Electrical conductivity detector

Abstract

To provide an electrical conductivity detector that is less susceptible to noise.SOLUTION: An electrical conductivity detector 1 includes a waveform output unit 11 that supplies an AC signal to an object to be measured 12, and a control device 100 that controls the waveform output unit 11. The control device 100 is configured to execute a multiplication process 105 that multiplies a detection wave obtained when the AC signal passes through the object to be measured 12 by a reference wave for detecting electrical conductivity. The control device 100 is configured to execute a phase difference detection process 102 that causes the waveform output unit to generate a waveform as an AC signal in which a dummy peak is superimposed on an input wave and detects a corresponding phase difference between a detection wave obtained and the AC signal. The control device 100 is configured to execute a process 103 of causing the waveform output unit to generate a waveform as an AC signal in which a dummy peak is not superimposed on the input wave, and correct the phase difference against the reference wave, and to execute an electrical conductivity calculation process 108 in which an electrical conductivity is calculated based on a result of the multiplication process.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to electrical conductivity detectors. [Background technology]

[0002] An electrical conductivity detector is used, for example, as a detector for detecting each component (ion) of a sample separated in a column in an ion chromatograph. The electrical conductivity detector applies a voltage between a pair of electrodes arranged on either side of a flow path of an eluate (sample liquid) from an ion chromatograph, and measures the current flowing between the two electrodes. This allows the electrical conductivity of the eluate flowing between the two electrodes to be calculated, and the electrical conductivity detector detects the components separated by the ion chromatograph based on the electrical conductivity. Here, if the voltage applied between the two electrodes is a direct current, polarization occurs between the electrodes, making it impossible to measure the electrical conductivity accurately, so an alternating current (e.g., a sine wave) with a constant amplitude and frequency is used as the voltage.

[0003] WO 2017 / 208561 (Patent Document 1) discloses a measuring instrument that is an example of such an electrical conductivity detector. This measuring instrument has a sensor unit that outputs an AC signal, a reference signal output unit that outputs a reference signal having the same frequency and phase as the AC signal, a multiplication unit that multiplies the AC signal by the reference signal, an A / D conversion unit placed before or after the multiplication unit, and an integrating unit that integrates the digital signal that is the output of the multiplication unit. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2017 / 208561 Summary of the Invention [Problem to be solved by the invention]

[0005] Some electrical conductivity detectors perform lock-in measurements. When performing lock-in measurements, a process is carried out in which a detection wave and a reference wave (= the same frequency and phase as the drive wave) are multiplied. Depending on the object being measured, a phase difference may occur between the detection wave and the reference wave, causing an error in the multiplication result. For this reason, the phase difference between the drive wave (reference wave) and the detection wave is calculated, and the phase of the reference wave is aligned with the phase of the detection wave to eliminate the above error. However, noise components may be superimposed on the drive wave, making it impossible to accurately detect the phase difference, resulting in reduced accuracy in detecting electrical conductivity.

[0006] An object of the present disclosure is to provide an electrical conductivity detector that solves the above problems and is less susceptible to the effects of noise. [Means for solving the problem]

[0007] A first aspect of the present disclosure relates to an electrical conductivity detector. The electrical conductivity detector includes a waveform output unit that supplies an AC signal to a measurement object, and a control device that controls the waveform output unit. The control device is configured to execute a multiplication process in which a detection wave obtained by passing the AC signal through the measurement object is multiplied by a reference wave for detecting electrical conductivity. The control device is configured to cause the waveform output unit to generate a waveform in which a dummy peak is superimposed on an input wave as an AC signal, and to execute a phase difference detection process in which a phase difference between the correspondingly obtained detection wave and the AC signal is detected. The control device is configured to cause the waveform output unit to generate a waveform in which a dummy peak is not superimposed on an input wave as an AC signal, and to execute a process to correct the phase difference with respect to the reference wave, and to execute an electrical conductivity calculation process in which an electrical conductivity is calculated based on the result of the multiplication process. Effect of the Invention

[0008] In the electrical conductivity detector of the present disclosure, a dummy peak serving as a marker for detecting the phase difference is superimposed on the input wave, so that the phase difference can be accurately detected even in the presence of some noise, thereby improving the detection accuracy of the electrical conductivity. [Brief description of the drawings]

[0009] [Figure 1] 1 is a block diagram showing a configuration of an electrical conductivity detector according to an embodiment of the present invention. [Diagram 2] 11A and 11B are diagrams for explaining lock-in measurement when there is a phase error; [Diagram 3] FIG. 13 is a diagram for explaining lock-in measurement when there is no phase error. [Figure 4] FIG. 11 is a diagram for explaining a phase difference. [Diagram 5] 11 is a diagram for explaining the influence of spike noise on a peak position. FIG. [Figure 6] FIG. 13 is a diagram showing an example of the shape of a dummy peak. [Figure 7] FIG. 13 is a diagram showing the relationship between spike noise and dummy peaks. [Figure 8] 10 is a flowchart for explaining details of a phase difference detection process. [Figure 9] 10 is a flowchart for explaining details of an electrical conductivity calculation process. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, embodiments will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference characters and their description will not be repeated.

[0011] Fig. 1 is a block diagram showing the configuration of an electrical conductivity detector according to the present embodiment. The electrical conductivity detector 1 shown in Fig. 1 includes a D / A converter 11, a cell 12, an amplifier 13, an A / D converter 14, and a control device 100 that controls a waveform output unit (D / A converter 11). The cell 12 and the D / A converter 11 function as a waveform output unit that supplies an AC signal to a measurement target. The cell 12 contains a sample liquid that is a measurement target. The amplifier 13 amplifies a detection wave. The A / D converter converts the output of the amplifier 13 into a digital signal.

[0012] The control device 100 is configured, for example, with at least one central processing unit (CPU), at least one application specific integrated circuit (ASIC), at least one field programmable gate array (FPGA), or a combination thereof.

[0013] The control device 100 includes a storage device. The storage device includes, for example, a read only memory (ROM), a random access memory (RAM), a non-volatile memory, a solid state drive (SSD), or a hard disk. The storage device stores programs for implementing various processes. The control device 100 implements the following various processes by executing the programs stored in the storage device.

[0014] The control device 100 is configured to execute a multiplication process in which a detection wave obtained by passing an AC signal through a measurement target (sample liquid in the cell 12) is multiplied by a reference wave for detecting electrical conductivity. The multiplication process is executed by a multiplier 105.

[0015] The control device 100 is configured to cause the waveform output unit to generate a waveform in which a dummy peak is superimposed on an input wave as an AC signal, and to execute a phase difference detection process to detect a phase difference between the correspondingly obtained detection wave and the AC signal. The phase difference detection process is executed by a phase difference detection unit 102.

[0016] The control device 100 is configured to cause the waveform output unit to generate a waveform that does not have a dummy peak superimposed on the input wave as an AC signal, and to execute a process of correcting the phase difference with respect to the reference wave. The process of correcting the phase difference is executed by the phase correction unit 103.

[0017] The control device 100 is configured to execute an electric conductivity calculation process for calculating electric conductivity based on the result of the multiplication process. The electric conductivity calculation process is executed by an electric conductivity calculation unit 108 after the output of the multiplier 105 is integrated by an integration unit 106 and the numerical value is smoothed by a filter unit 107.

[0018] According to the electrical conductivity detector 1, the accuracy of correction for matching the phase of the reference wave with the detection wave is improved by the phase corrector 103, so that the electrical conductivity can be detected with high accuracy.

[0019] The phase correction will be described in detail below. In electrical conductivity detectors, lock-in measurement is generally performed. Fig. 2 is a diagram for explaining the lock-in measurement when there is a phase error. Fig. 3 is a diagram for explaining the lock-in measurement when there is no phase error.

[0020] A fixed-frequency sine wave is often used as the drive wave, as shown in Figures 2 and 3. In the configuration shown in Figure 1, a D / A converter 11 is used to generate the sine wave, and an A / D converter 14 is used to read the waveform.

[0021] In the case of single-phase detection, the detection wave and reference wave (=same frequency and phase as the drive wave) are multiplied by multiplier 105. However, as shown in Fig. 2, depending on the measurement target, a phase difference occurs between the detection wave and the reference wave, causing an error in the calculation result.

[0022] Fig. 4 is a diagram for explaining the phase difference. As shown in Fig. 4, normally, a peak detection process is performed before measurement to calculate the phase difference between the peak of the drive wave (or reference wave) and the peak of the detection wave, and the above error is eliminated by correcting the time of the phase difference to align the phase of the reference wave with the phase of the detection wave.

[0023] As shown in Figure 2, if there is a phase difference between the drive wave (= reference wave) and the detection wave, the correct calculation result will not be obtained. Due to the phase shift, there will be areas where the signs of the reference wave and the detection wave differ, and the correct area will not be obtained even if the multiplication results are integrated. On the other hand, if there is no phase difference between the drive wave (= reference wave) and the detection wave, as shown in Figure 3, the correct calculation result will be obtained.

[0024] Therefore, a correct calculation result can be obtained by detecting the phase difference by peak detection processing before normal measurement and feeding back the detected phase difference to the reference wave to change the state from that of FIG. 2 to that of FIG.

[0025] However, when using an inexpensive D / A converter, spike noise can sometimes get into the output voltage. Figure 5 is a diagram to explain the effect of spike noise on the peak position. Figure 5 shows a drive wave that includes spike noise (solid line) and a drive wave that does not include spike noise (dashed line) superimposed on each other. As shown in Figure 5, the peak position changes depending on the presence or absence of spike noise, so the introduction of spike noise causes errors in the detection of the sine wave peak.

[0026] Therefore, an error occurs when detecting the peak as shown in Figure 4. This becomes a phase error, and the phase error directly becomes an error in the electrical conductivity.

[0027] Spike noise is averaged over the sampling time of the A / D converter to produce a broad peak, so there are many situations where processing using general threshold judgment is difficult, such as when the amplitude of the detected wave is small. On the other hand, errors can be improved by using a glitch-free D / A converter that does not produce spike noise, but such D / A converters are expensive.

[0028] Although spike noise can be reduced by inserting an analog filter into the output of the D / A converter, it is necessary to take into consideration the delay and attenuation of the detection signal caused by the filter, and the additional components lead to increased costs.

[0029] Therefore, when detecting the phase difference, the electrical conductivity detector of this embodiment uses a dummy peak. Fig. 6 is a diagram showing an example of the shape of the dummy peak. Fig. 7 is a diagram showing the relationship between spike noise and the dummy peak.

[0030] In this embodiment, when performing peak detection processing, the sine wave data output unit 101 creates data so as to add a dummy peak having an amplitude larger than that of the spike noise to the output signal of the D / A converter 11. As shown in Fig. 7, since the amplitude of the dummy peak is larger than that of the spike noise, the peak position is not shifted by the spike noise when peak detection is performed. On the other hand, during normal measurement, sine wave data without dummy peaks is used, so there is no effect on the analysis results.

[0031] The amplitude of the dummy peak should be larger than the amplitude of the expected spike noise and smaller than the amplitude of the AC signal, which is the input wave. For example, the amplitude of the dummy peak should be 5% to 15% of the amplitude of the AC signal. More specifically, the amplitude of the dummy peak should be 10%.

[0032] FIG. 8 is a flowchart for explaining the phase difference detection process in detail. First, in step S1, the control device 100 outputs sine wave data with dummy peaks to the D / A converter 11, and in step S2, the control device 100 obtains detection wave data from the A / D converter .

[0033] Next, in step S3, the control device 100 performs peak detection of the detection wave. In peak detection, the control device 100 detects the time when the data value of the waveform converted into a numerical value by the A / D converter 14 is maximum. Next, in step S4, the control device 100 calculates a phase difference (or delay time) from the difference between the time of the peak of the input wave and the time of the peak of the detection wave, and in step S5, sets the phase difference as phase correction data.

[0034] FIG. 9 is a flowchart for explaining the details of the electrical conductivity calculation process. First, in step S11, the control device 100 outputs sine wave data without dummy peaks to the D / A converter, and obtains data of the detection wave that has passed through the cell 12 from the A / D converter 14. Then, in step S12, the control device 100 outputs phase-corrected square wave data, and in step S13, the detection wave data and the square wave data are multiplied together. The multiplication result is then integrated over a half cycle or one cycle of the sine wave to calculate the area.

[0035] At this time, if the square wave data is not corrected, there will be parts where the sign of the square wave and the sign of the detection wave are different, as shown in Figure 2, so the multiplication result will be a negative value and the area cannot be correctly integrated. In this embodiment, the phase of the square wave data is corrected when multiplying the square wave data by the detection wave data. As a result, the sign of the detection wave data and the sign of the square wave data will match over the entire range, as shown in Figure 3, and the area can be correctly calculated by integrating the result of multiplying them.

[0036] Thereafter, in step S15, a filtering process is performed, and in step S16, the converted result is multiplied by a coefficient corresponding to the size of the cell, etc., as necessary, and the converted result is output as the electrical conductivity.

[0037] As described above, the electrical conductivity detector of this embodiment provides the following effects.

[0038] Since there is no need to use a glitch-free D / A converter that does not produce spike noise, and there is no need to implement a filter circuit for removing spike noise, the electrical conductivity detector can be manufactured at low cost.

[0039] By simply changing the input wave data, the calculation cost remains almost unchanged, so there is no need to change the hardware (CPU, PLD, etc.), and it is easy to introduce processing that improves the accuracy of phase correction.

[0040] [Aspects] It will be appreciated by those skilled in the art that the exemplary embodiments described above are examples of the following aspects.

[0041] (Item 1) A first aspect of the present disclosure relates to an electrical conductivity detector. The electrical conductivity detector includes a waveform output unit that supplies an AC signal to a measurement object, and a control device that controls the waveform output unit. The control device is configured to execute a multiplication process in which a detection wave obtained by passing the AC signal through the measurement object is multiplied by a reference wave for detecting electrical conductivity. The control device is configured to cause the waveform output unit to generate a waveform in which a dummy peak is superimposed on an input wave as an AC signal, and to execute a phase difference detection process in which a phase difference between the correspondingly obtained detection wave and the AC signal is detected. The control device is configured to cause the waveform output unit to generate a waveform in which a dummy peak is not superimposed on an input wave as an AC signal, and to execute a process to correct the phase difference with respect to the reference wave, and to execute an electrical conductivity calculation process in which an electrical conductivity is calculated based on the result of the multiplication process.

[0042] According to the electrical conductivity detector of the first aspect, the accuracy of correction for matching the phase of the reference wave with the detection wave is improved, so that electrical conductivity can be detected with high accuracy.

[0043] (Item 2) In the electrical conductivity detector according to item 1, the input wave is a sine wave, and the reference wave is a square wave.

[0044] According to the electrical conductivity detector of the second aspect, the accuracy of correction for matching the phase of the rectangular wave, which is the reference wave, with the sine wave, which is the detection wave, is improved, so that electrical conductivity can be detected with high accuracy.

[0045] (3) In the electrical conductivity detector according to the 2nd paragraph, the waveform output unit includes a D / A converter. The control device is configured to transmit the sine wave data to the D / A converter in the electrical conductivity calculation process. The control device is configured to transmit data obtained by adding dummy peak data to the sine wave data to the D / A converter in the phase difference detection process.

[0046] According to the electrical conductivity detector in the third paragraph, since the waveform is generated by a D / A converter, it is easy to switch between superimposing a dummy peak on the input wave and not superimposing it on the input wave.

[0047] (Item 4) In the electrical conductivity detector described in item 3, the control device is configured to detect a peak value of the detection wave in the phase difference detection process, and detect a phase difference based on the amount of delay from a dummy peak of the AC signal to the peak value of the detection wave.

[0048] According to the electrical conductivity detector of the fourth paragraph, since a dummy peak is superimposed, the time difference between peaks is less susceptible to the influence of noise, and the accuracy of the correction for matching the phase of the reference wave with the detection wave is improved, so that the electrical conductivity can be detected with high accuracy.

[0049] (Item 5) In the electrical conductivity detector described in item 2, the control device is configured to calculate the area of ​​the overlapping portion of the reference wave and the sine wave by integrating the result of the multiplication process, and convert the calculated area into electrical conductivity.

[0050] According to the electrical conductivity detector of paragraph 5, when the input signal is an AC signal, the accuracy of correction for matching the phase of the reference wave with the detection wave is improved, so that electrical conductivity can be detected with high accuracy.

[0051] (Item 6) In the electrical conductivity detector according to item 1, the height of the dummy peak is 5% or more and 15% or less of the amplitude of the AC signal.

[0052] According to the electrical conductivity detector of paragraph 6, the dummy peaks are larger than the noise and are within a range in which the dummy peaks do not affect the AC signal, so that the time difference between the peaks is less susceptible to the effects of noise and the detection accuracy of the electrical conductivity can be improved.

[0053] (Item 7) In the electrical conductivity detector according to item 6, the height of the dummy peak is 10% of the amplitude of the AC signal.

[0054] According to the electrical conductivity detector of paragraph 7, the dummy peaks are larger than the noise and are within a range in which the dummy peaks do not affect the AC signal, so that the time difference between the peaks is less susceptible to the effects of noise and the detection accuracy of the electrical conductivity can be improved.

[0055] (Item 8) In the electrical conductivity detector according to item 1, the width of the dummy peak is shorter than half the period of the AC signal.

[0056] According to the electrical conductivity detector of item 8, since the dummy peaks are within a range that does not affect the AC signal, the time difference between peaks is less susceptible to the effects of noise, and the detection accuracy of electrical conductivity can be improved.

[0057] (Item 9) In the electrical conductivity detector according to item 1, a measurement object is a sample liquid for ion chromatography. The electrical conductivity detector further includes a cell for accommodating the sample liquid. The cell includes a first electrode that is immersed in the sample liquid and receives an AC signal from the waveform output unit, and a second electrode that is immersed in the sample liquid and outputs a detection wave.

[0058] According to the electrical conductivity detector of the ninth aspect, it is possible to improve the detection accuracy of electrical conductivity used in ion chromatography.

[0059] The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present disclosure is defined by the claims, not by the description of the embodiments described above, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0060] 1 Electrical conductivity detector, 11 D / A converter, 12 cell, 13 amplifier, 14 A / D converter, 100 control device, 101 sine wave data output section, 102 phase difference detection section, 103 phase correction section, 105 multiplier, 106 integrating section, 107 filter section, 108 electrical conductivity calculation section.

Claims

1. A waveform output unit that supplies an AC signal to the object to be measured; A control device for controlling the waveform output unit, the control device is configured to execute a multiplication process of multiplying a detection wave obtained by passing the AC signal through the measurement object by a reference wave for detecting electrical conductivity, the control device is configured to cause the waveform output unit to generate a waveform in which a dummy peak is superimposed on an input wave as the AC signal, and to execute a phase difference detection process to detect a phase difference between the detection wave and the AC signal obtained in response to the generated waveform, the control device is configured to cause the waveform output section to generate a waveform that does not superimpose the dummy peak on the input wave as the AC signal, to perform a process of correcting the phase difference with respect to the reference wave, and to perform an electrical conductivity calculation process of calculating the electrical conductivity based on a result of the multiplication process.

2. The electrical conductivity detector of claim 1 , wherein the input wave is a sine wave and the reference wave is a square wave.

3. the waveform output unit includes a D / A converter; The control device is configured to transmit sine wave data to the D / A converter in the electrical conductivity calculation process, 3. The electrical conductivity detector according to claim 2, wherein the control device is configured to transmit data obtained by adding the sine wave data to the dummy peak data, to the D / A converter, in the phase difference detection process.

4. 4. The electrical conductivity detector according to claim 3, wherein the control device is configured to detect a peak value of the detection wave in the phase difference detection process, and to detect the phase difference based on an amount of delay from the dummy peak of the AC signal to the peak value of the detection wave.

5. 3. The electrical conductivity detector according to claim 2, wherein the control device is configured to calculate an area of ​​an overlapping portion of the reference wave and the sine wave by integrating a result of the multiplication process, and convert the calculated area into the electrical conductivity.

6. 2. The electrical conductivity detector according to claim 1, wherein a height of the dummy peak is equal to or greater than 5% and equal to or less than 15% of an amplitude of the AC signal.

7. The electrical conductivity detector according to claim 6 , wherein the height of the dummy peak is 10% of the amplitude of the AC signal.

8. The electrical conductivity detector according to claim 1 , wherein a width of the dummy peak is shorter than half a period of the AC signal.

9. The measurement object is a sample solution for ion chromatography, The electrical conductivity detector further includes a cell for containing the sample liquid.

2. The electrical conductivity detector according to claim 1, wherein the cell includes a first electrode immersed in the sample liquid and receiving the AC signal from the waveform output unit, and a second electrode immersed in the sample liquid and outputting the detection wave.

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