Method and device for examining pulp for the presence of shives

In a method for examining pulp for shives present therein a suspension of the pulp is passed through a measuring duct with transparent walls and two mutually perpendicular light beams are directed through the measuring duct in a plane perpendicular to the direction of flow in the measuring duct. The intensities of the light beams after their passage through the measuring duct are measured by means of two photo detectors and the output signals of the photo detectors are multiplied to provide a combined signal representing the product of the output signals of the photo detectors. This combined signal is analyzed with respect to the temporary amplitude reductions occurring therein due to the presence of shives in the pulp suspension flow passing through the measuring duct. Preferably, the light beams consist of light having a wavelength within the infra-red range.

The present invention is related to a method and a device for testing and 
examining pulp for the presence of shives therein. 
A "shive" is a larger fiber bundle consisting of two or more fibers which 
adhere to each other and which have not been completely separated from 
each other during the production of the pulp. The shives differ from the 
fibers in the pulp primarily in that they have a larger cross-section 
dimension than the fibers and thus generally also a somewhat larger length 
in average than the fibers. Whereas the fibers generally have a 
cross-section dimension (thickness) of 10-50 .mu.m the shives, or what is 
generally called shives, have a corresponding cross-section dimension from 
80-150 .mu.m and upwards. The lower limit for the cross-section dimension 
of what is called "shives" is often dependent on the capability of the 
available measuring apparatus of distinguishing between thin shives, i.e. 
shives having a comparatively small cross-section dimension, and fibers. 
The average length of the shives is generally 1.5 to 2 times larger than 
the average length of the fibers, but this value depends on the type of 
the pulp. 
The presence of shives in a paper pulp is an important factor for the 
quality of the pulp. One reason for this is that each shive will cause a 
weak spot or an initiative to rupture in the paper web being manufactured 
from the pulp and therefore increase the risk of rupture of the paper web 
during its manufacture. Further, a shive located in the surface of the 
manufactured paper will also impair the printing qualities of the paper, 
for instance in that the shive accepts and adsorbs the printing ink in a 
manner different from the surrounding paper or in that the shive comes 
loose from the paper surface during the printing process and possibly 
adheres to the printing form or printing plate. Consequently, it is of 
important interest to be able to examine pulp with respect to the presence 
of shives therein, the primary interest being to determine the total 
amount of shives in a given quantity of pulp but also to obtain 
information on the size or the size range of the shives present in the 
pulp. 
A prior art method for examining paper pulp for the presence of shives 
therein comprises the steps of passing a suspension of the pulp through a 
measuring duct having transparent walls and directing a substantially 
parallel beam of light from a light source located at one side of the 
measuring duct through the measuring duct towards a photo detector located 
on the opposite side of the measuring duct in such a manner that the 
direction of the light beam is substantially perpendicular to the 
longitudinal direction of the measuring duct, i.e. to the direction of 
flow of the pulp suspension. A shive present in the pulp suspension will, 
when it passes through the light beam, give cause to a reduction in the 
intensity of the light received by the photo detector and thus to a 
corresponding reduction in the amplitude of the output signal of the photo 
detector. The magnitude of this reduction in intensity and amplitude, 
respectively, constitutes a measure of the cross-section dimension of the 
shives in a direction perpendicular to the light beam, whereas the 
duration of the reduction in intensity and amplitude, respectively, is a 
measure of the length of the shive, as the shives orient themselves in the 
pulp suspension flow with their longitudinal direction substantially 
coinciding with the direction of flow. By analyzing the output signal of 
the photo detector with respect to the amplitude variations in the signal 
it is consequently possible to obtain information of the presence of 
shives in the pulp. As the cross-section of a shive is often rectangular, 
i.e. the shive is thin and broad, it is the preferred practice to direct 
two light beams through the measuring duct at right angles to each other 
and in a common plane perpendicular to the longitudinal direction of the 
measuring duct. These two light beams are, after their passage through the 
measuring duct, received by two corresponding photo detectors and the 
output signals of these photo detectors are combined to a combined signal, 
which is subsequently analyzed, as mentioned above, with respect to the 
occurrence of amplitude variations therein caused by shives in the pulp. 
It will be appreciated that in a measuring process of this type it is of 
fundamental importance that it is possible to distinguish between, on the 
one hand, the reduction of the intensity of the light beam caused by a 
shive passing through the light beam and, on the other hand, the reduction 
of the intensity of the light beam caused by the fibers in the pulp 
suspension flow, which are at the same time present within the light beam. 
Consequently, it is essential that the measuring process has a large 
sensitivity to shives but at the same time a low sensitivity to fibers. 
This can also be expressed by saying that it shall be possible to detect a 
shive even if at the same time a large number of fibers are present in the 
measuring duct illuminated by the light beams. The importance of this 
condition is illustrated by the fact that a typical value for the ratio 
between the number of fibers and the number of shives in a paper pulp is 
that the number of fibers is of the order 10.sub.5 larger than the number 
of shives. This value corresponds to a proportion of shives in the pulp of 
about 1% by weight, which is even a comparatively high value for many pulp 
qualtities. 
Prior art measuring processes of the kind described in the foregoing and 
prior art measuring devices operating according to these measuring 
processes are rather unsatisfactory in the above-discussed respect. 
The object of the present invention is therefore to provide an examination 
method of the type described in the foregoing and a corresponding device, 
which provide a substantially increased sensitivity to the shives with a 
maintained insensitivity to fibers, i.e. a substantially increased 
possibility of detecting a shive passing through the light beams even if a 
large number of fibers are at the same time present within the light 
beams. 
According to the invention this is achieved primarily in that the output 
signals from the two photo detectors are combined by multiplication so 
that a combined signal representing the product of the output signals of 
the two photo detectors is provided, this combined signal being analyzed 
with respect to amplitude variations occurring therein due to shives in 
the pulp. 
The manner of combining the output signals of the two detectors which 
presents itself immediately and which is most obvious, is a simple 
addition of the signals to each other. This is also the method used in the 
prior art. However, it has been found that if the combined signal is 
instead producted by multiplication of the output signals of the two photo 
detectors with each other, a substantially increased ratio between the 
measuring sensitivity to shives and the measuring sensitivity to fibers 
will be obtained. 
According to the invention light within the infrared wavelength range is 
preferably used, since it has been found that this gives an additional 
substantial increase in the ratio between the sensitivity to shives and 
the sensitivity to fibers as compared to the result obtained when using 
visible light.

FIG. 1 shows very schematically and only in principle a device for 
examining pulp for the presence of shives therein, comprising a measuring 
duct 1 with transparent walls, through which a flow of a suspension of the 
pulp to be examined is passed, as indicated by an arrow 2. From light 
sources with associated optical systems (not shown in the drawings) two 
mutually perpendicular light beams 3 and 4 are directed through the 
measuring duct 1 in a common plane perpendicular to the longitudinal 
direction of the measuring duct 1. Each of these light beams 3 and 4 
consists of substantially parallel light rays and is shaped by the optical 
system associated with the light source so as to have a comparetively thin 
rectangular cross section so that the light beam has substantially the 
form of a thin ribbon disposed in the plane perpendicular to the 
longitudinal axis of the measuring duct 1. After their passage through the 
measuring duct 1 the two light beams 3 and 4 are received by photo 
detectors 5 and 6, respectively, which consequently will provide output 
signals proportional to the intensities of the light beams 3 and 4, 
respectively, after their passage through the measuring duct 1 and the 
flow of pulp suspension present in the measuring duct. It will be 
appreciated that if a shive is present in the pulp suspension, this shive 
will, when passing through the light beams 3 and 4, "cast a shadow" on 
each of the photo detectors 5 and 6 so that the light intensities received 
by these photo detectors are reduced. It will also be appreciated that the 
magnitude of this reduction in intensity and thus the magnitude of the 
corresponding amplitude reduction in the output signals of the photo 
detectors is a measure of the breadth or width of the shive in the 
directions perpendicular to the light beam 3 and the light beam 4, 
respectively, i.e. in two mutual perpendicular directions. Consequently, 
in this way the "breadth" as well as the "thickness" of the shive are 
measured, as the shive tends to orient itself in the pulp suspension flow 
in the measuring duct 1 with its longitudinal direction coinciding with 
the flow direction. It will also be appreciated that the duration of the 
intensity reduction and thus the duration of the amplitude reduction in 
the output signals of the two photo detectors 5 and 6 will be a measure of 
the length of the shive. 
The output signals from the two photo detectors 5 and 6 are supplied to a 
signal multiplier 8 which provides an output signal corresponding to the 
product of the signals from the photo detectors 5 and 6. It will be 
appreciated that also the output signal from the signal multiplier 8 will 
display a temporary amplitude reduction when a shive in the pulp 
suspension flow in the measuring duct 1 passes through the two light beams 
3 and 4. The magnitude of this temporary amplitude reduction will be a 
measure of the cross-section area of the shive, whereas the duration of 
the amplitude reduction will be a measure of the length of the shive. The 
output signal from the signal multiplier 8 is supplied to a signal 
analyzing and displaying unit 7, in which the signal is analyzed with 
respect to the temporary amplitude reductions occurring therein due to 
shives in the pulp suspension flow, as will be described more in detail in 
the following. 
By multiplying the output signals from the two photo detectors 5 and 6 and 
analyzing a combined signal corresponding to the product of the two output 
signals from the photo detectors the possibility of detecting shives in 
the pulp suspension flow, in spite of the fact that pulp suspension flow 
contains at the same time also fibers affecting the light beams 3 and 4, 
becomes much larger than if the output signals of the photo detectors 5 
and 6 were combined by simple addition of the signals to each other. 
A further substantial improvement in this respect is achieved, when 
according to a preferred embodiment of the invention light within the 
infra-red wavelength range is used instead of visible light. The reason 
for this is probably that it has been found that the relation between the 
reduction in the intensity of the light beam caused by a shive and the 
thickness of the shive is a substantially linear function when using 
visible light but, on the contrary, a non-linear function when using 
infra-red light. This phenomenon is illustrated graphically in FIG. 2, 
which shows the relation between the transmission of a light beam, i.e. 
the percentage ratio between the exit intensity of the light beam leaving 
the measuring duct and the incident intensity of the light beam directed 
towards the measuring duct, as a function of the thickness of a shive in 
the measuring duct affecting a light beam; the curve A illustrating this 
relation when using visible light, or more exactly light from a halogene 
lamp, and the curve B illustrating the relation obtained when using 
infra-red light, or more exactly light from a luminiscence diode having 
the wavelength 930 nm. As immediately obvious from a comparison between 
the two curves A and B, the use of infra-red light (the curve B) provides 
a much stronger influence upon the transmission of the light beam from 
shives having a thickness from about 80 .mu.m and upwards as compared to 
the influence upon the transmission caused by fibers which have a 
cross-section dimension of the order of 10-50 .mu.m, than the case is when 
using visible light (the curve A). 
Experiments and calculations have been made for determining the maximum 
number of fibers that can be permitted to affect the two light beams 3 and 
4, if it shall be possible at the same time to detect a shive. This has 
been made for four different cases: (1) visible light and addition of the 
output signals of the photo detectors, (2) visible light and 
multiplication of the output signals of the photo detectors, (3) infra-red 
light and addition of the output signals of the photo detectors, and (4) 
infra-red light and multiplication of the output signals of the photo 
detectors. These experiments and calculations gave the values given in the 
following table for the maximum number of fibers than can be permitted to 
affect the two light beams without making it impossible to detect a shive 
at the same time. The experiments and the calculations were made for two 
different shive thicknesses; on the one hand 100 .mu.m and on the other 
hand 200 .mu.m. 
______________________________________ 
Added signals 
Multiplied signals 
Shive thickness 
Visible IR- Visible IR- 
.mu.m light light light light 
______________________________________ 
100 3 6 8 40 
200 5 7 20 50 
______________________________________ 
As immediately obvious from this table, multiplication of the output 
signals of the photo detectors instead of an addition of the signals to 
each other as well as the use of infra-red light instead of visible light 
produces a pronounced improvement of the sensitivity to shives relative to 
fibers. However, by far the best result is obtained, if both 
multiplication of the output signals of the two photo detectors and 
infra-red light is used simultaneously, in which case a sensativity to 
shives relative to fibers is obtained, which is about 10 times larger than 
when using addition of the output signals of the photo detectors and 
visible light. 
In the most simple embodiment of the invention the analyzer unit 7 can be 
designed to count the number of temporary amplitude reductions in the 
output signal from the signal multiplier 8 over a given period of time, 
which temporary amplitude reductions are caused by shives in the pulp 
suspension flow. This gives information of the number of shives in the 
quantity of pulp which has passed through the measuring duct 1 during the 
said period of time. 
According to a preferred embodiment of the invention the signal analyzer 
unit 7 is designed to determine also the magnitude and the duration of 
said temporary amplitude reductions in the output signal of the signal 
multiplier 8, which are caused by shives. This gives information on the 
cross-section dimension (thickness) and the length of the shives, since as 
mentioned in the foregoing the magnitude of the amplitude reduction is a 
measure of the cross-section dimension of the shive causing the amplitude 
reduction, whereas the duration of the amplitude reduction is a measure of 
the length of the shive. Preferably the analyzer unit may then be designed 
to grade the amplitude reductions in the output signals of the signal 
multiplier 8 into classes with respect to their magnitude and duration and 
to count the total number of amplitude reductions in each such class over 
a given period of time. This gives information on the total number of 
shives in the pulp within a number of size classes of shives. As an 
example the shives may be classed in 16 thickness/length classes having 
for instance the thickness ranges 80-150 .mu.m, 150-250 .mu.m, 250-500 
.mu.m and &gt;500 .mu.m and the length ranges 0-1 mm , 1-2 mm, 2-4 mm, and &gt;4 
mm.