WO1984002396A1

WO1984002396A1 - Method and apparatus for measuring the content of suspended substances in a flowing medium

Info

Publication number: WO1984002396A1
Application number: PCT/SE1983/000458
Authority: WO
Inventors: Krister Lundberg; Goeran Tidstam; Daniel F Pope
Original assignee: Bonnierforetagen AB
Current assignee: Bonnierforetagen AB
Priority date: 1982-12-15
Filing date: 1983-12-15
Publication date: 1984-06-21
Anticipated expiration: 1986-06-15
Also published as: SE443235B; FI87277B; US4641969A; EP0128189A1; EP0128189B1; JPS60501125A; FI843209A0; SE8207180D0; DE3382336D1; FI87277C; FI843209A7; SE8207180L

Abstract

Method and apparatus for measuring the content of suspended substances in a flowing medium by illuminating the medium with light from a light source (1). Said light is reflected in at least two detectors (2-4), at least one (2) of which measures directly reflected light and the other (3; 4) measures multiply reflected light. The detector (2) is disposed at a distance from the light source (1) such that a linear relationship between its output signal and the concentration of the flowing medium is obtained. The detector (3; 4) is disposed at a distance from the light source (1) such that its output signal mainly will be constant in relation to the concentration of the flowing medium within a large interval. A measure of the concentration of the flowing medium being independent of disturbing factors such as for example pulp colour is subsequently obtained from the quotient of the signal provided by direct reflection divided by the signal provided by multiple reflection.

Description

Method and apparatus for measuring the content of suspended substances in a flowing medium

The present invention relates to a method and an apparatus for measuring suspended substances in a flowing medium by illuminating the medium with light from a light source, the light being detected in at least two detectors, one for measuring directly reflected light and one for measuring multiply reflected light, said directly reflected light refers to the light received from an area of the medium that is directly illuminated by the light source and said multiply reflected light is received from an area of the medium that is not directly illuminated, but is illuminated by light that has been scattered from the beam of direct illumination. The apparatus is preferably intended for use in so-called consistency measurement, i.e. measuring the content of fibres in fibre suspensions, such as are to be found in the pulp and paper industry. The consistency is defined as the weight of fibres per volumetric unit. It is therefore a definition of concentration of suspended solids, henceforth for generality called concentration, since the present invention may be used to measure on any material that scatters light in the flowing medium.

A number of different methods have been developed for measuring the concentration of fibres in fibre suspensions. Methods based on mechanical apparatus have been primarily utilized. The shear force principle particularly has come into a great deal of use during the last 50 years. In shear force measurement, the network strength of the fibre suspension is measured first with the aid of a blade, a rotating sensor or the like. The relationship between the concentration and the network strength is then worked out by a special calibration method. During later years, however, optical methods have begun to be used for measuring the concentration in fibre suspensions. There are several reasons for such a development having taken place. In the first place, the mechanical meters of the shear force or viscosity type normally require a great deal of maintenance. In the second place, the accuracy, particularly with stationary measurement, is relatively poor with mechanical apparatus. In the third place, great technical development has taken place in the optical and electronic fields, particularly in the latter, which has facilitated and cheapened the manufacture of meters based on optical principles. In the fourth place, an accuracy may be obtained with electro-optical techniques which is at least as good as, and in some cases even better than, that obtained with corresponding mechanical apparatus.

The failing in the use of electro-optical methods has, however, been the difficulty of measuring at higher concentrations. One has therefore normally been limited to measurements in the interval under 10 g/litre (1%) . This is due to the fact that the primary choice has been to use transmission measurement, i.e. the light transmitted through the fibre suspension has been detected. Since the intensity of the light passing through the suspension declines according to an exponential function, the signal it provides will be close to the signal noise threshold for even relatively low concentrations. Meters based on transmitted, polarized light also suffer from the same failing.

The present invention relates to a method and apparatus of the kind mentioned in the introduction, said method being generally distinguished in that the detector for the directly reflected light is arranged at a distance from the light source such that a linear relationship between the output signal from the detector and the concentration of the flowing medium is obtained up to a maximum is reached from which the signal does not increase at even higher consistencies, that the detector for the multiply reflected light is disposed at a distance from the light source such that the output signal from the detector will be independent of the concentration of the flowing medium within a large interval, a signal being obtained from the quotient of the signal provided by direct reflection divided by the signal provided by multiple reflection, this signal being a measure of the concentration of the flowing medium.

With the aid of the present invention the disadvantages burdening concentration transducers known up to now have been circumvented by measuring the reflected light. In such measurements, good accuracy at concentrations well above the 1% level may be obtained, and with the present invention concentrations of 6% may be measured without deteriorated accuracy. On some types of pulp fibre good linearity continues to 10% concentration or more.

The invention will now be described below in detail with reference to the appended drawings, on which Fig. 1 is a section through a measuring head or insertion probe in a measuring apparatus according to an embodiment of the invention. Fig. 1A is a side view section of the probe showing the beam of illumination from the light source and to the detectors. Fig. 2 is a side view, partly in section, of two designs of measuring apparatus according to the invention, Fig. 3 is a side view in section, and to an enlarged scale, of an insertion probe included in another embodiment of the invention. Fig. 4 is a longitudinal section to an enlarged scale through the outer end of the insertion probe. Fig. 5 is a cross section along the line A-A through the outer end of the probe illustrated in Fig. 4, Fig. 6 is a partial section along the line B-B in the probe end illustrated in Fig. 5, Fig. 7 is a partial view of a longitudinal section through another embodiment of the probe end and Fig. 8 illustrates the signals I₂₁, I₄₁, I₂₁/I₄₁ and I₂₁/I₄₁+k·I₁, as a function of the concentration of the flowing medium. In the embodiment illustrated in Fig. 1, a measuring head or insertion probe 10 includes four fields, field 1a assigned to a light source 1 , field 2a assigned to a detector 2 for direct reflection and fields 3a, 4a, assigned to detectors 3, 4 for multiple reflection. These fields comprise one end 13 of optical fibre conductors. The end 13 includes a window 9 , behind which there is a flange 5 attached to the end portion with the aid of unillustrated attachment screws in attachment holes 6, the flange 5 being intended to centre and fix the optical fibre conductors. The apparatus functions in the following manner. Light is taken through an optical fibre conductor and out through the field 1a towards the fibre suspension flowing in a pulp line 11 (see Fig. 1). The reflected light is detected in three different detectors 2-4 via fields 2a-4a, which are at different distances from the field 1a emitting the light. The detector field 2a, which is nearest to the field 1a of the light source, primarily accepts directly reflected light for measurement by the detector 2. The other two detector fields 3a, 4a accept light for measurement by the detectors 3, 4 which is reflected a number of times in the fibre suspension and has undergone so-called multiple reflection. The beam of illumination from field 1a into stock line 11 through the window 9 is detailed in Fig. 1A, along with the angles of view of detectors 2, 3, 4 through the fields 2a, 3a, 4a. The angles of these light beams are restricted by placement of the light source and detectors with respect to the ends of the optical fibre conductors inside a casing 12.

As is shown, the angle of view for detector 2 always includes some part of the direct illumination of the flowing medium regardless of the depth of penetration of this illumination, which decreases with increasing concentrations; detector 3 is barely able to receive a direct reflection except at low concentrations. As is shown by shaded lines, the angle of view for detector 4 includes practically no direct illumination of the medium at technical concentrations appropriate for this probe.

Providing that its field is sufficiently close, the detector 2, which has its field 2a closest to the light source field 1a, will respond more or less linearly to the concentration up to a maximum proportional to the reflectance of the medium. The detector 3 will also respond linearly to C, up to a maximum, which also is proportional to the reflectance, and then will begin to decrease linearly with C. About the maximum the detector response will be approximately constant over a wide interval in concentration. The detector 4 will behave similarly to detector 3, but will reach a maximum at a lower concentration and decrease linearly with increasing C more rapidly than detector 3.

If the detectors 2, 3, 4 respond linearly to light then they will output signals proportional to I. and varying with concentration C as described above and illustrated in Fig. 8. By performing the division specified, for example I₂₁/I₄₁, variations in I₁ have no effect on the result. Also, attenuation in light caused by color in the medium or dirt on the window 9 will affect both detector outputs nearly proportionally, so again there will be no effect on the result.

As previously mentioned, the light is taken out via fibre optics towards the pulp fibres and reaches the pulp via the window 9 in the probe 10, as seen in Fig. 2. Analogously, this light is led back to an electronic transducer including detectors 2, 3, 4 situated in the casing 12. The probe 10, accommodating the fibre optics via a probe tube 8 and terminating at the common end 13, extends from the casing 12 to the pulp line 11 inside a protective tube 7. To enable cleaning the probe 10 while it is in operation, there is a sluice valve 14 in the protective tube 7, this valve being in an open position when the probe 10 is inserted, but in conjunction with the probe 10 being removed from the tube 7 the valve 14 closes and seals against the fibre suspension. In Fig.1-2 where optical fibres are used, the respective beam path is separated by an intermediate wall in the form of a blind baffle, e.g. of metal.

The different parts forming the insertion probe 10 are apparent from the embodiment having no optical fibres, illustrated in Fig.3-7. An electrical cable 18 (Fig.3) terminates in an optical unit 19 , which includes the light source 1 and the detectors 2-4. Outside these there is the window 9 fastened against the unit 19 with the aid of a fastening sleeve 20.

From Fig. 4 will be apparent the way in which the window 9 is fixed with the aid of the fastening sleeve 20 to the optical unit 19, in which the light source 1 and the detector 4 are retained with the aid of a sealing fastening plate 21. A reflecting means 15 in the form of the tip of a screw 17 going through the wall of the probe 10 comes a small distance into the light flow from the light source 1. The screw tip 15 thus reflects a portion of the light coming from the light source 1 via a mirror 16 directly into the detector 4 , giving rise to a signal which is added to the signal I₄₁ from the detector 4 according to the relationship I₂₁/I₄₁+k·I₁, where the constant k is obtained optically. Instead of the reflecting means 15, an extra detector can be arranged in front of the light source 1 in its path of light, whereby the constant k in this latter case is obtained electronically. By this arrangement there is obtained the linearity for the concentration values C of the flowing medium, these values being mainly below 1%, as will be seen from the graphical depiction in Fig. 8. From Fig. 5, which is a section through the end of the probe 10 along the line A-A in Fig. 4, will be seen the position of the light source 1 in relation to the detector 2 for the directly reflected light.

Fig. 6 is a section along the line B-B in Fig. 5 and illustrates how the light opening to the detector 2 for the directly reflected light is laterally displaced in relation to the detector 2 itself.

In the embodiment illustrated in Fig. 7, a lens system 22 has been arranged in the beam path of the light source 1 between the window 9 and said source 1. The focal length of the lens system 22 is selected such that the light going through it is focused at a point outside the window 9 , the measuring apparatus thus being insensitive to possible dirtying of the window 9 , since there is no reflection able to directly reach detector 2 from the small area of directly illuminated window surface.

Figure 8 illustrates the different relative signals as a function of the concentration C of the flowing medium. It will be seen from the graphs that the relationship I₂₁/I₄₁ is substantially linear for fibre concentrations above a critical concentration C' and that the relationship I₂₁ /I₄₁ +k·I₁ provides a graph which is substantially linear for fibre concentrations both under and above the critical concentration C' .

Advantages obtained by adjustment of all the specified optical parameters include the extended linearity as compared with sensors using direct reflection alone, minimization of the effects of the medium color, surface dirt on window 9, and variations in light intensity I₁.

Electronic means for powering the light source and taking the outputs from the detectors and combining them to perform the necessary division and outputting scaled signals to display or control devices should be apparent to those versed in the art. The present embodiments use, but are not restricted to, silicon photodiodes of large area for detectors 2, 3, 4 and a gallium arsenide light emitting doide of wavelength 940 nm operating in a pulsed manner for light source 1.

Claims

1. A method of measuring the content of suspended substances in a flowing medium by illuminating the medium with light from a light source (1 ) , the light being detected in at least two detectors (2, 3, 4), one for measuring directly reflected light and one for measuring multiply reflected light, c h a r a c t e r i z e d i n that the detector (2) for the directly reflected light is adapted at a distance from the light source (1) such that a substantially linear relationship is obtained between the output signal (I₂₁) from the detector and the concentration (C) of the flowing medium, that the detector (3; 4) for the multiply reflected light is disposed at a distance from the light source (1) such that the output signal (I₃₁; I₄₁) from the detector (3; 4) will mainly be constant in relation to the concentration of the flowing medium within a large interval, a signal (I₂₁/I₄₁) being obtained from the quotient resulting from dividing the signal provided by direct reflection by the signal provided by multiple reflection, this signal representing a measure of the concentration of the flowing medium and being independent of disturbing factors such as for example pulp colour.

2. A method as claimed in claim 1, c h a r a c t e ri z e d i n that a signal is added to the signal (I₄₁) from the detector (3; 4) for the multiply reflected light, the former signal being obtained from the part of the light from the light source (1) which is led directly and optically to the detector (3; 4), the signal being formed according to the relationship I₂₁/I₄₁+k·I₁ , where the constant (k) is obtained by optical means to provide the linearity for the concentration values (C) of the flowing medium, these values being mainly below 1%.

3. A method as claimed in claim 2 , c h a r a c t e r i z e d i n that the signal being obtained from the part of the light from the light source (1 ) is led directly and electronically to the detector (3; 4), said signal being formed according to the relationship I₂₁/I₄₁+k·I₁,where the constant (k) is obtained electronically.

4. Method as claimed in any of the preceding claims, c h a r a c t e r i z e d i n that the light from the light source (1) is focused at a point outside the window (9) which is between the light source (1) and the medium flowing in the line (11).

5. Apparatus for carrying out the method in accordance with any of claims 1-4, including a light source (1) and at least two detectors (2, 3; 4), said means (1-4) being disposed in a probe (10) thrusting into a pipe line (11), said probe having a window (9) sealing against the medium at its end (13) facing towards the medium flowing in the line (11), one of said detectors (2, 3; 4) measuring the directly reflected light and one measuring the multiply reflected light, c h a r a c t e r i z e d i n that the detector (2) for the directly reflected light is disposed in conjunction with the light source (1) while the detector (3; 4) for the multiply reflected light is disposed at a distance from the light source ( 1 ) .

6. Apparatus as claimed in claim 5, c h a r a c t e ri z e d i n that a detector is disposed in the light beam of the light source (1) between the source (1) and window

(9) for measuring the intensity (I₁) of the light source (1) before light proceeds into the flowing medium.

7. Apparatus as claimed in either of claims 5 or 6, c h a r a c t e r i z e d i n that a reflecting means (15) is disposed in the light flow of the light source (1), between the light source (1) and the window (9), said means being adapted for optically conducting a part of the light beam from the light source (1) at right angles to the longitudinal direction of the probe (10) via an obliquely disposed mirror surface (16) into the detector (4) for the multiply reflected light.

8. Apparatus as claimed in claim 7, c h a r a c t e ri z e d i n that the reflecting means consists of the tip (15) of a screw (16) going through a casing of the probe (10).

9. Apparatus as claimed in any of the preceding claims, c h a r a c t e r i z e d i n that the light opening of the detector (2) for the directly reflected light is laterally displaced at the end (13) of the probe (10), a predetermined distance in relation to the detector (2) .

10. Apparatus as claimed in any of the preceding claims, c ha r a c t e r i z e d i n that a lens system (22) is disposed in the light beam of the light source ( 1 ) between the window (9) and light source (1), the focal length of said lens system being selected such that the light going through the system (22) is focused at a point outside the window (9) .