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Description  |
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The present invention relates to a method for indicating the content of
suspended substances, existing is a flowing medium.
Especially in forest industry it is of a great interest to be able to
measure the percentage of suspended substances, in the first place fibres
and fibre fragments in various wastewaters. In nowadays used standard
methods suspended substances mean such material that may be removed
mechanically by filtration. Such suspended substances may consist of many
different components, primarily fibre material, and different filling
agents from paper manufacture possibly being of very varying size.
The instruments now available for measuring suspended substances, such as
turbidity meters, being based on the general light scattering ability of
the sample, or measuring devices operating with polarized light, permit
determination of the percentage of suspended substances, provided that
variations in composition do not exist. Typical for the waste discharge
from forest industry is, however, that such variations exist, especially
in so-called occasional waste discharges due to disturbances or errors in
the manufacturing process.
Turbidity measuring instruments are often much more sensitive to discharges
from fine fractionation (for instance fibre fragments) than for discharges
of first-rate fibres, and, therefore, an increase in the output signal
from an instrument of said type may indicate a large waste discharge of
first-rate fibres or also a considerably smaller discharge from fine
fractionation.
Since fibres have a great value, it is of interest to try to be able to
re-use the fibres in wastewater in the production. Said fibres can, if
they leave the external waste treatment plant of the factory, owing to
their size gather in fibre bars near the waste outlet, in contrast to fine
fractionation, that can be kept suspended in water for a long time,
possibly causing environmental disturbances of other kinds. An increased
number of first-rate fibres in wastewater is often an indication that
something in the system has got out of order and must be corrected.
Therefore, it is often desirable to be able to selectively measure the
quantity of first-rate fibres in wastewater, and there is also an
instrument on the market, which substantially only responds to the
quantity of first-rate fibres in wastewater, but for which the quantity of
fine fractionation does not give a noticable indication.
Existing guide-lines for the control of activity dangerous to the
environment in forest industry state that the percentage of suspended
substances, delivered by a factory to a receiving body, must be measured
and reported to the authorities. The increased requirements as to control
have also emphasized the need of an automatic, possibly continuously
recording instrument, which can discover rapid variations of the
percentage value.
Up to now measurements of suspended substances in forest industry have in
most cases taken place in the laboratories of the factory by means of
mechanical separation of a sample, which has usually been obtained by
means of an automatic sampler which, at regular intervals, collects
partial samples into a collecting sample (in most cases a 24 hours
sample). Said method is slow and comparatively expensive, since it
requires manual handling, and gives different results for different
compositions of sample. The advantage of said method is, that it functions
and is simple. The accuracy is satisfactory, but care is required for its
maintenance. The method is, however, only convenient for sample test
control.
It is true that there is also an already known method where a light source
illuminates a suspension and where the light passing through the liquid in
a certain direction is detected. According to said method the direct
voltage level of the signal is measured and, furthermore, the number of
times is counted in which the voltage exceeds a preset threshold level
during a fixed time. Said method in itself operates in a satisfactory way,
but certain calibration difficulties exist, since the accuracy of
measurement is dependent on both the preset threshhold value and on
different coefficients, with which the various indicated values must
match. Said apparatus also gives a measuring value which is dependent on
the flow velocity of the liquid.
The object of the present invention is to obtain a method by which the
total percentage of suspended substances is accurately achieved,
independent on the particle size distribution, and by which the advantages
of the various methods described above are combined while obviating the
weakness of said methods in measuring the total percentage of suspended
substances. The method according to the invention, in addition to
producing an indication of the total percentage of suspended substances,
also makes it possible to state a measure of the particle size
distribution. The method of the invention gives a possibility of a more
continous control of the variation in time of various components, enabling
an early tracing of changes.
Naturally, the method of the invention is not only suitable for use in
forest industry, but it is also applicable in many other connections,
where it is desirable to find out the percentage of suspended substances
in a liquid. This especially applies to situations where fractions of
different size appear in the liquid, which is often the case.
The invention will be better understood by reference to the accompanying
drawings, in which
FIG. 1 is a diagram which illustrates for short fibres and for long fibres,
detected light intensity transformed into voltage as a function of
frequency;
FIG. 2 illustrates the squared effective value of the alternating voltage
portion of the signal as a function of fibre length in the suspended
substance for the whole alternating voltage portion and the effective
value for the alternating voltage portion after the signal has first
passed a highpass filter having three different limit frequencies and also
illustrates the direct voltage level of the received signal relative to a
fixed level as a function of the average fibre length in the suspension;
FIG. 3 illustrates a first embodiment of a device for carrying out the
method of the invention;
FIG. 4 illustrates the shape of a curve used for explaining a part of the
invention; and
FIGS. 5 and 6 illustrate two other different embodiments of devices for
carrying out the mthod of the invention.
The method of the invention is based on the fact, that in a suspension of a
fibre material in fine fraction, i.e. fibres with a short fibre length,
the fibre material is more evenly distributed throughout a liquid volume
than if the liquid contains a coarse fraction of fibre material, i.e.
fibres with a long fibre length. The liquid will for the same percentage
of fibre material be more turbid if it contains a fine fraction than if it
contains a coarse fraction. This implies that if a flowing liquid
containing a suspension is illuminated with light approximatively
perpendicularly to the direction of flow, and said light is focused in the
liquid with a solid angle .omega. and if the radiation passing through the
liquid on the other side is detected either in the elongation of the
direction of incidence or at a certain angle in relation to said
direction, it is possible to obtain, if the intensity of the detected
light is transformed into an electrical signal, for the same percentage of
suspension with a coarse fraction, a signal with strong variations with a
relatively low frequency and with a relatively small difference in direct
current value compared with a signal without any suspension in water, and
with a fine fraction, a signal, with high frequency variations and with a
relatively large direct current component. That this is really the case
can be easily realized, if first a suspension with a coarse fraction is
regarded, where evey fibre passing the light flow gives rise to a distinct
change in the received signal of a relatively large duration, obtained
whether a fibre stands in the way of the light beam from the light source
for the detector, or reflects light towards the detector. The more
sparsely the the fibres are distributed in the suspension, the greater is
the chance that each fibre passing the beam between the light source and
the detector will give rise to a single signal change, and that two fibres
do not give rise to signal changes simultaneously, thus resulting in a
pulse formed received signal. The more fibres that simultaneously appear
in the part of the liquid just passing the beam between the light source
and the detector, the greater is the probability that more fibres
simultaneously influence the light towards the detector, bringing about
that the signal will vary more complexly with a larger number of frequency
components but still with a predominance of low frequencies.
If, on the other hand, the fibres of the coarse fraction are ground into
smaller particles, it is easily understood that each fibre part in the
suspension so obtained when passing the beam between the light source and
the detector will give rise to a relatively short pulse, but that a larger
number of fibre parts will pass after each other and beside each other, so
that even a low percentage will give a relatively "turbid" signal with a
strongly varying frequency content and with a certain predominance of
higher frequencies compared with the coarse fraction and with a relatively
large direct voltage component.
In FIG. 1 two curves are shown of the detected light transformed into
voltage as a function of frequency, but with the omission of the direct
voltage component, for two different suspensions with a predetermined
percentage of suspended substances. Curve I refers to the frequency
variation for long fibres and curve II for short fibres. Naturally, the
frequencies are directly dependent on the velocity of the flow of the
liquid. As is apparent from the diagram the curves are intersecting at the
frequency f.sub.0. Curve I for long fibres shows a higher voltage within
the low frequency range of frequencies and a lower voltage within the high
frequency range than curve II. For higher percentages than that shown by
the curves, these will be displaced approximatively parallelly in an
upward direction, and the intersections will approximatively lie above
each other, however, with a certain displacement to the right, since the
alternating voltage component as such will decrease faster for a fine
fraction than for a coarse fraction, while, on the other hand, the direct
voltage component will increase faster for a fine fraction than for a
coarse fraction, owing to the above described increased turbidity of the
suspension for higher percentages. However, it is possible to find a
frequency range around the intersection point, which the effective value
of the voltage, i.e.
##EQU1##
is identical for both curves, and that said relationship with the same
limit frequencies will also be applicable with a good approximation for
other percentages the one illustrated in FIG. 1 within a limited
percentage range.
As is apparent from FIG. 1 much greater difference is obtained between
curve I and curve II for low frequencies than for high frequencies. In
consequence thereof it is possible to select the chosen frequency range in
such a way that a low-frequency part of the total frequency band is cut
off.
This result is not quite true for very short fibres, but for the indication
of the percentage of a suspension having a fraction above a predetermined
coarseness, the same value will be obtained for the effective value of
detected light transformed to voltage. This is readily apparent from FIG.
2 in which the solid curves show the effective value of the alternating
voltage portion of the signal obtained from the above-mentioned detector
after passage through a highpass filter with different limit frequencies
as a function of the fibre length in a suspension.
These curves are obtained for different suspensions with a fibre percentage
of 100 milligrams per litre with fibres of different lengths, by measuring
the effective value of the alternating voltage portion of the output
signal of the detector after the passage through different highpass
filters. As is apparent from the different curves, an almost horizontal
curve is obtained above a predetermined fibre length. The horizontal part
of the curve will be more extended to the left for shorter fibres, the
higher the limit frequency is selected, but the voltage value obtained
will simultaneously be strongly decreased for higher selected limit
frequencies. As is apparent from a comparison with FIG. 1, the largest
differences between the curves I and II appear at the leftmost part of the
curves, where the limitation is made.
The consequence thereof is that a fairly good approximation can be obtained
when measuring the fiber percentage in a suspension where the fibre
content can be expected to contain fibres which are mainly above a
definite length, by means of a highpass filter after the above mentioned
detector.
FIG. 2 also shows on an enlarged scale the direct voltage portion of the
signal delivered from a detector placed in the angular position 0.degree.,
i.e. just opposite the light source on the other side of the focusing
point, said signal being deducted from a direct voltage value obtained for
clear water as shown by a dashed line. As is apparent from the diagram,
the direct voltage signal is largest for fine fractionation, i.e. for
short fibres, and then decreases for coarse fractionation, i.e. for long
fibres, being fully in correspondence with the above discussion. On a
comparison between the curve for the direct voltage signal and that for
the squared effective value over the whole alternating voltage portion
there is left no room for doubt, that the curve for the direct voltage
signal and that of the unfiltered alternating voltage signal run in
opposite directions, and investigations have shown, that if the above
mentioned direct voltage difference signal multiplied by an appropriate
factor is added to the mean value of the squared alternating voltage
portion, a curve is obtained that is practically horizontal for all fibre
lengths. For different fibre percentages said curve is parallelly
displaced upwards for higher percentages of suspended material and
downwards for lower percentages. If the square of the effective value of
the alternating voltage part is added to the direct voltage signal
multiplied by the appropriate factor, the result will be fully independent
of the flow velocity of the liquid down to very small velocities, which is
a property of a great value making this embodiment of the method very
useful.
From the curves for the direct voltage difference signal and that of the
unfiltered alternating voltage signal it is also apparent that for
instance by dividing said signals it is possible to obtain a signal with
strong variation for different fractions which may be used for indicating
a change in the relation between coarse fraction and fine fraction in the
suspension being investigated.
FIGS. 3, 5 and 6 show three different embodiments of devices for carrying
out different embodiments of the method according to the invention. In the
device shown in FIG. 3 a light source 1, via an optical element 6 and a
diaphragm 7 illuminates a transparent tube 5, through which a liquid
containing suspended substance is flowing. On the other side of the tube
at an angle .theta. to a line from the light source 1 through the centre
of the tube a light detector 2 is placed. The angle .theta. is, if only
the alternating voltage part of the signal from the detector is to be
indicated, in no way critical, but may be selected between 0.degree. and
90.degree.. The magnitude of the signal is, however, influenced thereby.
The light intensity detected by the light detector 2 is transformed into
an electric voltage signal, which is fed through a filter 3 to a generator
4 of effective value. This generator 4 can be a voltmeter, indicating the
true effective value (true RMS-voltmeter), the suspension percentage,
however, not being linear with respect to the scale of the voltmeter. A
linear relation to the suspension percentage, at least for low values
thereof may, however, be obtained if the output signal from said generator
4 is squared in a squaring circuit 4' connected in the signal direction
after said generator.
If a measurement of the fibre percentage for fine fraction is desired, the
filter 3 is a bandpass filter, with its limit frequencies suitably
selected on each side of the frequency F.sub.0 in FIG. 1. This will give a
direct dependence between the velocity of flow of the liquid and the
frequency, and therefore a control of one of said magnitudes relative to
the other one must be made.
If it is less important to be able to indicate the content of a very fine
fractioned suspension, the filter 3 is instead a highpass filter, where
the lower limit frequency, as is evident from FIG. 2, is higher the more
fine fractioned suspension is to be indicated. The advantage of this
embodiment is that the dependence on the velocity of flow of the liquid
will not be critical. However, it should be observed that the lower limit
frequency of the filter is dependent on the velocity of flow, and
therefore instead of raising the limit frequency it is possible to
decrease the velocity of flow, and to get the same curves as in FIG. 2.
The measuring time, however, must be in proportion to the velocity of flow
in order that the same scatter of measuring data should be obtained.
Sometimes colour variations can occur in the liquid, flowing through the
transparent tube, and such variations must be compensated. Therefore, in
the device according to FIG. 3 a second light detector 8 is shown, which
is placed (at a definite) angle .phi. to the line through the light source
and the center of the tube.
If light with a definite solid angle .omega. is to incide in and is to be
focused in a liquid without content of light scattering particles, the
light radiating from the liquid is in principle evenly distributed over
the same solid angle as the inciding light on the other side of the
focusing point, so that a light detector, which is moved around the
focusing point would not be able to detect any light outside the solid
angle range. Within said range an even light flow would then be obtained,
provided that the inciding light flow is evenly distributed over the solid
angle range of the inciding light. A light detector which is moved around
the focusing point opposite the inciding light should thus give a
rectangular output pulse. This can never be obtained in practice, since
the light scattering phenomena always appear, by an output signal from a
detector, being moved around the focusing point, will be
Gauss-curve-shaped or bellshaped with the strongest signal opposite the
light source and with steep flanks when passing the range around the limit
of the solid angle range. The more light scattering particles the liquid
contains, the lower is the signal which is obtained opposite the light
source, and the flatter are the flanks of the Gauss-curve (see FIG. 4). At
the same light absorption in the liquid but with different scattering,
however, the obtained Gauss-curves always have the same intersection
points K, which has been proved when measuring conifer wood sulphate of
fractions > 16; 16-30; 30-50; 50-100; 100-200; < 200 Mesh for
concentrations up to 200 mg/l. If a light detector is placed in a
direction relative to the focusing point such that the Gauss-curves run
through the same point, the signal indicated by said detector will be
independent of the scattering phenomena and thus only dependent on the
absorption of the liquid of the light used, i.e. the colour variation of
the liquid. It should be observed that this applies to the direct voltage
portion of the signal only, so that consequently in said point, when
measuring a flowing liquid, a signal will be obtained which will fluctuate
around a constant value.
If an increase in the absorption in the liquid occurs, the two curves will
fall in relation to the increased absorption, and consequently also the
value in the point K at the angle position .phi.. This is utilized for
colour compensation, by placing the light detector 8 in the angle position
.phi. and by feeding the voltage signal emitted from said detector through
a filter 9, that filters out the alternating voltage portion, to a control
device 9' which in turn controls the drive voltage source 10 of the light
source 1 so that the direct current component fed from the detector 8 will
remain constant. Any other type of colour compensation may of course be
used.
Since the placement of the detector 2 is not critical, the detector 2 and
the detector 8 in said embodiment can be one and the same placed in the
angle position .phi., the output signal from said detector in this case
being divided into a direct current portion for colour compensation and
into an alternating voltage portion for measuring the suspension
percentage.
The device shown in FIG. 5 utilizes the fact that it is possible to obtain
a practically horizontal curve for all types of fractions if the squared
effective value of the alternating voltage portion of the signal from the
detector 2 is added to the difference between two direct voltage signals
from a light detector multiplied by a constant.
If in FIG. 4 curve .alpha. relates to clear water, i.e. to water without
any content of suspended substance and curve .beta. relates to water with
suspended substance, it is apparent that the difference between the output
voltage level for a detector placed at the angle position 0.degree. is x,
and for a detector, placed at the angle position .theta., is y. Trials
have shown that the proportion between y and x is approximatively a
constant for the low percentages of the suspension, for which the method
of the invention is suitable. Therefore, the direct voltage portion with
reference to a constant value can be used from a detector, which can be
placed in any angle position except just in the angle position .phi.. The
angle position .theta. should, however, lie on an easily definable part of
the curve for clear water, different from zero.
In FIG. 5 a detector 13 is placed in the angle position 0.degree.. The
output signal therefrom is fed to a filter 14, in which the alternating
voltage portion is filtered out. The output signal of the filter 14 is fed
to an amplifier 15, the amplification of which is adapted to the constant
with which the direct voltage signal is to be multiplied, and can
consequently have an amplification below 1. As in FIG. 3 the output signal
is fed from the light detector 2 to a filter 11. However, in this case
said filter filters out the direct current part only of the signal.
Thereafter the effective value of the signal is produced or generated in
the effective value generating device 12 and is squared in the squaring
circuit 12'. The signal from the element 12' and the signal from the
amplifier 15 are added in the adder 16.
Also in said embodiment a colour compensation is made, but instead this
takes place at the output signal. If the colour increases in the liquid,
i.e. for increasing absorption, the two curves are lowered in FIG. 4 and
the lowering of the curves at the angle position 0.degree. and at the
angle position .theta. will have a certain relation to each other. This is
utilized in such a manner that the signal from the detector 2, which in
this embodiment is placed in the angle position .theta., separated from
.phi., is fed to an additional filter 20 filtering out the alternating
voltage portion of the signal. The signals from the filter 20 and from the
amplifier 15 or alternatively directly from the filter 14 are each one fed
to an individual input of a signal treating device 17 calculating the
given relation between the signals, and in dependence thereon a signal is
fed to a control input of an evaluation circuit 18, the output signal
obtained from the adder 16 being thus colour compensated. Instead of using
the signals from the detectors 2 and 13, a detector 8 can, of course, also
in this case be placed in the angle position .phi., and its output signal
can be fed to the control input of the evaluation circuit 18 for the
colour control.
As is apparent from the above statements, the detectors 2 and 13 can of
course for the measuring of the suspension percentage be one and the same
detector, the output signal of which is divided into a direct voltage
portion and an alternating voltage portion and is treated individually in
the circuits 11, 12, 12', 14 and 15, and this is also shown in FIG. 6. In
accordance with said Figure the colour compensation takes place exactly as
in FIG. 3 with a detector 8 placed in the angle position .phi.. In said
embodiment also the output signal from the filter can be fed to the
control input of an evaluation circuit 19.
Many different modifications are possible within the scope of the
invention. The transparent tube, for instance, must not have a circular
section but may be of a square type if a compensation of the light beam is
performed. It is also possible not to have any tube at all, but to lower
the light source with the optical elements and the detectors in the liquid
proper. Of course, the method according to the invention is not only
applicable for measuring a flowing liquid but also for measuring particles
existing in a flowing gas.
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