he Schopper Riegler apparatus was the first drainability tester to be designed and the application is virtually identical with that of the Canadian Freeness Tester. Although, in principle, suitable for testing all kinds of pulp in aqueous suspension, the test only provides acceptable results if a sufficiently dense matt of fibre of the correct weight is deposited on the wire screen. Due caution should therefore be exercised when pulps contain high percentages of debris or fines and when SR values are outside the range 10 to 90 SR. The Schopper Riegler number scale is the scale on which a discharge of 1000 ml corresponds to a SR number of zero and zero discharge to a SR number of 100