FI89412C - Method and polarimeter for measuring widening of the polarization plane in sugar or other solution - Google Patents

Description

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Method and Polarimeter for measuring the rotation of the level of polarization of a sugar or other solution

The invention relates to a method for measuring the rotation of the polarization plane of a sugar or other solution 5, in which method a monochromatic polarized measuring beam is guided through a sample, the rotation of the polarization plane of which is measured after the sample with respect to the original polarization plane. The invention also relates to a polarimeter carrying out the method.

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A polarimeter measures the rotation of the polarization plane of polarized monochromatic light from a sugar sample dissolved in water. The magnitude of the light rotation angle is directly proportional to the amount of sugar in the solution. In addition, based on the direction of rotation, different sugars can be distinguished.

In sugar production, the polarimeter plays a key role in monitoring both beet and cane sugar-based production lines. All available commercial measuring devices 20 utilize ordinary, narrowband filtered light. Measuring dark solutions in particular is therefore difficult due to poor permeability. In this respect, laser light offers superior properties compared to previous measuring devices. The implementation of polarimeters utilizing laser light has only been at the design stage.

The structure of the most commonly used polarimeters today is described in NATURE, December 22, 1956 Vol. 178, E.J.

Gillham. Shown here is a polarimeter in which the polarized light 30 is guided through a sample cuvette and the change in rotation angle therein is compensated by a so-called Faradayn magnetomagnetic compensation cell. The rotation of the polarization plane caused by the solution is compensated by a magnetic field generated by the alternating current, whereby the required compensating current is directly proportional to the rotation angle and the sugar content. Because the magnetic field of the earth is inhomogeneous, it is possible to measure or obtain different results depending on the latitude with the same device. Therefore, calibration using standard sugar solutions is necessary.

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The known polarimeter technique has been described in the following publications: Hans Wenking, Göttingen, "Zeitschrift fiir Instrumen-tenkunde" 66. Jahrgang, Heft, January 1, 1958, and von K. Zander et al., ZUCKER, No. 12, 1974, vol. 642-647. Known commercial polarimeters require high voltage technology because a photomultiplier tube is used as the sensing element.

The use of a laser in polarimeters is described in A.L. Cummings et al., "Lasers and Analytical Polarimetry" pp. 291-302 and 10 ICUMSA 17th session Montreal 1978 subject 5 pp. 56-57, French Patent 2,393,296 and Brazilian Patent Application PI 7803313. In these, the use of laser light has achieved higher transmission and slightly higher accuracy due to the narrowband nature of the laser light. However, the detection technology is old-15 and no replacement solution has been found for the Faraday cell and photomultiplier tube.

It is an object of the present invention to provide a new method for measuring the rotation of a plane of polarization and a corresponding polarimeter, which is more accurate than before and insensitive to variations in the magnetic field of the earth. The characteristic features of the method according to the invention are set out in claim 1 and the characteristic features of a polarimeter carrying out the method are set out in claim 4.

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According to the invention, no attempt is made to determine the moment when the signal goes out after the analyzer level, but the phase difference is measured between the triggers of the clock counter placed on the rising parts of the signals. For this reason, cheap and simple 30 light emitting diodes (PINs) can be used as sensor elements. A significant additional advantage is obtained by using a temperature-stabilized laser diode, which reduces the size of the polarimeter and increases the beam transmittance compared to the use of mercury lamps. In addition, with temperature stabilization, the laser diode is kept in exactly the same single operating mode.

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The laser polarimeter according to the invention is made compact, mechanically simple and operated at low voltage.

In the following, the invention is illustrated by means of the accompanying exemplary embodiment, the principle of which is shown in the accompanying figure.

Figure 1 shows the basic structure of a polarimeter. Figure 2 shows level-corrected signals received from PIN diode detectors. Figure 3 shows a block diagram of a polarimeter. By means of temperature stabilization, the laser diode 2 is held TEM ^ form.

15 The test sample is placed in a cuvette 1 and a measuring beam 11 is passed through it. Here the light beam is produced by a laser diode 2, collimated parallel by a lens 3 and doubled by directing it through an inclined glass plate 4, leaving about 4% of the reflected reference beam 12. When the reference beam 12 has been turned by means of the prism 9 again in the direction of the measuring beam 11 (substantially), both beam beams are polarized by means of a polarizer plate 5.

A stronger measuring radius 11 passes through the sample cuvette 1 and, especially when measuring dark solutions 25, it is strongly attenuated. Increasing the thickness of the cuvette 1 is directly proportional to the increase in the angle of rotation and thus also to the resolution, so that the size of the sample column is optimized for each application. For a device designed for process use, the transmission thickness is thought to be about 30 20 mm. The measurement of clear liquids allows the use of a cuvette larger than a decade.

The level of polarization of the light beam passing through the sample rotates in proportion to the sugar content. Both the measuring and reference beams 11 and 12 pass through the analyzer plate 6, which is rotated via the shaft 10, and arrive at the light emitting diodes (PINs) 7 and 8, which act as detectors.

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Based on the experiments performed, the rotational speed of the analyzer has been chosen to be 50 revolutions per second in order to obtain sufficiently small rise times for the LEDs. Periodic signals (Malus' law) are obtained from the detectors, the phase difference of which indicates the change in the angle of rotation in the solution. Due to the symmetry of the polarizing plate 6, the laser beams are turned off by diodes at 180 * intervals, and the frequency of the signals becomes 100 Hz. The phase difference is measured using two counters. The reference counter counts the pulses arriving for the entire half cycle, while the measurement counter counts the pulses for the duration of the phase difference.

Figure 2 shows the intensity signals from the detector as a function of angle, after the level of the signals has been set equal. With amplifier technology, the signals can be smoothed in the range of 1:20. Still taking into account that the reference radius was only 4%, the attenuation of the sample can be compensated in the range of 1: 400. Since the analyzer plate 6 is rotated at a constant angular velocity, time could be used as the Y-axis quantity instead of the angle.

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The laser polarimeter according to the invention is insensitive to variations in the magnetic field of the earth. In addition, with each rotation of the analyzer, the instrument compares the phase difference of the reference and sample signals with the total rotation time, so no calibration is required. 25 The instrument provides the absolute rotation angle value twice during the analyzer plate rotation.

The CFD (constant fraction 30 discrimination) technique known per se is used to determine the trigger levels that trigger the clock counters. With this, the clocks are started exactly, for example, at a quarter of the periodic wave rise, in the intensity level Ir in Figure 2. Triggers are generated from the signals of both LEDs in each half-cycle, whereby the reading of the clock counter is stored. Figure 2 shows the trigger times tref <1, tref 2 for the reference signal and t1, t2 for the measurement signal. Half-cycle duration is obtained as the difference between the readings of two consecutive counters recording the reference signal and the phase difference time as the difference between the triggers of the measurement and reference signals „09412. The phase difference, i.e. the rotation of the polarization, is finally obtained by dividing the time between the triggers of the different signals by the total time of the period and multiplying this relative rotation by the whole circle, i.e. 360 °.

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The laser polarimeter takes only 20 milliseconds for one measurement, which consists of a 10 millisecond measurement period and another corresponding time when the data is read from the counters. This allows the same sample to be quickly analyzed 10 several times in a row, and thus a statistical error estimate of the results can be calculated on a computer.

Referring to Figure 3, the equipment can be divided into functional parts. The laser diode 2 and its temperature stabilizing means are assembled in their own unit 13. The optics, polarizing elements and rotating device, NTC resistor and PIN diodes are assembled in their own unit 14, which is mechanically firmly placed on the same body of the laser diode unit 13. The rest of the equipment is mainly signal processing and computing electronics.

The signals from the PIN diodes are fed to a CFD converter 16, from which the triggers are obtained as standard pulses. These pulses are applied to a timer 4, which results in two readings, one proportional to the total time of the cycle 25 and the other to the phase difference time. As a result of several measurements, the central unit 18 calculates the polarization rotation in the sample solution. The central unit 18 can also be used to process the result further. The control unit 19 includes a few pushbuttons from which the device is controlled. The display unit 30 includes an LED numeric display to display the result.

The CPU 18 communicates via the serial port 20 with both the process 22 and the NTC resistor that measures the temperature of the sample. The NTC resistor measuring the temperature of the sample is connected to 35 serial buses by means of a measuring interface unit 21.

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The light emitting diodes used as light detectors operate at low voltages, unlike current polarimeters, whose photomultiplier tubes require the use of high voltage. For safety reasons, the industry will try to avoid high voltage 5 instruments where possible.

According to the invention, the periodic rotation of the second polarizing plane can also be a reciprocating movement. It is not necessary to turn the reference radius exactly in the direction of the measuring radius. 10 What is essential here is to obtain a beam proportional to the original main beam, which is made to pass through both polarizing plates but past the sample.

Claims (4)

7 b 9 41 2 A method for measuring the rotation of a plane of polarization of a sugar or other solution, wherein method 5. produces a monochromatic light beam and divides it into a measuring beam (11) and a reference beam (12), passing the measuring beam (11) through a sample (1) and a reference beam (12) passing each beam before the sample through the first polarizing plane 10 (5) and after the sample through the second polarizing plane, i.e. the analyzer (6), which is rotated at a constant angular velocity, the phase difference of the beam intensities after the analyzer (6) is measured, the intensities being squared 15-shaped functions, characterized in that a certain trigger level (It) initiating the clock counters is determined from the intensities, which is adapted to the substantially steep ascending phases of the intensities, and that the phase difference time and the time proportional to the total time of the same 20 intensities are measured. Measuring method according to claim 1, characterized in that the obtained monochromatic beam 25 is collimated, polarized and doubled to obtain a reference beam (12), the duplicated beam is turned back parallel to the measuring beam (11), the measuring beam (11) is passed through the sample and both beams (11) , 12) is passed through a common rotating polarizer (6) to its own sensor elements (8, 7), from the signals of which the above-mentioned phase difference is calculated. A polarimeter for carrying out the method according to claim 1, comprising a laser diode (2), a first polarizing plate (5), a main beam duplication device (4) for providing a reference beam 35 (12), a sample cuvette (1) aligned with the measuring beam (11), at each radius, a second polarizing analyzer plate (6) and sensor means (7) for recording the intensities of the measuring (11) and reference beams (12), and wherein the analyzer plate with the measuring radius is located on a different side of the sample cuvette and in which one analyzer plate is rotatable; that the device comprises a prism (9) for rotating the reference beam (12) parallel to the measuring beam 5 (11), for determining the trigger level of the constant proportion transducer (16) from signals from the sensor means (7), clock counters (17) for measuring intervals between triggers, 9) and one analyzer plate (6) is thus set, e In this case, both the measuring radius (11) and the reference radius (12) pass through the same analyzer plate, which is adapted to be rotated. Laser polarimeter according to Claim 3, characterized in that the device comprises temperature stabilizing means for the laser diode (2). 9 89412