Electrical conductivity measuring device and ion chromatograph

Abstract

The challenge is to improve the accuracy of measuring the electrical conductivity of solutions. [Solution] The electrical conductivity measuring device 1 comprises an input voltage supply unit 2 that supplies a sinusoidal input voltage, electrodes 3 placed in the solution to be measured, an amplification circuit 5, a measurement unit 7 that measures the amplified output voltage, and a calculation unit 8 that calculates the electrical conductivity of the solution based on the measured output voltage. The amplification circuit 5 includes a plurality of first feedback circuits 61 to 63 connected in parallel, each having switches S1 to S3 and first feedback resistors R1 to R3, each having parasitic capacitance. The electrical conductivity measuring device 1 further comprises a bias voltage supply unit 4 that supplies DC bias voltage to the plurality of first feedback circuits. The gain of the amplification circuit is adjusted by switching the connection of each feedback circuit to the amplification circuit by operating each switch ON / OFF.

Description

Technical Field

[0001] The present invention relates to a conductivity measuring device for measuring the electrical conductivity of a solution and an ion chromatograph including the conductivity measuring device.

Background Art

[0002] There is a conductivity measuring device for measuring the electrical conductivity of a solution. The conductivity measuring device is used, for example, for measuring the ion concentration of sample components in ion chromatography. In the conductivity measuring device, a sinusoidal voltage is supplied as an input signal to a pair of electrodes disposed in the solution to be measured. An amplifier circuit is provided downstream of the electrodes. After amplifying a signal based on the current flowing between the electrodes, it is measured as the voltage of the output signal in the measuring unit. Then, the resistance between the electrodes is determined based on the voltage measured in the measuring unit. The electrical conductivity is represented by the reciprocal of the electrical resistance. Thereby, in the conductivity measuring device, the electrical conductivity of the solution is measured.

[0003] In the above-described conductivity measuring device, in order to widen the measurement range of the output signal, an amplifier circuit configured to be able to switch the magnitude of the feedback resistor may be used. A plurality of feedback circuits each having a feedback resistor are connected in parallel to the amplifier circuit. The connection of the plurality of feedback circuits to the amplifier circuit is switched by turning on / off a switch. With such a configuration, the gain of the amplifier circuit can be configured to be variable. Patent Document 1 below discloses an amplifier circuit having a variable gain.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] As described above, the gain of the amplification circuit can be made variable by switching the connection of multiple feedback circuits using a switch. Here, since there is inter-terminal capacitance in the switch, multiple feedback circuits connected in parallel form a parallel RC circuit. As a result, the impedance of the feedback circuit decreases at the cutoff frequency. Alternatively, because the input signal is a sine wave, a phase lag occurs in the output signal at the cutoff frequency. These phenomena reduce the measurement accuracy of the output signal, which is voltage. Consequently, this reduces the measurement accuracy of the electrical conductivity of the solution by the electrical conductivity measuring device.

[0006] The objective of this invention is to improve the accuracy of measuring the electrical conductivity of a solution using an electrical conductivity measuring device. [Means for solving the problem]

[0007] An electrical conductivity measuring device according to one aspect of the present invention is an electrical conductivity measuring device for measuring the electrical conductivity of a solution, comprising: an input voltage supply unit that supplies a sinusoidal input voltage; an electrode connected to the input voltage supply unit and placed in the solution to be measured; an amplification circuit connected to the electrode; a measuring unit that measures the output voltage amplified by the amplification circuit; and a calculation unit that calculates the electrical conductivity of the solution based on the output voltage measured by the measuring unit. The amplification circuit includes a plurality of first feedback circuits connected in parallel, each having a switch and a first feedback resistor, each having a parasitic capacitance. The electrical conductivity measuring device further comprises a bias voltage supply unit that supplies a DC bias voltage to the plurality of first feedback circuits of the amplification circuit. The gain of the amplification circuit is adjusted by switching the connection of each feedback circuit to the amplification circuit by the ON / OFF operation of each switch.

[0008] An ion chromatograph according to another aspect of the present invention comprises the above-described electrical conductivity measuring device. [Effects of the Invention]

[0009] The present invention aims to improve the accuracy of measuring the electrical conductivity of a solution using an electrical conductivity measuring device. [Brief explanation of the drawing]

[0010] [Figure 1] This figure shows an ion chromatograph equipped with an electrical conductivity measuring device according to this embodiment. [Figure 2] This is a circuit diagram showing the electrical conductivity measuring device according to this embodiment. [Figure 3] This graph shows the relationship between the load voltage and terminal capacitance of a photomos. [Modes for carrying out the invention]

[0011] Next, an embodiment of the present invention, consisting of an electrical conductivity measuring device and an ion chromatograph, will be described with reference to the attached drawings.

[0012] (1) Configuration of an ion chromatograph Figure 1 shows an ion chromatograph 10 equipped with an electrical conductivity measuring device 1 according to this embodiment. The ion chromatograph 10 is composed of a mobile phase tank 11, a liquid delivery pump 12, a sample injection unit 13, a separation column 14, an electrical conductivity measuring device 1, and a control unit 16.

[0013] The liquid transfer pump 12 delivers the mobile phase stored in the mobile phase tank 11 to the analysis channel 15. The sample injection unit 13 injects the sample into the analysis channel 15 through which the mobile phase flows. The separation column 14 separates the sample flowing through the analysis channel 15 into its components. The separated sample flows downstream through the analysis channel 15 together with the mobile phase and is supplied to the electrical conductivity measuring device 1.

[0014] The electrical conductivity measuring device 1 measures the electrical conductivity of the solution containing the sample and mobile phase. The ion chromatograph 10 detects the ion concentration of the sample components contained in the solution based on the electrical conductivity measured by the electrical conductivity measuring device 1. The control unit 16 controls the entire ion chromatograph 10, including the electrical conductivity measuring device 1.

[0015] (2) Configuration of the electrical conductivity measuring device Figure 2 is a circuit diagram showing the electrical conductivity measuring device 1 in this embodiment. As shown in Figure 2, the electrical conductivity measuring device 1 comprises an input voltage supply unit 2, electrodes 3, a bias voltage supply unit 4, an amplification circuit 5, a measurement unit 7, and a calculation unit 8.

[0016] The input voltage supply unit 2 supplies a sinusoidal input voltage. The electrode 3 is connected downstream of the input voltage supply unit 2. The input voltage supplied by the input voltage supply unit 2 is applied to the electrode 3. The pair of electrode plates of the electrode 3 are placed in the cell 31 to which the solution 32 to be measured is supplied. The solution 32 is a solution containing the sample and mobile phase supplied to the electrical conductivity measuring device 1 in the ion chromatograph 10 described above.

[0017] In this embodiment, the amplifier circuit 5 is an inverting amplifier circuit. The inverting input terminal (-) of the amplifier circuit 5 is connected to the downstream side of electrode 3. A negative feedback circuit 6 is also connected to the inverting input terminal of the amplifier circuit 5. The non-inverting input terminal (+) of the amplifier circuit 5 is grounded. The bias voltage supply unit 4 is connected to the inverting input terminal of the amplifier circuit 5 via resistor 41. The bias voltage supply unit 4 supplies a DC bias voltage to the negative feedback circuit 6 of the amplifier circuit 5.

[0018] The negative feedback circuit 6 includes four feedback circuits 61, 62, 63, and 64. The feedback circuits 61 to 64 are connected in parallel. Feedback circuit 61 includes a series-connected switch S1 and a feedback resistor R1, and is connected in parallel to the amplifier circuit 5 when switch S1 is switched ON. Feedback circuit 62 includes a series-connected switch S2 and a feedback resistor R2, and is connected in parallel to the amplifier circuit 5 when switch S2 is switched ON. Feedback circuit 63 includes a series-connected switch S3 and a feedback resistor R3, and is connected in parallel to the amplifier circuit 5 when switch S3 is switched ON. The connection / disconnection of the feedback circuits 61 to 63 to the amplifier circuit 5 can be switched by individually operating switches S1 to S3 under the control of the control unit 16. Feedback circuits 61, 62, and 63 are examples of the first feedback circuits according to the present invention. Feedback resistors R1, R2, and R3 are examples of the first feedback resistors according to the present invention.

[0019] In this embodiment, switches S1 to S3 are PhotoMOS relays. As shown in FIG. 2, a PhotoMOS relay is a semiconductor relay in which an LED is used for an input terminal and a MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) is used for an output terminal.

[0020] The feedback circuit 64 includes a feedback resistor R4. The feedback circuit 64 is not provided with a switch and, unlike the feedback circuits 61 to 63, is always configured to be connected in parallel with the amplifier circuit 5. The feedback circuit 64 is an example of the second feedback circuit according to the present invention. The feedback resistor R4 is an example of the second feedback resistor according to the present invention. In this embodiment, as an example, a case where the resistance values of the feedback resistors satisfy R4 > R3 > R2 > R1 will be described.

[0021] When all of the switches S1 to S3 are OFF, the amplifier circuit 5 is in a state where only the feedback resistor R4 is connected. In this state, the feedback resistor is at its maximum and the gain of the amplifier circuit 5 is at its maximum. When only the switch S3 is ON, the amplifier circuit 5 is in a state where the feedback resistors R3 and R4 are connected in parallel. In this state, the feedback resistor is the second largest and the gain of the amplifier circuit 5 is also the second largest. Similarly, when only the switch S2 is ON, the amplifier circuit 5 is in a state where the feedback resistors R2 and R4 are connected in parallel, and the gain of the amplifier circuit 5 is the third largest. When only the switch S1 is ON, the amplifier circuit 5 is in a state where the feedback resistors R1 and R4 are connected in parallel, and the gain of the amplifier circuit 5 is at its minimum. Of course, control may be performed such that any two of the switches S1 to S3 are ON.

[0022] As described above, the amplifier circuit 5 is configured such that the feedback resistor is variable by the ON / OFF operations of the switches S1 to S3. Thereby, the gain of the amplifier circuit 5 can be adjusted according to the measurement target, and the measurement range can be widened.

[0023] The measurement unit 7 is connected to the output terminal of the amplification circuit 5. The measurement unit 7 measures the output voltage amplified by the amplification circuit 5. The calculation unit 8 calculates the electrical conductivity of the solution 32 based on the output voltage measured by the measurement unit 7. The measurement unit 7 includes an A / D conversion processing unit that converts the acquired analog output voltage value into a digital value. The calculation unit 8 calculates the resistance value of the solution 32 by finding the reciprocal of the electrical conductivity of the solution 32.

[0024] (3) Influence of capacitance between switch terminals As described above, each of the feedback circuits 61 to 63 is equipped with a photomoss relay switch S1 to S3. Since photomoss relays have inter-terminal capacitance (parasitic capacitance), when any of the switches S1 to S3 are OFF, the feedback resistor R4 and the parasitic capacitance of switches S1 to S3 are connected in parallel. Therefore, the impedance of the feedback resistor of the amplifier circuit 5 decreases at the cutoff frequency of the RC circuit formed by the feedback resistor R4 and the parasitic capacitance of switches S1 to S3. For example, when all of the switches S1 to S3 are OFF, the impedance of the feedback resistor of the amplifier circuit 5 should ideally be R4, but when the frequency of the input voltage is lower than the cutoff frequency, the impedance becomes lower than R4 due to the effect of parasitic capacitance. Also, since the input voltage is a sine wave, a phase lag occurs in the output voltage at the cutoff frequency. In other words, when the frequency of the input voltage is lower than the cutoff frequency, a phase lag occurs in the output voltage measured by the measurement unit 7. A decrease in the impedance of the feedback resistor or a phase lag in the output voltage will reduce the accuracy of the measurement of the electrical conductivity of solution 32, which is calculated based on the output voltage and the feedback resistor.

[0025] Here, if we let Rx be the feedback resistor of the amplifier circuit 5 and Cf be the capacitance between the terminals of the switch, the cutoff frequency fc is expressed by the following equation 1. fc=1 / (2πRxCf) (Formula 1) From equation 1, it follows that the larger the feedback resistance Rx, the smaller the cutoff frequency fc becomes, and therefore the greater the influence of the terminal capacitance Cf.

[0026] In this embodiment, the electrical conductivity measuring device 1 is provided with a bias voltage supply unit 4 that supplies a DC bias voltage to the negative feedback circuit 6. By applying a DC bias voltage to switches S1 to S3 via the bias voltage supply unit 4, the inter-terminal capacitance of switches S1 to S3 can be reduced.

[0027] Figure 3 is a graph showing the relationship between the load voltage (DC bias voltage) and terminal capacitance of a certain photomos product. In this example, it can be seen that applying a DC bias voltage of about 10V to the photomos reduces the terminal capacitance by about half.

[0028] As described above, the electrical conductivity measuring device 1 of this embodiment is configured to apply a DC bias voltage to the negative feedback circuit 6, thereby increasing the cutoff frequency represented by Equation 1 compared to the case where no DC bias voltage is applied. This reduces problems that occur at the cutoff frequency, namely, problems such as a decrease in the impedance of the feedback resistor or a phase lag in the output voltage. By keeping the inter-terminal capacitance small, a large resistance can be used as the feedback resistor when a certain frequency is assumed for the input voltage (input signal) supplied by the input voltage supply unit 2. This makes it possible to increase the gain of the amplification circuit 5 and widen the measurement range of the electrical conductivity measuring device 1. Alternatively, when a certain resistance value is assumed for the feedback resistor of the amplification circuit 5, it becomes possible to increase the frequency of the input voltage.

[0029] (4) Adjustment of DC bias voltage The voltage value of the DC bias voltage supplied by the bias voltage supply unit 4 is preferably set based on the maximum feedback resistance of the amplifier circuit 5. As explained using Equation 1, the larger the feedback resistance Rx, the smaller the cutoff frequency and the greater the effect of the terminal capacitance Cf. In the above embodiment, it is preferable to adjust the magnitude of the DC bias voltage based on the case where switches S1 to S3 are switched to OFF and the feedback resistance is R4.

[0030] Furthermore, in the above embodiment, the gain of the amplification circuit 5 changes by switching the feedback resistors R1 to R4. This also changes the gain of the DC bias voltage, which affects the output voltage. Therefore, by adjusting the magnitude of the DC bias voltage based on the largest feedback resistor R4, it is possible to prevent the amplified voltage from exceeding the circuit's rating.

[0031] (5) Other embodiments As described above, the electrical conductivity measuring device 1 of this embodiment is equipped with a bias voltage supply unit 4, so the DC bias voltage is added to the output voltage measured in the measuring unit 7. For this reason, the amplitude of the voltage to be measured may be relatively small compared to the value of the DC bias voltage. If A / D conversion is performed in the measuring unit 7 on such a measurement signal, the resolution of the voltage change of the voltage to be measured will deteriorate. Also, since the feedback resistance is variable by switching switches S1 to S3, the gain in the amplification circuit 5 is also configured to be variable. For this reason, the gain of the DC bias voltage also changes, and the output voltage also changes. Therefore, a high-pass filter may be provided in front of the A / D conversion processing unit in the measuring unit 7 to cut the DC component (DC bias voltage) from the amplified output voltage.

[0032] In the above embodiment, the case in which photomos relays with inter-terminal capacitance are used as switches S1 to S3 was described as an example. The electrical conductivity measuring device 1 of this embodiment has the same effect for all switches that have parasitic capacitance. For example, the same effect is obtained even when switches S1 to S3 are mechanical relays that have parasitic capacitance.

[0033] In the above embodiment, the example described was one in which no switch is provided in the feedback circuit 64. In other words, the example described was one in which only the feedback resistor R4 is always connected in parallel to the amplifier circuit 5. In other embodiments, a switch may also be provided in the feedback circuit 64. In other words, a configuration in which switches are provided in all feedback circuits is also possible.

[0034] (6) Aspect Those skilled in the art will understand that the above-described exemplary embodiments are specific examples of the following embodiments.

[0035] (Section 1) An electrical conductivity measuring device according to one aspect of the present invention is: An electrical conductivity measuring device for measuring the electrical conductivity of a solution, An input voltage supply unit that supplies a sinusoidal input voltage, The electrode connected to the input voltage supply unit and placed in the solution to be measured, An amplification circuit connected to the aforementioned electrode, A measuring unit for measuring the output voltage amplified by the aforementioned amplification circuit, A calculation unit that calculates the electrical conductivity of the solution based on the output voltage measured by the measurement unit, Equipped with, The aforementioned amplification circuit is Each has a switch and a first feedback resistor, each with parasitic capacitance, and multiple first feedback circuits are connected in parallel. Includes, A bias voltage supply unit that supplies a DC bias voltage to the plurality of first feedback circuits of the amplification circuit, Furthermore, The gain of the amplification circuit is adjusted by switching the connection of each feedback circuit to the amplification circuit by operating each switch ON / OFF.

[0036] This can reduce problems that arise at the cutoff frequency, such as a decrease in the impedance of the feedback resistor or a phase lag in the output voltage.

[0037] (Section 2) In the electrical conductivity measuring device described in paragraph 1, The aforementioned amplification circuit is Inverting amplifier circuit, It may include.

[0038] Even at high input voltage frequencies, stable operation enables amplification of the output voltage.

[0039] (Section 3) In the electrical conductivity measuring device described in paragraph 1, The aforementioned switch is Photomos Relay, It may include.

[0040] This allows for stable switching of the feedback resistor over a long period of time.

[0041] (Section 4) In the electrical conductivity measuring device described in paragraph 1, The aforementioned amplification circuit is A second feedback circuit connected in parallel, without a switch and including a second feedback resistor, It may include.

[0042] The number of switches can be reduced.

[0043] (Section 5) In the electrical conductivity measuring device described in paragraph 1, The second feedback resistor may have a higher resistance value than any of the first feedback resistors.

[0044] The feedback resistor that achieves the maximum gain can be configured to be always connected.

[0045] (Section 6) In the electrical conductivity measuring device described in paragraph 5, The magnitude of the DC bias voltage may be set based on the state in which each switch is set to OFF and only the second feedback resistor is connected to the amplification circuit.

[0046] The DC bias voltage can be set based on the maximum resistance at which the effect of parasitic capacitance becomes significant.

[0047] (Section 7) In the electrical conductivity measuring device described in paragraph 1, A high-pass filter may be provided to remove the DC component from the output voltage amplified by the aforementioned amplification circuit.

[0048] This can improve the measurement accuracy of the output signal.

[0049] (Section 8) An ion chromatograph according to one aspect of the present invention comprises an electrical conductivity measuring device as described in any one of paragraphs 1 to 7.

[0050] This can improve the accuracy of measuring the ion concentration of sample components in ion chromatography. [Explanation of symbols]

[0051] 1: Electrical conductivity measuring device 2: Input voltage supply unit 3: Electrode 4: Bias voltage supply section 5: Amplifier Circuit 6: Negative feedback circuit 7: Measuring part 8: Arithmetic section 10: Ion Chromatography 31: Cell 32 :Solution 61-64: Feedback circuit R1~R4: Feedback resistors S1~S3: Switch

Claims

1. An electrical conductivity measuring device for measuring the electrical conductivity of a solution, An input voltage supply unit that supplies a sinusoidal input voltage, The electrode connected to the input voltage supply unit and placed in the solution to be measured, An amplification circuit connected to the aforementioned electrode, A measuring unit for measuring the output voltage amplified by the aforementioned amplification circuit, A calculation unit that calculates the electrical conductivity of the solution based on the output voltage measured by the measurement unit, Equipped with, The aforementioned amplification circuit is Each has a switch and a first feedback resistor, each with parasitic capacitance, and multiple first feedback circuits are connected in parallel. Includes, A bias voltage supply unit that supplies a DC bias voltage to the plurality of first feedback circuits of the amplification circuit, Furthermore, An electrical conductivity measuring device in which the gain of the amplification circuit is adjusted by switching the connection of each feedback circuit to the amplification circuit by turning each switch ON / OFF.

2. The aforementioned amplification circuit is Inverting amplifier circuit, An electrical conductivity measuring device according to claim 1, including the following:

3. The aforementioned switch is Photomos Relay, An electrical conductivity measuring device according to claim 1, including the following:

4. The aforementioned amplification circuit is A second feedback circuit connected in parallel, without a switch and including a second feedback resistor, An electrical conductivity measuring device according to claim 1, including the following:

5. The electrical conductivity measuring device according to claim 1, wherein the second feedback resistor has a greater resistance value than any of the first feedback resistors.

6. The electrical conductivity measuring device according to claim 5, wherein the magnitude of the DC bias voltage is set with reference to a state in which each switch is set to OFF and only the second feedback resistor is connected to the amplification circuit.

7. The electrical conductivity measuring device according to claim 1, further comprising a high-pass filter that removes the DC component from the output voltage amplified by the amplification circuit.

8. An ion chromatograph comprising an electrical conductivity measuring device according to any one of claims 1 to 7.

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