The cleanliness of the pulp is measured in square
millimeters of impurities per kilogram of produced
pulp accordingly to the UNE/ISO 5350/2 standard.
The measurement of impurities is typically performed
manually in the pulp industry accordingly to a test card
shown in Fig. 1. The human inspector has a transparent
calibration sheet that is used to determine the size of
the impurity by comparing them to the dots in the
calibration sheet and thereafter classify the impurity
into a type accordingly to the size. The minimum size
of the impurity to be detected is 0.04 mm2, which is at
the limit of the human visual resolution, resulting in a
slow and tedious work. A regular time for inspecting a
50 cm 50 cm. sheet is around 8 min. These inspection
processes are usually implemented at a maximum
sample rate of one sheet each half an hour per
production line.
Since the regular speed of the product can reach
7.5 m2/s, the mentioned sample rate of a human
inspector is only about 0.0018% of the total produced
surface. The sparse inspection data is then extrapolated
in order to determine the cleanliness of the
produced unit of pulp.
A few automatic systems have been tested to
replace the described manual inspection procedure, as