Measuring apparatus and method

Abstract: A method for registering the state of a moving suspension containing particle fractions of mutually very different sizes, comprising illuminating the suspension with a beam of light and detecting light exiting from the suspension for the purpose of producing an electric signal which corresponds to the intensity of the detected light. The suspension is illuminated with the aid of a beam of light which is narrow in relation to the interspaces between large particles and detecting light exiting from the suspension within a narrow angular range. An extreme value (peak or bottom value) of the electrical signal is indicated within predetermined time intervals; and starting values are formed within each extreme value time-interval for the purpose of calculating at least one pre-selected property of the suspension.

Description: The present invention relates to a method of the kind set forth in the preamble of claim 1, and an apparatus of the kind set forth in the preamble of claim 7. The amounts in which material is suspended in different kinds of aqueous suspensions is an important measuring parameter, not least from the aspect of environmental care and protection. By suspended material is meant here generally such substances as those which can be separated mechanically by filtration and centrifugation. Especially in the forest industries, the suspended material may consist of many different components such as fibres, fibre fragments and various fillers and coating agents. These components may vary widely in size, from a few millimeters in length and some tens of microns in width (fibres) down to a particle diameter of about 1 micron and smaller (filler). The concentration of a suspension can vary within wide limits, from some mg/l to tens of g/l. It is not only important to measure the content of suspended material, since it is important in many cases also to indicate when the size distribution of the suspended material changes and the extent of this change. One area of use in this respect is to indicate the effect of the addition of floculating chemicals. The majority of instruments at present available on the market for continuously measuring suspended materials are based on optical measurement principles, e.g. on light absorption, light scattering and the influence of polarized light. The most common method comprises measuring the turbidity of the suspension, in which attenuation or scattering of light is used as a measurement of the suspended-material content. The extent to which light is scattered, however, does not only depend on the concentration of the suspended material, but also on the particle size, shape, surface structure and refraction index of the material concerned. Thus, suspensions in which the particle-size distribution varies considerably can give misleading information with regard to the concentration of the suspension. This is illustrated in Figure 22, which discloses the result of turbidity measurements in which attenuation of the light transmitted through suspension was measured. The result (the output signal from the measuring apparatus) is shown as a function of the concentration of cellulose fibres (large particles) and for clay (small particles). As will be seen, such an instrument is far more sensitive to clay than to fibres for one and the same concentration. US 4 110 044 describes an optical method which, in principle, enables the concentration of suspended material to be determined independently of particle size. This measuring principle is based on recording not only the mean value of the light transmitted, but also on measuring fluctuations in light intensity, in the form of a signal which includes the square of the true effective value of an alternating voltage component of the measurement signal obtained. As a function of particle size, this value has a mirror-reversed behaviour in relation to the signal which is based on the direct voltage component of the measurement signal. Consequently, the sum of these two signals will provide a measurement of the amount of material in the suspension independently of particle size. Furthermore, it is possible to obtain a relative measurement of the particle-size distribution of the suspended material, by forming the quotient of these two signals. This method is called the TP-method and is quite effective, particularly in the case of low concentrations, where good linearization of both the direct voltage signal and the square of the true effective value can be obtained. However, in the case of suspensions having a particle-size distribution in which the particles are predominently large particles, the signal formed by the square of the effective value is influenced to a greater extent by the large particles than by the small particles. Another drawback, and one which is often serious in practice, is that the TP-method is relatively difficult to calibrate. Another method which indicates the direct voltage component of the measuring signal and the effective value of the alternating voltage component of the measuring signal is described in US 3 879 129. This has the same drawbacks as the TP-method. In order for the TP-method, and similar methods based on measuring the effective value of the measuring signal to provide good accuracy with respect to particle content and good resolution with respect to particle size, it is necessary (depending on particle content and particle size) for the measured volume to be small and the light beam to be narrow. If essentially each particle is to produce a significant indication of the alternating voltage component of the signal, the illuminating light beam should be narrow and preferably collimated or focused, and the reflected light should be detected within a narrow angular range. The variations in alternating voltage are smoothed out when the light beam has a broad path. It is often desired to measure the suspended-material content of suspensions in which the proportion of suspended material is relatively high. In order to enable light radiated through the suspension to be indicated at all, the light source must be located comparatively close to the light detector. Consequently, it is usual to place the measuring apparatus in a narrowed or necked part of the measuring cell. This narrowed area of the measuring cell is prone to become blocked with large particles, suspended material-agglomerates and the like. Consequently, these narrowed measuring cell-areas are equipped with back-flushing devices and the like by means of which said area can be rinsed clean, when necessary. One example of an arrangement of this kind is described in the article LASER OPTICAL METHOD FOR DYNAMIC FLOCCULATION TESTING IN FLOWING DISPERSIONS, by W. Ditter et al, BASF Aktiengesellschaft from the book The Effect of Polymers on Dispersion Properties, Academic Press, London 1982, pages 353-342. The suspension is led through a narrowed or necked measuring cell equipped with a measuring head arrangement. Light from a laser is focused onto the measuring cell and light exiting from the measuring cell is led to a photosensor. The main object of the present invention is to provide a particularly favourable method for evaluating the signal produced by a measuring head operation to measure a flowing suspension. Still another object of the invention is to provide measuring apparatus, which will illuminate the flowing suspension with a narrow light beam and detect the light, transmitted or scattered by the suspension within a narrow angular range. The light beam may be collimated or focused. The main object of the invention is achieved with inventive method for registering the state of a moving suspension having the characterizing features set forth in claim 1. Further significant features of the invention and further developments thereof and an apparatus for registering the state of a moving suspension is set forth in the claim 7. Yet another object of the present invention is to provide a measuring head operative to measure a flowing suspension with which the risk of blockages due to solid particles is minimized. The main object of the invention is achieved with inventive method for registering the state of a moving suspension having the characterizing features set forth in claim 1. Further significant features of the invention and further developments thereof and an apparatus for registering the state of a moving suspension are set forth in the remaining claims. The width of the measuring beam is adapted so as to be the same size as or smaller than the interspaces between the larger particles in the suspension on which measurements are carried out. When measuring is effected with the intention of establishing the mutual distribution between the fine fraction and the coarse fraction of the suspended material, the width of the beam is adapted to the desired boundary line between the coarse and the fine fractions. The width of the measuring gap is adapted so that the multiple scattering effect of the particles present in the suspension will be as small as possible. Consequently, the width of the measuring gap will be small when the concentration of suspended material is high, and large when the concentration of suspended material is low. Naturally, the gap width will be larger than the size of the anticipated largest particles in the suspension on which measurements are carried out. The constructional design of the measuring apparatus may vary widely, for instance with respect to the measurement indicator body intended for immersion in a channel or passageway or for embodiment in the walls of a tube intended for the throughflow of media, etc. The measuring apparatus may also be provided with several optical measuring channels, e.g. channels which operate with and without polarized light, light of mutually different wavelength, beams of mutually different diameters, etc. In this respect, it is possible to obtain appropriate information for respective measuring positions from the detector signal, with the aid of appropriate signal processing and correlation-measuring techniques. Measuring apparatus which have one or more optical measuring channels may also be provided with measuring detectors operative to measure scattered light, which can provide more information about the composition of the suspension. Simultaneous measurement of the transmitted and scattered light can also be employed to compensate for absorption effects in the suspension or in coatings on the optical surfaces, or to improve characterization of the composition of the suspension. A reference detector can be used to compensate for variations in intensity of the light source. Optical glass-fibres can be used with or without protection against the suspension being measured, depending on purpose and on measuring position. The ends of the fibres may also be provided with polarization filters, lens/index-lens, and protected with the aid of a quartz window or sapphire window etc., without departing from the basic concept of the configuration of the measuring channel and measuring gap of the inventive measuring apparatus. When the measurement indicating body used is intended for immersion in flowing medium, the body will preferably be configured in a manner which will ensure that good flow conditions are achieved around the body, at least in the vicinity of the measuring gap or throat, so that the suspension will truly pass through the gap and so that the gap will be essentially self-cleaning. The region around the measuring gap is therefore provided with smooth surfaces and rounded edges. The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 is a front view of a first embodiment of a measuring head to be used together with the method according to the invention; Figure 2 is a view taken from beneath the apparatus illustrated in Figure 1; Figure 3 is a longitudinal section view of a second embodiment of a measuring head, taken on the line III-III in Figure 4; Figure 4 is a view from beneath of the embodiment illustrated in Figure 3; Figure 5 is an alternative view from beneath of the embodiment illustrated in Figure 3; Figure 6 is a sectional view of a first embodiment of a measuring head with mechanical cleaning; Figure 7 is a sectional view of a second embodiment of a measuring head with mechanical cleaning; Figure 8 is a view of the side with the measuring channel in Figure 7, taken from above; Figure 9 illustrates still another embodiment of a measuring head; Figure 10 illustrates a fourth embodiment of a measuring head; Figure 11 is a diagram which illustrates the output signal from the detector of an embodiment of the measuring apparatus according to the invention and showing a number of parameters; Figure 12 is a block schematic of an embodiment of signal processing apparatus according to the invention; Figure 13 is a diagram of a curve scale obtained when testing the invention; Figure 14 is a diagram comprising a number of curves obtained when carrying out experiments with the invention with successively increasing concentration of suspended materials, and Figure 15 illustrates the result of measuring the concentration of suspended material of different particle sizes carried out with a known concentration meter. The actual invention is shown in Figures 11 to 14 and a thorough description of it is given in the description of those Figures. Figures 1 to 17 show some suitable measuring heads to be used in the inventive method. The measuring apparatus, or meter, illustrated in Figures 1 and 2 has a measurement indicating body in the form of a four-sided plate 1 with bevelled edges. The plate 1 forms a probe which is intended for immersion into a flow of suspension, with the broad side of the plate 1 facing towards the flow direction. The bottom surface of the plate has provided therein a groove 2 which functions as a measuring channel. The plate 1 has on one broad side thereof two rounded and comparatively deep grooves which connect at one end thereof with a respective side of the measuring channel and which extend parallel with the undersurface of the plate precisely at said connection. Each of the grooves has moulded therein a fibre-optic light guide 3, 4. The inner end of the guide 3 is connected to a light source 5, such as a photodiode or a small laser, which is also moulded in the plate. The inner end of the guide 4 is connected to a light detector 6 which is adapted to the light emitted by the light source and which is also moulded in the plate 1. The electrical guides or cables leading to the light source 5 and extending from the light detector 6 are also moulded in the plate and extend to and pass through a tube 7 screwed into the top of the plate. Because it is necessary to extend the fibre-optic light guides in a gentle arc, so as not to influence their performances, the channels carrying the guides 3 and 4 are given a gentle curved configuration. Both the light source 5 and the light detector 6 are temperature responsive components. Consequently, a temperature sensor 8 is moulded in the plate 1 and an electric cable or fibre bunch extending from the sensor 8 is also passed through the tube 7 and connected to a signal processing circuit (not shown in the Figure) for the purpose of compensating for temperature when calculating the measuring result with the aid of the signal arriving from the light detector 6 in a manner described in more detail hereinafter. The surfaces of the plate 1 will preferably be smooth and also repellent, so as to avoid adherence thereon of material flowing past the plate, to the greatest possible extent. Naturally, the material from which the plate is made should be inert with respect to the medium in which the material is suspended, and also to the suspended material. The plate 1 is preferably made of a plastics material or some like material, although it may also be made of stainless steel. The essential features of the measurement head lie in the configuration of the measuring channel together with the optics used. The width of the measuring gap between the side-walls is contingent on the type of medium on which measurements are to be carried out. For instance, if measurements are to be carried out on fibre suspensions in waste water that derive from the paper manufacturing industry, a width of about 3 mm may suffice. The edges of the gap will preferably be low and the fibre-optic light guides will be placed as close as possible to the outer edge of the gap, for manufacturing reasons. The underside of the plate, which holds the fibre-optic light guides in place, is preferably bevelled in the region of the measuring channel 2, whereas the end of the fibre-optic is preferably cutoff transversely to the direction of propagation and the severed end polished. The invention is primarily conceived for the detection of light transmitted directly through a suspension, and hence the fibre-optic light guides are placed with their respective ends opposite one another. However, as will later become apparent from the description made with reference to Figure 16, light which is scattered within a given angle can also be detected. As beforementioned, the optic, i.e. the ends of the fibre-optic light guides 3, 4 of the illustrated embodiment, shall be placed close to the outer edge of the measuring gap. As shown in Figure 2, the channel inlet and outlet are also gently rounded in the flow direction, as is also the small part of the outer edge of the channel located externally of the cptic. The fibre-optic will preferably have a given depth beneath the channel, such that the effect of friction between the flowing suspension and the bottom cf the channel will have no appreciable influence on the suspension flowing past the fibre-optic. The presence of sharp edges on the channel bottom should be avoided. All of these measures contribute towards providing a measuring head in which the risk of the channel becoming blocked is minimized. Preferably, that part of the measuring apparatus located in the medium or suspension, at least in the vicinity of the measuring channel, is given a somewhat streamlined configuration so as to reduce the risk of a change in form of the suspension passing through the measuring gap in relation to the actual or true composition of the suspension. In Figures 3 and 4 the measuring body has a rotational-symmetrical configuration. Figure 3 is a section view taken on the line III-III in Figure 4, and Figure 4 is a view seen from beneath in Figure 3. These Figures also illustrate an embodiment in which the fibre-optic light guides 10 and 11 are passed completely through the body 9 and through a tube 12 screwed into the upper end of the body 9, although it is conceivable, of course, to incorporate the light source, light detector and temperature sensor in the body. The body 9 is preferably pear-shaped, so that the fibre-optic light guides 10 and 11 can be extended in gentle curves from the measuring channel 13 up to and through the tube 12. Figure 4 illustrates a suitable embodiment of the channel 13, having a broad inflow and outflow part and a narrowed or necked measuring gap part precisely in the region of the openings of the fibre-optic light guides 10 and 11 into the channel. The body 9 is preferably manufactured in two parts, which are glued together along an interface 14. One of said parts, in Figure 4 the bottom part, has cast or milled therein grooves for accommodating the fibre-optic light guides 10 and 11. The body 9 of the Figure 5 embodiment has an elongated configuration in the flow direction. The section between the fibre-optic light guides is, in this case, the same as in Figure 3. An even more streamlined configuration is conceivable. In the Figure 3 embodiment, a light source 15 for the light fibre-guide 10 and a light detector 16 for the light fibre-guide 11 are positioned in a part located on the end of the tube 12 externally of the suspension medium. If this location is stable with respect to temperature, there is no need to provide a temperature sensor for temperature compensating purposes. The light source 15 is supplied form a voltage source 17. The electrical output signal from the detector 16 is supplied to an evaluating and calculating unit 18. Appropriate calculations will be discussed hereinafter when describing Figs. 11 to 15. The calculation result is passed to a presentation unit 19, which may have tne form of a display screen or a curve printer for continuous performance. The apparatus is intended primarily to provide quickly information concerning tendencies or trends towards changes in a suspension, so that monitoring personnel are able to notice immediately when something happens to change the suspension content of the monitored medium and to observe the effects of remedial measures taken. The aforedescribed measuring units may be intended for immersion in a flowing suspension e.g. a suspension flowing in an open channel or passageway. Figure 6 illustrates a variant of the measuring head illustrated in Figure 3, this variant incorporating mechanical cleaning of the head. The fibre light guides 22 and 23, illustrated solely by full lines in the Figure, extended to the measuring channel 21 are located relatively close to the periphery of the measuring body 20. The cleaning arrangement is placed in a central hollow or cavity in the body 20. The cavity has a first cylindrical part 24 whose diameter corresponds essentially to the measuring gap of the measuring channel 21, e.g. a width of about 3 mm, a second cylindrical part 25 of somewhat larger diameter, e.g. 5 mm, than the diameter of the first cylindrical part, and a third cylindrical part 26 of still larger diameter, e.g. 12 mm. A cylindrical brush 27 of essentially the same configuration as the dental brushes used to clean between the teeth has its stem 28 moulded in a plunger 29 which moves in the third cylindrical part 26 and which has essentially the same diameter as said part. The brush faces towards the outlet of the first cylindrical part 24. The second cylindrical part 25 functions as a brush guide, such as to prevent the brush from bending when moved outwardly through the opening at 24. The brush may also be caused to rotate during a cleaning operation. A helical spring 34 is positioned between the plunger 24 and the shoulder formed at the junction between the second and third cylindrical parts 25 and 26. A ringshaped seal 30 is fitted to the periphery of the plunger 29. The plunger can be operated either pneumatically or hydraulically, by supplying gas or liquid to the part of the plunger 24 distal from the measuring gap 21, through a closed channel 35 provided in the stem of the measuring body 20. A nippel 31 is screwed into the neck of the body 20, adjacent the third cylindrical part 26. Fitted onto the nippel is a hose which is connected to a pressure-medium arrangement 36 which when brushing is to take place supplies pressure-medium to the rear side of the plunger 29, so as to push the brush 27 out through the opening 24. The stem 28 of the brush projects outwardly through a given distance from the plunger 24. When cleaning the brush 27 or replacing the brush together with plunger, the nippel 31 and hose are removed. The end of the brush-stem is gripped with a suitable tool, e.g. snipe-nose pliers or the like, and the brush and piston are withdrawn through the cavity formed therewith. The measuring process effected with the measuring head is carried out on a flowing suspension. Despite the particular configuration of the measuring gap with low, gently rounded edges coatings will nevertheless form and consequently the measuring gap must be cleaned, preferably mechanically. The frequency at which the gap must be cleaned will depend on the composition of the suspension. A pressure-medium arrangement 36 operative to supply and remove pressure medium for operating the plunger 34 is controlled by a control circuit 35, which may be program controlled, time controlled, manually controlled or controlled by a combination of these types of control. The spring 34 is sufficiently strong to be able to move the brush back to a withdrawn position, even when part of the suspended material has adhered to the bristles of the brush. It may happen that particularly large suspended substances adhere to the bristles, so as to prevent the brush from being withdrawn completely into the measuring gap, or so that a fibre part projects from said gap. This will result in a marked indication in the measuring signal indicated by the evaluating and calculating unit (not shown in Figure 6), and this unit will then supply a control signal to the control circuit 35, which subsequently causes the pressure-medium arrangement 36 to commence a new brushing cycle. If one or more additional brushing cycles fail to give the result desired, the evaluating and calculating unit can be constructed to deliver an alarm signal indicative of the fact that the brush must be cleaned or replaced. Figures 7 and 8 illustrate the measuring body 37 at present preferred since this body can be manufactured and serviced in a simple fashion. The measuring body 37 of this embodiment is in the form of a flat plate whose periphery has an irregular configuration. The fibre-optic light guides 33 and 40 of the measuring body 37 are drawn to the measuring gap 38, these guides being shown schematically in the Figure in the form of a full line in those cases when the fibres lie on the same level as the illustrated section and in broken lines when said fibres lie on a different level. The fibre guides are placed in gently rounded channels and are passed through a nippel 41 screwed into the body and supporting one end of a hose 32, from where the guides are extended to an evaluating and calculating circuit (not shown in Figure 7). The hose extends in a tube 33 screwed to the top of the body 37. In this case, the measuring gap is placed on a surface 43 which is oblique in relation to the surface along which supply lines are drawn to control and evaluating circuits. Figure 8 is a view of the surface 43 containing the measuring-gap channel and the measuring gap 38 and the inlet of a channel 42 provided with a brush. The through channel 42 accommodating the brush arrangement has essentially the same configuration as the channel illustrated in Figure 6, but is obliquely positioned in relation to said channel, since the channel of the Figure 8 embodiment extends perpendicularly to the oblique surface 43. This provides a readily accessible opening on a side-part parallel to the surface 43 , through which the brush arrangement can be serviced opposite the oblique surface. This opening is provided with internal screw threads and can be closed by means of a screwable lid or cover 44. A pressure-medium channel 45, optionally in the form of a tube moulded in the body, is provided between the channel 42 and a nippel 46 screwed into the body, said nippel having attached thereto a hose 47 which extends to a pressure-medium arrangement (not shown). The fibre-optic light guide 40 and the channel 42 are located at different levels in the body 37. This measuring head includes a further channel 48 which extends between a nippel 49 screwed into the body and the channel 42, close to the measuring gap 38. A cleaning medium is supplied through the hose connected to the nippel 49, immediately prior to and optionally also during a brushing operation, for the purpose of improving removal of material adhering to the measuring gap. This will also enable the excessive adherance of suspended material to the brush to be avoided. The channel 48 can also be flushed clean between the individual brushing cycles. The cleaning medium may be either a gas, such as air, or a liquid, such as water, optionally admixed with chemicals. Figure 9 illustrates a third embodiment of a measuring apparatus which is intended to be inserted in a measuring cell or a tube through which a suspension flows. The measuring apparatus has the form of a tube 50 whose diameter is the same as the diameter of the measuring cell/tube in general. A measuring channel 51 is configured in a thickened part on the inside of the tube. The tube wall has placed therein two fibre-optic light guides 52 and 53 whose respective one ends are each connected to a respective side of the measuring channel 51, the other ends of said guides being connected respectively to units in the form of a light source 54 and a light detector 55. Since the units 54 and 55 are positioned so as to be influenced by the temperature of the medium being measured, a temperature indicator 56 is placed in the proximity of said units. A voltage source 57 is connected to the light source 54. Similar to the aforedescribed measuring heads, the signals from the units 55 and 56 are applied to an evaluating and calculating circuit 58, which is operative to calculate desired suspension data, e.g. concentration, particle-size distribution, fibre content etc in accordance with the invention. The calculations are carried out continuously with corrections being made for the temperature indicated by the indicator 56, and supplied to a presentation unit 59. The presentation unit 59 may, for instance, be a curve printer operative to print continuously curves representing one or more of the calculated properties of the suspension. Monitoring personnel can be made aware of trends occurring towards changes in the suspension by changes in the curve or curves, and may quickly observe the result of measures taken to change the composition of the suspension. These measures may involve the addition of flocculating chemicals to waste water with the intention of precipitating solid material. The tube 50 may include a plurality of measuring channels 51, for the purpose of measuring different properties of the suspension. These measuring channels may, for instance, be disposed sequentially in the flow direction, or located at mutually different positions around the tube circumference (not shown). For instance, the particle-size distribution of a suspension may be measured with the aid of two or more measuring channels with mutually different diameters of the transmitted beam path. Figure 10 illustrates a further embodiment of the measuring apparatus. This apparatus is a variant of the apparatus intended for measuring cell/tube throughflow illustrated in Figure 9, although the principle can also be applied to variants of the other aforedescribed measuring apparatus. Instead of a fibre light-guide for the emitted light beam, the Figure 17 embodiment has a straight channel 90 extending to the measuring channel 88 in the thickened part of the measuring cell/tube 89. A laser which sends a narrow light beam through the channel 90 is placed at the other end of the channel. Located on the opposite side of the measuring channel 88 is a fibre light guide 92 which passes around the tube 89 and leads to a light detector 93, the electrical output signal of which is applied to a calculating and presentation unit 94, as is also the output signal from a temperature sensor 92. A drive-voltage source 96 is connected to the laser 91. When using a narrow beam in relation to the interspaces between large particles in the suspension, the alternating voltage component of the detector signal is able to give an indication of the suspended material that flows past, with good resolution. The novel signal-processing method according to the invention has been found very suitable for use with the measuring head described above. since said head is able to provide the well-demarcated and narrow beam path and a measuring gap of small width required for said signal-processing method when measuring suspensions typical of the pulp and paper manufacturing industry for instance. The inventive method is based on a first assumption that the suspension will in principle consist of two components, large and small particles. The large particles can then be considered as a relatively transparent network in which many small particles are suspended. When a minor part-volume of such a suspension is considered in accordance with statistic distribution principles, the number of small particles will be large and relatively constant. The number of large particles, on the other hand, is very small and varies greatly. A suspension of fibres 1 mm in length and 20 microns in width and having a concentration of 1000 mg/l will, for instance, contain approximately two particles per mm 3 . If 100 mg/l clay having a particle size of 1 micron in diameter is added to this suspension, the number of particles will increase to 7.5*10 4 per mm 3 . When measuring light attenuation in a minor volume of this suspension, a mean value of the intensity of the transmitted light will be obtained. Deviations from this mean value are caused primarily by fluctuations in the fibre material (the fibres enter the light beam from time to time). In principle, the greatest signal height is obtained when no fibres are present in the light beam and the light beam is attenuated solely by the clay particles. This enables the proportion of large or small particles in the suspension to be determined by measuring the mean value V DC of the light transmitted and the maximum value of said light, i.e. peak values V P , over a predetermined time period. (In this connection, light-absorbing, dissolved substances in the suspension function in the same manner as clay and can be determined by measuring the peak value V P . Lambert Beer's law is used to calculate the turbidity or concentration of the suspension, this law giving the intensity of transmitted light in accordance with the following relationship: V = Vo * e -alN where Vo = the intensity of the light transmitted for clear water, i.e. water having no particles suspended therein. V = the mean value of the light intensity attenuated by the presence of particles. a = the scattering coefficient. l = the length of the light beam-path through the suspension. N = the number of particles per unit of volume. The turbidity is normally defined as γ = N * a = ln(VO/V)/l It will be seen from the aforegoing that the turbidity is directly proportional to the concentration of the suspension for one and the same type of particle. The following relationship applies in the case of suspensions which comprise a plurality of different types of particle: γ = Σ N i * a i = (1/l) * Σln (V 0 /V i ) where i is an index for different types of suspensions, the summation being made over all particle types. The concentration of the suspension in mg/l is therewith: conc = (1/l) * Σ C 1 * ln(V 0 /V i ) where C 1 are constants for converting the turbidity to concentration in mg/l for respective particle types. If the suspension concerned consists of a mixture of two completely different types of suspensions, i.e. a suspension having very small particles, e.g. clay, and a suspension having relatively large particles, e.g. fibres, these particles will have a mutually different effect on the light transmitted, as mentioned above. Roughly speaking, it can be said that the suspension part containing the small particles will have an affect on the deviation of the maximum signal height from the signal obtained when measuring clear water, whereas the suspension-part containing the large particles will result in a clear variation (changing) of the signal. Figure 18 is a diagram which illustrates the alternating voltage signal obtained from the inventive measuring head. The levels of the clear-water signal V O (these levels having been measured earlier when calibrating the instrument by measuring on clear water), the direct voltage level V DC of the signal and the peak level V P of the signal are shown in the Figure. Since, in accordance with the aforedescribed theory, the small particles will have no appreciable influence on the variation of the resultant signal, but primarily cause the peak value of the signal to be lowered from the clear-water level to the peak value V P there is formed for this part of the suspension ln(V O /V P ), whereas ln(V P /V DC ) is formed for the suspension-part containing the large particles. Thus, the application of Lambert Beer's law for a "model suspension" consisting of two particle types - large and small - will give the following result for the signals in Figure 17: conc. = a*ln(V O /V P ) + b*ln(V P /V DC ) where a and b are constants, a*ln(V O /V P ) is the concentration of small particles and b*ln(V P /V DC ) is the concentration of large particles. The above equation (5) can be rewritten as follows: conc = (ln(V O /V P ) + c 1 *ln(V P /V DC ))*c 2 where c 1 is the sensitivity coefficient of the measuring apparatus and c 2 is a constant for converting the measurement values to concentration in mg/l. It is found that these constants can be readily obtained, by calibrating the measuring apparatus. These constants were readily obtained experimentally, by measuring two suspensions having the same concentration, containing on one hand the smallest particles and on the other hand the largest particles; (ln (V O /V P )) s + c 1 (* ln (V p /V DC ) s = ln (V O /V P ) l + c 1 * (ln (V P /V DC )) l . where s signifies small particles and l signifies large particles c1 = (ln(V O /V P )) s - (ln(V O /V P )) l (ln(V P /V DC )) l - (ln(V P /V DC )) s c2 = conc. lab (ln(V O /V p +c 1 * ln(V P /V DC ) where conc.lab is a value of the suspension concentration obtained when determining measurements in the laboratory in accordance with standard methods. Several such measuring processes may be carried out, of course, preferably with different concentrations, so as to obtain a result which is more positive statistically. In (V O /V P ) and ln (V P /V DC ) can be used together with the calculated conc-value for the purpose of calculating the percentual composition of the suspension with respect to small and large particles respectively, as will be described in more detail hereinafter with reference to Figure 20. The quotient of ln (V P /V DC ) and in (V O /V P ) will provide another relative measurement of particle-size distribution. Figure 19 illustrates an embodiment of apparatus for carrying out the inventive method. A light beam L from a light source 101 illuminates a flowing suspension S. The diameter of the light beam L is small in relation to the interspaces between the large particles in the suspension, but large in relation to the interspaces between the small particles. In practice, this will often lead to an endeavour to obtain a narrow light bundle. The transmitted light is detected by a photosensor 102. The sensor signal is amplified in an amplifier 103. Connected to the output of the amplifier 103 is a filter or mean-value former 104, which produces on its output the direct current component V DC of the signal, this component providing a measurement of the mean value of the transmitted light. Connected to the amplifier output is also a peak detector 105, which registers the largest signal V P for a given measuring period. The measuring period is set with the aid of a timer 106. At the end of each measuring period, the timer 106 sends a first signal to a sample and hold circuit 107, the input of which is connected to the output of the peak detector 105, so as to store temporarily the prevailing output signal from the peak detector 105, and after a slight delay sends a second signal to the peak detector 105 for the purpose of setting said detector to zero in readiness for a new measuring period. Consequently, the peak values V P for the immediately preceding period will always be present on the output of the sample and hold circuit 107. It is also possible to control the light source 101 in a pulsating manner, and to carry out measurements solely during the pulses. This control (not shown) is effected by the unit 108, which controls the circuits 105 and 107 in accordance therewith. The signals V DC and V P are supplied to a control and calculating unit 108, suitably a microprocessor, for instance an IBM-compatible PC, which carries out the aforedescribed calculations and presents the results on at least one presentation unit 109, 110, e.g. a monitor, printer and the like. The measuring period of the timer 106 is adapted so that in all probability a peak value will occur within each measuring period, but is nevertheless kept so short that changes in the suspension will be indicated as soon as possible after they occur, and so that variations of the changes in time can be followed continuously. This enables the measuring periods to be kept short when the coarse-fraction concentration is low, although these periods should be longer with increasing concentrations of the coarse fraction. The duration of the measuring period can be made variable in accordance with predetermined parameters, which may optionally depend on measuring results earlier obtained. It may also be suitable to control a sample and hold circuit 113 with the aid of the timer 106 in the same manner as the sample and hold 107 downstream of the filter 104. The sample and hold circuits 107 and 113 are of the kind operative to transmit their values in digital form to the digitally operating unit 108. It will be obvious to the person skilled in this art that a part of the circuits 104-107 and 113, or all of said circuits, can be simulated by software in the unit 108, although the signal or signals supplied to the unit 108 will, of course, be converted to the signal format with which the unit 108 operates. Such conversion units have no significance in the present context and are therefore not shown. For instance, the filter 104 may consist of a calculating or aritmethical routine which calculates the mean value of the signal during each measuring period. The signal from the amplifier 103 may be sampled by the units 104 and 105 and all calculations effected digitally in a manner well known to the skilled person. As also illustrated in Figure 19, calculations can also be effected in accordance with the abovementioned TP-method or may be based on solely effective-value measurements. It may be to advantage to calculate mutually the same properties of the suspension in accordance with two different methods and to continuously compare the results and send an alarm signal to an alarm unit 114, e.g. an audio alarm or an optical alarm, as soon as the calculations made in accordance with the two methods deviate from one another in a pre-specified manner. Comparisons with methods based on effective-value measurements do not involve significant additional measures, since it is possible to utilize the same fundamental signal from the measuring head 101, 102, 103. Consequently, a filter 115 is shown connected to the output of the amplifier 103. The filter 115 is operative to filter-out the alternating voltage component of the signal and to supply this component to a unit 116 operative to measure the true effective value of the signal arriving from the filter 115 and to supply this value to the unit 108, optionally via a sample and hold circuit 118 of the same type and operational function as the circuits 107 and 113. The signal paths are shown in broken lines, to indicate that the elements 115,116 and 118 can be excluded. It will be understood that the elements 115, 116 and 118 can also be simulated by program loops in the unit 108. Figure 19 illustrates only one measuring head 101, 102. In this case, a division is made into solely two particle-size ranges, large and small. An indication in several size classes can be obtained, however, by using several measuring heads having beam paths of mutually different widths. Measuring equipment having these properties for the TP-method is described in SE 7806922. The described signal processing method according to the invention can be applied on the signals from all of the heads. In this case, the control and calculating unit 109 obtains a V DC -signal and V P -signal from each measuring head and is provided with a program which calculates the particle concentration within a number of particle-size ranges. For the purpose of testing the inventive method experimentally on particle sizes, experiments were carried out initially on suspensions which contained very small particles. Different flocculating chemicals were then added to these suspensions while agitating or stirring the suspensions continuously, such as to form large particle agglomerations or flocs. The results of these measuring operations are given in Figure 20. The curve A illustrates turbidity measurement which corresponds to C 3 * ln (V O /V DC ), where C 3 is a constant. The curve B shows the results obtained when measuring the concentration of suspended material in accordance with the invention, i.e. conc =c 2 *(ln(V O /V P )+c 1 *ln(V P -/V DC )). The curve C represents b *ln(V P /V DC ), i.e. the concentration of large particles in the suspension. It is found that, in principle, In V P /V DC is independent of variations in light intensity and also dirtying and scratches of the optical surfaces. The curve D represents a.ln(V O /V P ), i.e. the concentration of small particles in the suspension. The curves have been marked with different types of signs. Each sign, or marking, is placed on a measuring time-point, which is marked along the abscissa. There is a time period of 15 seconds between each measuring time-point. The measurements were carried out in the laboratory on white water obtained from a paper machine. The water was untreated at measuring time-point 1. Different flocculating chemicals were added to the water on two occasions, immediately prior to measuring time-points 2 and 6. As will be seen from curves C and D, the first chemical, added at measuring time-point 2, had a relatively small flocculating effect, whereas the second chemical, added at measuring-point 6, had a relatively large effect. This is also shown by the curve A obtained in accordance with the earlier known turbility method, although not as clearly. The curve C is practically a mirror-image of the curve D. These curves show that the flocs are degraded slowly subsequent to the heavy formation of flocs at the measuring time-point 6, i.e. the curve D falls and the curve C rises. A certain tendency in this direction is also observed after measuring time-point 2. Curve B, which is a weighted addition of the curves C and D, lies practically on the same level, which can be taken as an extraordinarily good result, since this curve would show the total concentration, which has been held constant during the laboratory measuring process. The curves obtained when practising the inventive method indicate that clear information can be obtained continuously with regard to the state of, e.g., a flowing suspension, and that an indication of the result of measures taken when adding different chemicals can be obtained rapidly and positively. As shown by curve D, it can be seen when the chemical addition is such that the addition of further chemicals will not have a greater effect with regard to the flocculation of small particles. When chemicals were added at time-point 6, the curve D fell so low as to lie relatively close to the x-axis. The operator can endeavour to lower the curve still further, by adding more flocculating chemicals. If the curve fails to fall to a lower level, the addition of further chemicals will have had no effect on the flocculation of residual small particles, this circumstance being indicated immediately. Other agents can then be tested, the effect of these agents also being shown on the curve immediately after their addition. Thus, the inventive method will enable the metering of chemical flocculating agents, e.g. into white water obtained from a paper mill and when pulp cleaning to be controlled in a more optimum fashion. Furthermore, the effect produced by different chemicals can be quickly seen, thereby enabling the chemical best suited for the purpose to be used. Figure 21 illustrates the results obtained when practicing the inventive method with various mixtures of fibre and clay suspensions of known concentration. Similarly to Figure 20, the curve A shows the results of turbidity measurements, the curve B shows the results obtained when measuring the concentration of suspended material in accordance with the invention, i.e. C 2 * (in (V O /V P ) + C 1 *ln (V P /V DC )), the curve C b *ln (V P /V DC ) and the curve D a*ln/V O /V P ). The curves E and F illustrate respectively the measured and calculated concentrations of large particles in relation to the total concentration. There were added successively to a starting suspension of pine sulphate pulp known contents of clay, clay, clay, pine sulphate, pine sulphate, clay, birch sulphate, birch sulphate. When applying the inventive method, the measured concentration, which was found to increase almost linearily, correlated very well with the concentration determined in the laboratory (abscissa). The measured content of large particles (curve E) also coincided very well with the calculated proportion of fibre material according to the mixing procedure (curve F). As will be evident from curve A, in the case of the variations in composition of the suspensions used here, these variations being realistic within the forest industries, turbidity would definitely be the wrong measuring parameter to use when wishing to obtain knowledge of the events taking place with the suspension, both with respect to the content of suspended material and to the particle size distribution. The inventive principle can also be applied when measuring in accordance with the scattering principle. This can apply, for instance, when the suspension contains very low proportions of suspended material. In the case of this measuring procedure, there is measured a small signal relative to the background at the O-level instead of a small signal change relative to a high background level (the clear-water level) as in the case of transmission measuring processes. This can involve indicating the bottomn value V B of the alternating voltage signal instead of indicating the peak value in Figure 18, and placing said bottom value in relation to the zero level instead of to the clear-water level for the suspension-part containing the small particles. For the suspension-part containing the large particles, the bottom value V B is placed in relation to the mean value level V DC . These relationships, however, are more complicated than the simple logorithmic quotient expressions for transmission measurements and, inter alia, are contingent on the geometric configuration of the measuring apparatus. When measuring typical suspensions, such as white water or backwater deriving from a paper mill, the inventive measuring method requires a relatively narrow measuring beam which is substantially collimated or focused. The suspensions are often relatively concentrated, which requires a small measuring gap in order to enable light transmitted through the suspension to be indicated and in order to provide a processable, received signal. Known measuring heads possessing these properties are liable to become blocked and must be cleaned often. It will be understood that many modifications are possible within the scope of the invention. For instance, the measuring channels can be constructed for use with radiation other than light radiation, e.g. for β-radiation. The embodiments illustrated in Figures 9 and 17 may also be provided with an arrangement for cleaning the measuring gap of essentially the same kind as those illustrated in Figures 6 and 7.

Claims: A method for registering the state of a moving suspension containing particle fractions of mutually very different sizes, comprising illuminating the suspension with a beam of light and detecting light exiting from the suspension for the purpose of producing an electric signal which corresponds to the intensity of the detected light, characterized by illuminating the suspension with the aid of a beam of light which is narrow in relation to the interspaces between large particles and detecting light exiting from the suspension within a narrow angular range; indicating an extreme value (peak or bottom value) of the electrical signal within predetermined time intervals; and by forming starting values within each extreme value time-interval for the purpose of calculating at least one pre-selected property of the suspension. A method according to Claim 1 with indication of light transmitted through the suspension, characterized in that the extreme value is the highest value, the peak value (V P ) of the signal during said time interval; and in that the fine fraction of the suspension is indicated by placing the peak value in relation to a voltage level (V o ) obtained by measuring clear water. A method according to Claim 1 or 2 when indicating light transmitted through the suspension, characterized in that the extreme value is the highest value, peak value (V P ) of the signal during said time interval; in that the mean value (V DC ) of the signal is indicated; and in that the course fraction of the suspension is indicated by placing the peak value in relation to the mean value of said signal. A method according to Claim 2, characterized in that the fine-fraction concentration is indicated by forming the signal a*ln(V O /V P ), where a is a constant obtained by calibration against a suspension having a pre-known fine-fraction concentration. A method according to Claim 2 or 4, characterized in that the course-fraction concentration is indicated by forming the signal b*ln(V P /V DC ), where b is a constant obtained by calibration against a suspension having a pre-known course-fraction concentration. A method according to any one of Claims 2 to 5, characterized in that the total concentration of substances suspended in the suspension is indicated by forming the signal conc = (ln(V O /V P ) + c 1 *ln(V P /V DC ))*c 2 where the constant c 1 is the sensitivity coefficient of the measuring apparatus, and c 2 is a constant for converting measurement values to concentrations in mg/l, the constants being obtained by calibration against suspensions having pre-known fractions. Apparatus for registering the state of a moving suspension containing fractions having particles of mutually very different sizes, said apparatus including a light source (1) which is operative to illuminate the suspension with a beam of light which is narrow in relation to the interspaces between large particles in the suspension, and which further includes a light detector (2) operative to detect light exiting from the suspension within a narrow angular range and also operative to produce an electric signal corresponding to the intensity of the light detected, characterized by a signal processing arrangement (104-108) which is operative to indicate an extreme value (peak or bottom value) of the signal obtained from the detector (102) within predetermined time intervals, and which is also operative to calculate at least one preselected property of the suspension on the basis of said extreme value. Apparatus according to Claim 7, characterized in that the signal processing arrangement (104 -108) is also operative to indicate the direct voltage level (V DC ) above which the detector signal varies and to calculate a value from an algorith which includes the extreme value and the indicated direct voltage level and adapted to the configuration of a measuring head including the light source (101) and the light detector (102).

Inventors: FLADDA GERDT HEINRICH, FLADDA, GERDT HEINRICH
Applicants: FLADDA GERDT H, FLADDA, GERDT HEINRICH
Application year: 1990
Main classification: G01N21
IPC codes: G01N21, G01N33, G01N15