Ultrasonic sensor to characterize wood pulp during refining.

A novel sensor concept has been developed for measuring the degree of refining, the water retention value (WRV), and the weight percentage of wood pulp during the refining process. The measurement time is less than 5 min and the sensor can operate in a slip-stream of the process line or as an at-line instrument. The degree of refining and the WRV are determined from settling measurements. The settling of a pulp suspension (with a weight percentage less than 0.5 wt%) is observed, after the mixer, which keeps the pulp uniformly distributed, is turned off. The attenuation of ultrasound as a function of time is recorded and these data show a peak at a time designated as the "peak time." The peak time T increases with the degree of refining, as demonstrated by measuring pulp samples with known degrees of refining. The WRV can be determined using the relative peak time, defined as the ratio T(2)/T(1), where T(1) is an initial peak time and T(2) is the value after additional refining. This method offers an alternative WRV test for the industry to the current time-consuming method.