the process was found to be the most critical step in the whole analysis procedure, while the individual steps of the testing procedure were less critical, and variations were almost independent of the freeness level. Although the freeness test is quite a reliable measurement in general, all laboratory technicians have their own operative characteristics in the testing procedure, which may lead to significant systematic and stochastic differences between results. The accuracy of the analysis depends greatly on the analytical skill and routine of the laboratory technician. The most questionable step in the whole procedure is the sampling of pulp from the process. The sampling procedure and sampling devices have not been standardized, which may cause poor representativeness if inadequate sampling methods are used.