Consistency Measurement: The Foundation for Accurate Dilution Control at All Levels Throughout the Pulp and Paper Making Process, 1991 Process Control, Electrical & Info. Conference Proceedings

Keeping stock consistency constant - at different levels for different steps in the pulp and paper making process - has always been, and will remain, one of the most important control functions.

No consistency control loop can ever be better than the sensor that measures the fiber content in a moving pulp slurry. The signal we get from consistency sensors depends on the relation between certain pulp properties and the dry fiber content per volume unit of the suspension. The best correlation with true consistency is usually found in the fiber network strength and the optical characteristics of a given pulp. Most in-line consistency sensors are designed to work on these principles. They are either rotating or stationary mechanical sensors for shear force measurement, or electro-optical sensors for the measurement of light reflection. These pulp properties vary with consistency, but unfortunately, they also change with process variables which are not consistency dependent, e.g. flow variations, variations in filler contents, and blend changes.

Due to the great economic significance of accurate consistency control, it is essential to fully understand the possibilities and limits of the various applications. This paper will concentrate on the practical aspects of dilution, and discuss the benefits of different sensors.