CONCENTRATION METER

Abstract: A concentration meter for use in measuring paper pulp consistency within the concentration range of 0.5 to 10%. The meter measures the depolarization of light passed through a pulp slurry and utilizes a pulsating infra-red light emitting diode (1) having a narrow beam in conjunction with a digital electronic circuit to provide a continuous concentration measurement. The instrument can be modified to form an in pipe probe to measure pulp concentration during the paper making process.

Description: CO CE-TTRATION METER This invention relates to a concentration meter for measuring volume concentration of fibrous solids in liquids. In particular, this meter has application in measuring stock consistency of wood fibre slurries. It is known that polarized light incident on a single fibre will be depolarized by a certain amount; several fibres will depolarize a given quantity of light more than a single fibre. An estimation of stock consistency can be made if polarized light is passed through a stream of stock, and the amount of depolarization is measured. U.S. Patent 4,171,916 (Simms and Madsen) discloses apparatus for measuring fibre concentration in a pulp suspension in which polarized light is passed at right angles through a flow cell through which the sample is allowed to pass. Two detectors are used one having an axis of transmission parallel to and the other crossed with the plane of polarization of the light beam. The detectors provide electrical signals which are processed to give a consistency measurement of the sample. This patent discloses using a large continuous beam of light n the visible range. U.S. patent 3,653,767 also discloses the use of polarized light in this case to determine particle size distribution by measuring the degree of depolarization that occurs at different wavelengths. It is also disclosed in patent 4,134,879 that the volume of particles suspended in a fluid can be measured by measuring the depolarization of light having a wavelength approximately two times the particle size of the particles to be measured. The measurement of the size of particles is also disclosed in U.S. patent 4,140,395 which patent discloses the use of a pulsed light source in measuring particle size of suspended solids. The reason for using a pulsed light source is to ensure that the same particle is not the subject of each measurement and to also lower energy consumption. Measurement of particle size as suggested in this patent is based on the measurement of back scattering. It is an object of this invention to provide an improved means of using polarized light to measure stock consistency. To this end the present invention provides apparatus for measuring volume concentration of optically active solids suspended in a liquid comprising a cell for containing the suspension to be measured, a pulsed infra-red light source which emits a narrow beam of polarized light at right angles to the cell, a first photo-detector on the opposite side of the cell to the light source, a second photo- detector with an associated polarizing filter located on the opposite side of the cell to the light source, computing means to analyse the signals received from said first and second photo-detectors and produce a signal corresponding to the ratio of signals from said first detector to said second detector and optional display means fro displaying or recording in' suitable form the measurement so obtained. The unique features of "this invention are: l. The apparatus uses infra-red or red light preferably in the wavelength range of 600 to 650 nm which gives a linear response. It is preferred that the band width is narrow as in a laser. Colour and suspended solids have less effect on consistency estimation at longer wavelengths. A consequence of the use of an infra-red source is that a semi-conductor diode detector can be used and this gives the consequent advantage of lower engergy use. 2. The light source is pulsed. The combination of digital electronics with a pulsed source allows the inherent problems of analogue electronics to be overcome, i.e. drift, offset and sensitivity which develops with, for example, ageing of the equip- ment. An automatic intensity control is not required. 3. The response rate of the equipment is measurable. Microprocessor control allows the response rate and delay associated with all electronic devices to be measured and accounted for in calculations within the instrument. 4. The light beam is fine. The optical non- linearities of the- system have very little effect, particularly with changing temperatures . It has 10 been shown that the narrow beam may permit develop¬ ment of this instrument to measure flocculation and fibre length distribution. In a preferred form the apparatus functions as follows. The light passes through a polarizer, a measuring 1 cell and a beam splitter. One of the resulting beams is collected while the other passes through another polarizing filter before being collected. The collection is done by semiconductor diodes . Stock flows through the measuring cell at 90° to the light beam. The electrical signals produced 20 by the detectors in response to the pulsed source and the stock variations are used in three ways. (a) The ratio of the detector signals is used to over¬ come any change in light intensity produced by the source. ^- (b) The difference between the ratios of detector signals when the lamp is on and off overcomes drift and offset in any components of the system. ( c) The delay from when power is applied to the source to when the output of the analogue section reaches 30 an adequate level is measured and stored for later use. This facility allows replacement of compon¬ ents without the need for recalibration. All the functions except the detector output amplification and analogue to digital conversion are done by 35 microprocessor. This has resulted in a substantial reduction in overall size, weight and cost. It uses a small flow and is more reliable. Paper puip is passed through a glass vessel which is constricted in one dimension to obtain a pulp cross-sec¬ tion of approximately 1mm. Light from a light-emitting diode is polarized with a polarizing filter before reaching the pulp. After passing through the pulp, part of the light is collected as a reference and the rest is passed to a main detector through another polarizing filter which is set at 90° to the first filter. The pulp density is a function of the ratio P MATN / P R π-τ where P MΛTTJ I S he. optical power received by the main detector and P REF is that received by the reference detector. When water is passed through the measurement tube, the ratio P MAIN P REF is a inimuιn - Attenuation of the light beam by dark samples or impurities decreases both Pjy * AIN a < ^ ^ EF atld ^ ence t * le rat i° P MA -./P _-__, remains constant and representative of the pulp density. As the pulp density increases, cellulose depol¬ arizes the light and the signal from the main detector increases. To obtain the ratio t * e t o detector signals IM ΔTM /I P EF are am pHfi e d and the zero light offsets --OMAIN and i 0REF are removed. This gives absolute measures of light intensity falling on the detectors through the sample chamber. For a photo-detector whose output current is linearily related to the incident optical power, P MAIN = L MAIN ~ L 0 MAIN P REF. 1 REF- ~ 1 0 REF. This gives us the measurement of pulp density. The ranges of concentration over which the instrument gives a linear response is 0.05 to 2.57. and .possibly up to 10%. A preferred form of the detector system is shown in Figures 1, 2 and 3 of the drawings and figures 4 to 6 show measurement results for particular suspensions. Another form of the invention is shown in figures 9, 10 and 11 which illustrate an in pipe probe. The entire transducer is machined from a solid piece of 2" Delrin plastic rod. All mountings are achieved by slots, channels or holes cut into the plastic. Alignment of the optical components in the proto¬ type is achieved by drilling along a common axis. This allows the source, aperture, lens and main detector to be precisely centered. A half silvered mirror 7 allows part of the light to be collected at a reference detector 9 at 90° to the main beam. Polarization is achieved by using a Polaroid polarizing filter 2. A narrow beam red LED 1 is used as the source and illuminates the pulp 4. To maintain the same monitoring point in the pulp a brass bayonet acts as an aperture 5. A short focal length lens 6 then produces an image on the detectors 9 and 10. By using a half silvered mirror 7 after the lens, two images are obtained at the detectors. In front of the main detector 10 is another polar- izing filter 8. The polarizers are aligned by placing water or air in the sample chamber 3 and one of the polarizers is rotated until a minimum is achieved at the main detector. An extinct¬ ion ratio (the ratio of minimum to maximum light) of 1% was easily achieved. Figure 2 is a cross-sectional view . of a preferred form of the sample tube. The narrow section 21 comprises the flow through cell of the detector, and the wider sections 22 correspond to the diameter of the conduit for pumping the suspension. Figure 3A is a schematic plan of the instrument connected to the main stock flow line and figure 3B is a block diagram of the instrument. Samples for testing are collected from the main stock flow line 30 by the valve and dip tube sampling assembly 31. These samples pass into the sample head 33 via the input line 32 and are returned via line 34. Power is provided by cable 35 to the electronics board 36. Referring to figure 3B the electrical ouput signal of the photo detectors 40 and 41 are filtered by filters 42 and 43 to remove unwanted spectral information. Analogue to digital converters (A/D) 44 and 45 transform the filtered electrical signals into digital 8-bit binary parallel forms 47 by control of the microprocessor 52 through line 48. The pulsed light source 55 is an L.E.D. controlled through an adapting interface 56 by the microprocessor 54 running a light emission program in its peripheral read only memory. The light from the L.E.D. 55 is received at the photo detectors 40 and 41 after passing through the series of polarizing and depolarizing filters and the sample cell as described above. The output signals 49 and 50 of the converters 44 and 45 are produced concurrently. Output 50 from converter 45 is stored in a buffer memory 51 until the output 49 from converter 44 is read by microprocessor 54 through the parallel part 52. Control switches 53 allow the operator to present the equipment output signal in the desired form. The digital binary parallel output signal is converted to an analogue current by the output current source 57 and the condition of the instrument is displayed on the front panel indicators 58. The equipment is powered by a D.C. power supply 59. The instrument was used to measure pulp concen¬ tration for bleached unrefined pine kraft pulp, reslushed corrugating medium and waste paper stock for concentrations from 0 to 2 gms/dry wt. per 100 gms. The results are shown in figures 4 to 6 respectively and the milliamp outut of the photo detectors show a linear response to concentration, within the useful light intensity range of the detectors, for the three stock types. The limit of the device is determined by the opacity of the stock being measured and the lower limit of light sensitivity of the detector. This invention is not only applicable to measuring concentration of paper pulp but also for other optically active substances. If the substances are not optically active they can be treated to render the surface optically active. Figures 4 to 7 and 8 refer to tests made upon suspensions of bleached unrefined pine kraft (figure 4), reslushed corrugating medium (figure 5), waste paper stock (figure 6), raw wheaten starch (figure 7) and clay (figure 8) with the system of this invention set for maximum damping and maximum gain, with 2 volt maximum output range. The results of figures 4 to 8 demonstrate the linearity of the measuring system of the invention. The present invention can also be adapted for use as in pipe probe to provide processing control information. Such a probe is illustrated schematically in figures 9 to 11 in which figure 9 is a side view of the end of the probe figure 10 is a view at right angles to the view of figure 9 and figure 11 is a plan view of the end of the probe. The probe 39 has at its lower end a channel 40 bounded by two legs 41 and 42. The channel 40 is aligned in the direction of flow of the stock to be measured. The optical system is arranged in the legs 41 and 42 with the light source 43 in leg 41 and the photo-detectors 44 and 45 in leg 42. A sealing barrier 46 separates the optical system from the electronic circuting located either in the main body of the probe or in a separate unit. The optical system and the electronic system are the same as outlined above except they are adapted to fit the confined space of the probe. From the above it can be seen that the combination of measuring depolarization by comparison of two signals and identifying a narrow pulsed infra-red beam with digital analysis provides the basis for a robust means for obtaining concentration measurements.

Claims: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. Apparatus for measuring volume concentration of optically active solids suspended in a liquid comprising a cell for containing the suspension to be measured, a pulsed infra-red light source which emits a narrow beam of polarized light at right angles to the cell, a first photo-detector on the opposite side of the cell to the light source, a second photo- detector with an associated polarizing filter located on the opposite side of the cell to the light source, computing means to analyse the signals received from said first and second photo-detectors and produce a signal corresponding to the ratio of signals from said first detector to said second detector and optional display or recording means for displaying .or recording in suitable form the measurement so obtained. 2. Apparatus as claimed in claim 1 in which the light source is an improved light emitting diode in combination with a polarizing filter. 3. Apparatus as claimed in claim 1 in which the wavelength of the infra-red beam is 600 to 650 nm. 4. Apparatus as claimed in claim 1 wherein the first photo- detector is located on a light path at right angles to that for the second photo-detector a mirror being used to split the beam between the two detectors. 5. A method of measuring volume concentration of optically active solids suspended in a liquid comprising passing through said liquid pulses of a mirror beam of infra-red polarized light and detecting incident light which has passed through said liquid on two photo-detectors one of which has an associated polarizing filter, analysing the signals received from said first and second photo-detectors to produce a signal corresponding to the ratio of signals from said two detectors and subsequently recording or displaying the measurement so outlined. 6. A method as claimed in claim 5 wherein the light beam has a wavelength of 600 to 650 nm. 7. A method as claimed in claim 5 wherein a light emitting diode is used to provide the pulsed light beam. 8. A method as claimed in claim 5 wherein the difference between detector signals when the light source is on and off is used to overcome drift and offset any components used in the measuring system.

Inventors: BOLDISTON DAVID JOHN, CAHILL LAWRENCE WALTER, STUMPF ERICH, BOLDISTON, DAVID, JOHN, CAHILL, LAWRENCE, WALTER, STUMPF, ERICH
Applicants: APM LTD, AUSTRALIAN NEWSPRINT MILLS LTD
Application year: 1985
Main classification: G01N15
IPC codes: G01J1, G01N21