Pressure screens are used to filter out impurities and separate fibres in stock preparation. A rotating screen basket inside the filter does the actual filtering. Pressure screens have an inlet for the suspension, an outlet for accepted stock and a discharge for rejected stock and contaminants. The process conditions are characterized by pressure surges as well as abrasion and contaminants in the stock. Efficient screening requires a controlled pressure difference between inlet and outlet, which therefore requires continuous, reliable measurement.