JP7132014B2 - electrical conductivity meter - Google Patents

Description

The present invention relates to an electrical conductivity measurement technique for measuring the electrical conductivity of liquids.

A two-electrode electrical conductivity meter is known as a device for measuring the electrical conductivity (conductivity) of a liquid. A two-electrode electrical conductivity meter applies an AC signal such as a sine wave or square wave between two electrodes and detects the electrical signal generated between the electrodes to determine the electrical conductivity of the liquid. be. Patent Documents 1 to 3 disclose conventional techniques of a two-electrode electrical conductivity meter.

For example, in Patent Document 1, in a state in which two electrodes are immersed in a liquid to be measured, when an AC voltage is applied to one electrode, the current flowing into the other electrode is detected to detect the liquid to be measured. A two-electrode electrical conductivity meter for measuring the electrical conductivity from the electrical resistance is disclosed. Further, Patent Documents 2 and 3 disclose a two-electrode electrical conductivity meter in which two electrodes are formed in a rod shape.

Japanese Patent Publication No. 7-15490 Japanese Patent Application Laid-Open No. 2005-148007 Japanese Patent Application Laid-Open No. 2002-296312

In such a two-electrode type electrical conductivity meter, it is necessary to connect two electrodes and a circuit for deriving the electrical conductivity with a pair of wires, but the impedance of the wires increases depending on the length of these wires. , becomes non-negligible in the measurement of electrical conductivity. Therefore, there is a problem that the waveform of the signal detected by the electrodes is distorted due to the influence of the wiring impedance, and the measurement accuracy of the electric conductivity is lowered.

This point is discussed in Patent Document 1. FIG. 17 is a circuit diagram showing a signal processing circuit of a conventional electrical conductivity meter. FIG. 18 is an equivalent circuit between electrodes and cables in FIG.
In the signal processing circuit 50 shown in FIG. 17, the applied voltage Vg of the AC rectangular wave generated by the signal generating circuit 51 is stabilized to the applied voltage Vg' by the buffer amplifier U1, and then is applied via the terminal N1 and the electrode wiring LT1. is applied to electrode T1. A detection current It generated in the electrode T2 is input to the operational amplifier U2 and the feedback resistor Rf via the electrode wiring LT2 and the terminal N2, converted to a detection voltage Vt, and then rectified by the synchronous rectification circuit 52 to obtain a DC voltage Et. converted to output.

In the equivalent circuit shown in FIG. 18, Cp and Rp are the polarization capacitance and polarization resistance generated between the electrodes T1 and T2 when they come into contact with the liquid, and Rl is the liquid between the electrodes T1 and T2. is the liquid resistance with respect to Also, Cw is the line-to-line capacitance generated between the electrode wirings LT1 and LT2, and Rw is the wiring resistance of the LT1 and LT2.
As shown in FIG. 18, when the electrodes T1 and T2 are viewed from the signal processing circuit 50 through the terminals N1 and N2, the parallel circuit of the polarization capacitance Cp and the polarization resistance Rp and the series circuit of the liquid resistance Rl are Therefore, it seems that a series circuit of the line capacitance Cw and the wiring resistance Rw is connected in parallel.

In Patent Document 1, the output of the applied voltage Vg' is stopped at a timing when the influence of the distortion of the detected voltage Vt due to such Cp and Cw is small, for example, after the differential noise generated immediately after the start of the output of the applied voltage Vg'. Vt is sampled a plurality of times in the previous period, and Vt, which is not affected by Cp and Cw, is obtained by a calculation formula from a plurality of obtained sample voltages.
However, since the above formula assumes that the inter-line capacitance Cw can be ignored, there is a problem that if the electrode wirings LT1 and LT2 are long and Cw cannot be ignored, the electrical conductivity cannot be measured with high accuracy. rice field.

Generally, in order to suppress the influence of the polarization capacitance Cp, it is necessary to increase the frequency of the applied voltage Vg applied between T1 and T2. However, when the frequency of the applied voltage Vg is increased, the effect of the line capacitance Cw increases, and the distortion of the detected voltage Vt obtained from T1 and T2 increases, causing a decrease in the measurement accuracy of electrical conductivity. In addition, the longer the electrode wirings LT1 and LT2, the larger the Cw, the greater the distortion of the detected voltage Vt, and the lower the accuracy of electrical conductivity measurement.

The present invention is intended to solve such problems, and provides an electrical conductivity measurement technique that suppresses the influence of inter-line capacitance of electrode wiring that connects electrodes and can measure electrical conductivity with high accuracy. It is an object.

In order to achieve such an object, the electrical conductivity meter according to the present invention is an electrical conductivity meter for measuring the electrical conductivity of a liquid in a measuring tube, which has a constant amplitude at a preset signal frequency. a signal generating circuit for generating an alternating rectangular wave current as a rectangular wave signal; first and second electrodes attached to the measuring tube for applying the rectangular wave signal to the liquid; It comprises a detection circuit for detecting the amplitude of the detected voltage by sampling the detected voltage detected from the electrodes, and an arithmetic processing circuit for calculating the electrical conductivity of the liquid based on the amplitude.

Further, one configuration example of the electrical conductivity meter according to the present invention further includes a printed wiring board arranged near the first and second electrodes and on which the signal generating circuit is mounted.

In one configuration example of the electric conductivity meter according to the present invention, a buffer amplifier is installed near the first and second electrodes to stabilize the detected voltage and output the detected voltage to the detection circuit. The printed wiring board is further provided.

Further, in one configuration example of the electrical conductivity meter according to the present invention, the first electrode is a liquid-contact electrode that is in contact with the liquid, and the second electrode is provided on the outer circumference of the measuring tube. It consists of a non-wetted electrode formed and not in contact with said liquid.

In one configuration example of the electrical conductivity meter according to the present invention, the detection circuit samples the detected voltage at the central time position of the half cycle of the rectangular wave signal.

In one configuration example of the electric conductivity meter according to the present invention, the signal generation circuit includes a current detection circuit for detecting the magnitude of the rectangular wave signal, a clock signal indicating the signal frequency, and the current detection circuit. and an operational amplifier for maintaining the amplitude of the square-wave signal at a set current based on the detection result from.

According to the present invention, even if the signal frequency is relatively low, the slope of the detected voltage becomes linear, and the amplitude of the detected voltage can be stably detected. Therefore, as the signal frequency, it is possible to use a frequency that can suppress the influence of the line-to-line capacitance of the electrode wiring that connects the electrodes, and it is possible to measure the electrical conductivity with high accuracy.

1 is a block diagram showing a circuit configuration of an electrical conductivity meter according to a first embodiment; FIG. 1 is a side view of an electrical conductivity meter according to a first embodiment; FIG. 1 is a top view of an electrical conductivity meter according to a first embodiment; FIG. 1 is a perspective view of an electrical conductivity meter according to a first embodiment; FIG. It is another perspective view of the electrical conductivity meter according to the first embodiment. 4 is a front view showing a sub-board; FIG. FIG. 4 is a back view showing the sub-board; 4 is a signal waveform diagram showing the operation of the electrical conductivity meter according to the first embodiment; FIG. It is a configuration example of a rectangular wave current source. It is an equivalent circuit on the electrode side according to the first embodiment. FIG. 4 is a signal waveform diagram showing the operation of an electrical conductivity meter using a rectangular wave constant voltage signal; FIG. 4 is a characteristic diagram showing a correspondence relationship between amplitude data and electrical conductivity; It is a side view of an electrical conductivity meter according to a second embodiment. It is a top view of an electrical conductivity meter according to a second embodiment. It is a perspective view of an electrical conductivity meter according to a second embodiment. It is another perspective view of the electrical conductivity meter according to the second embodiment. FIG. 3 is a circuit diagram showing a signal processing circuit of a conventional electrical conductivity meter; FIG. 18 is an equivalent circuit between electrodes and cables in FIG. 17; FIG.

Next, embodiments of the present invention will be described with reference to the drawings.
[First embodiment]
First, an electrical conductivity meter 10 according to a first embodiment of the present invention will be described with reference to FIGS. 1 to 5. FIG. FIG. 1 is a block diagram showing the circuit configuration of the electrical conductivity meter according to the first embodiment. FIG. 2 is a side view of the electrical conductivity meter according to the first embodiment; FIG. 3 is a top view of the electrical conductivity meter according to the first embodiment; FIG. 4 is a perspective view of the electrical conductivity meter according to the first embodiment; FIG. 5 is another perspective view of the electrical conductivity meter according to the first embodiment.

As shown in FIGS. 1 to 5, the electrical conductivity meter 10 according to the present invention applies an alternating rectangular wave signal SG between two electrodes T1 and T2 attached to the measuring tube 3, It has a function of determining the electrical conductivity of the liquid in the measuring tube 3 based on the amplitude of the detected voltage Vt detected from .

As shown in FIG. 1, the electrical conductivity meter 10 includes a detection circuit 11, an arithmetic processing circuit 12, a setting/display circuit 13, a transmission circuit 14, a signal generation circuit 21, and a buffer amplifier 22 as main circuit units. there is

In the present invention, at least one or both of the signal generating circuit 21 and the buffer amplifier 22 among these circuit units are attached to the outer peripheral surface of the measuring tube 3 at positions near the electrodes T1 and T2. (printed wiring board) 2, and the electrodes T1 and T2 are electrically connected to the sub-board 2 via jumper wires J1 and J2. In the following, the detection circuit 11, the arithmetic processing circuit 12, the setting/display circuit 13, and the transmission circuit 14 are mounted on the main board (printed wiring board) 1, and the signal generation circuit 21 and the buffer amplifier 22 are mounted on the sub board 2. A case will be described as an example.

The detection circuit 11 controls the signal generation circuit 21 to generate an alternating rectangular wave current having a constant amplitude (set current Is) at a preset signal frequency fg for the electrodes T1 and T2 as a rectangular wave signal SG. That is, it has a function of applying a rectangular wave constant current signal and a function of detecting the amplitude of the detection voltage Vt generated at the electrodes T1 and T2 and outputting it to the arithmetic processing circuit 12 .

For variations of the electrical conductivity meter 10 according to the present invention, a non-wetted electrode that does not come into contact with the liquid may be used as the electrode T2, or a wetted electrode that comes into contact with the liquid may be used. In this embodiment, an example in which the electrode T2 is a non-wetted electrode will be described. A case where the electrode T2 is the liquid-contacting electrode will be described later in a second embodiment.

The detection circuit 11 includes a clock generation circuit 11A, a sample hold circuit (SH circuit) 11B, and an A/D conversion circuit (ADC circuit) 11C as main circuit portions.
The clock generation circuit 11A has a function of generating a clock signal CLKs for generating the rectangular wave signal SG and clock signals CLKh and CLKl for sampling control based on the clock signal CLK0 from the arithmetic processing circuit 12. .

The sample-and-hold circuit 11B samples and holds the output voltage Vt' from the buffer amplifier 22 by on/off controlling the switches SWh and SWl based on the clock signals CLKh and CLKl from the clock generation circuit 11A. It has a function of outputting VH and VL to the A/D conversion circuit 11C.
The A/D conversion circuit 11C has a function of A/D converting the differential voltage between VH and VL from the sample hold circuit 11B, ie, the amplitude voltage of Vt, and outputting the obtained amplitude data DA to the arithmetic processing circuit 12. is doing.

A connector CN1 mounted on the main board 1 is connected to a connector CN2 mounted on the sub-board 2 via a four-core connection wiring LC. Thereby, the main board 1 and the sub board 2 are electrically connected. Specifically, the clock signal CLKs is supplied from the terminal T11 of CN1 to the terminal T21 of CN2 via LC. Also, the reference voltage Vs is supplied from the terminal T12 of CN1 to the terminal T22 of CN2 via LC. Also, the ground voltage GND is supplied from the terminal T13 of CN1 to the terminal T23 of CN2 via LC. Also, the output voltage Vt' of the buffer amplifier 22 is supplied from the terminal T24 of CN2 to the terminal T14 of CN1 via LC.

Also, the sub-board 2 is electrically connected to the first and second electrodes T1 and T2 via jumper lines J1 and J2. Specifically, the pads (electrode connection terminals) P1 formed on the sub-board 2 are connected to the first electrodes T1 via jumper wires J1, and the pads (electrode connection terminals) formed on the sub-board 2 are connected to the first electrodes T1. ) P2 is connected to the second electrode T2 via a jumper J2. P1 is connected to the ground voltage GND on the sub-board 2 via a wiring pattern LP1 formed on the sub-board 2, and P2 is connected to the ground voltage GND on the sub-board 2 via a wiring pattern LP2 formed on the sub-board 2. , are connected to the signal generation circuit 21 and the buffer amplifier 22 .

The signal generation circuit 21 has a function of generating an alternating rectangular wave current having a constant amplitude (set current Is) at a preset signal frequency fg as a rectangular wave signal SG, ie, a rectangular wave constant current signal. Specifically, the signal generation circuit 21 is composed of a rectangular wave current source IG that operates on and off as a whole, is connected to the reference voltage Vs of T22 and the ground voltage GND of T23, and is based on the clock signal CLKs of T21. has a function of generating a rectangular wave signal SG having a set current Is and the same signal frequency fg as CLKs.

The buffer amplifier 22 is composed of an operational amplifier or a buffer circuit, for example, and has a function of stabilizing the detection voltage Vt detected from the electrodes T1 and T2 and outputting it as an output voltage Vt'. Specifically, in the buffer amplifier 22, the input terminal is connected to the pad P2 via LP2, and the output terminal is connected to T24.

The arithmetic processing circuit 12 has a function of calculating the electric conductivity of the liquid in the measuring tube 3 based on the amplitude data DA obtained by the detection circuit 11 by cooperating with the CPU and the program. ing. The arithmetic processing circuit 12 includes, as main processing units, an electrical conductivity calculation unit 12A and an empty state determination unit 12B.

The electric conductivity calculator 12A has a function of calculating the electric conductivity of the liquid in the measuring tube 3 based on the amplitude data DA obtained by the detection circuit 11. FIG. Specifically, the electrical conductivity corresponding to the amplitude data DA from the detection circuit 11 may be calculated using a preset electrical conductivity calculation formula. The correspondence relationship is measured in advance, the obtained characteristics are set in advance as a lookup table, and by referring to the lookup table based on the amplitude data DA from the detection circuit 11, the electrical Conductivity may be derived.

The empty state determining section 12B has a function of determining whether or not there is liquid in the measuring tube 3 based on the electric conductivity calculated by the electric conductivity calculating section 12A. Specifically, the empty state determination unit 12B compares the electric conductivity calculated by the electric conductivity calculation unit 12A with a preset threshold conductivity, and the calculated electric conductivity is equal to the threshold conductivity. , it is determined that there is no liquid in the measuring tube 3, that is, it is in an empty state.

The setting/display circuit 13 includes operation buttons and a display device such as an LED/LCD. It has the function of displaying
The transmission circuit 14 has a function of transmitting data between a host device (not shown) such as a controller via the transmission line LT, and the electrical conductivity and empty state determination result obtained by the arithmetic processing circuit 12 and a function of transmitting to a higher-level device.

[Structure of electrical conductivity meter]
Next, the structure of the electrical conductivity meter 10 according to the present embodiment will be described with reference to FIGS. 2 to 5. FIG. Hereinafter, for convenience, the direction in which the measuring tube 3 extends will be referred to as a first direction X, the left-right direction of the measuring tube 3 orthogonal to the first direction X will be referred to as a second direction Y, and the first and second The vertical direction of the measuring tube 3 perpendicular to the two directions X and Y is called a third direction Z. As shown in FIG.

The measuring tube 3 is made of a material having excellent insulating and dielectric properties, such as a cylindrical ceramic or resin, and is accommodated inside the lower case 4 . The lower case 4 is composed of a bottomed box-shaped resin or metal housing having an opening 4D on the upper side.

A pair of side surfaces 4A orthogonal to the first direction X of the side surfaces of the lower case 4 are provided with a metal plate capable of connecting a pipe (not shown) provided outside the electrical conductivity meter 10 and the measuring pipe 3. Tubular joints 5A and 5B made of material (eg, SUS) are provided. The measuring tube 3 is housed inside the lower case 4 along the first direction X, and joints 5A and 5B are respectively connected to both ends of the measuring tube 3 with a pair of O-rings OR interposed therebetween. .

At least one of the joints 5A and 5B functions as an electrode (first electrode) T1. For example, the joint 5A is connected to the ground voltage GND (common potential) to not only connect the external pipe and the measurement pipe 3 but also function as the electrode T1.
Thus, by realizing the electrode T1 with the joint 5A made of metal, the area where T1 contacts the liquid is increased.

As a result, even if foreign matter adheres or corrodes on T1, the area of the portion where foreign matter adheres or corrodes is relatively small with respect to the total area of T1, so the change in polarization capacity Measurement errors can be suppressed. Further, since the ground voltage GND is applied to the joint 5A, even if the external pipe connected to the joint 5A is made of metal, the external pipe does not act as an antenna and radiate electromagnetic noise. In addition, since the joint 5A is also used as the electrode T1, there is no need to provide a separate T1, and the electrical conductivity meter 10 can be made smaller.

On the other hand, a pair of side surfaces 4B of the side surfaces of the lower case 4 perpendicular to the second direction Y and the outer surface of the bottom surface 4E of the lower case 4 are attached with a shield 6 made of a metal plate having a U-shaped cross section. It is Thereby, the noise radiated outside from the electrical conductivity meter 10 can be reduced.

Further, on the outer peripheral surface 3A of the measuring tube 3, on the opposite side of the joint 5A with the sub-substrate 2 interposed therebetween, a plane electrode (second electrode) T2 made of a thin film conductor extends over the entire circumference of the measuring tube 3 and is not connected. It is patterned as a liquid electrode. A pad P3 is formed so as to protrude toward the sub-board 2 at the side end of T2 on the side of the sub-board 2 .

As described above, at least one of the signal generation circuit 21 and the buffer amplifier 22 is attached to the sub-board (printed wiring board) 2 on the outer peripheral surface 3A of the measuring tube 3 at a position near the electrodes T1 and T2. , and the electrodes T1 and T2 are electrically connected to the sub-board 2 via jumper wires J1 and J2.

FIG. 6 is a front view showing the sub-board. FIG. 7 is a back view showing the sub-board.
As shown in FIG. 6, pads P1 are patterned along the second direction Y on the substrate surface 2A on the side of the electrode T1 formed by the joint 5A of the sub-substrate 2, laterally of the pipe hole 2H. , J1 are connected to this P1 and T1. J1 is soldered to the outer surface of P1 and T1.

Further, as shown in FIG. 7, on the substrate surface 2B on the electrode T2 side of the sub-substrate 2, a pad P2 is patterned above the tube hole 2H along the third direction Z. The P2 and P3 are connected via. J2 is soldered to P2 and P3.
A circuit mounting region 2G is provided on the board surface 2B above the tube hole 2H including the pad P2, and a signal generation circuit 21, a buffer amplifier 22, and a connector CN2 are mounted, and the wiring pattern LP1 is provided. , LP2 (not shown).

As a result, the electrode wires connecting the sub-board 2 and the electrodes T1 and T2, that is, the lengths of the jumper wires J1 and J2 can be extremely shortened, and the impedance of J1 and J2 can be kept extremely low. Moreover, since the signal generation circuit 21 or the buffer amplifier 22 is mounted on the sub-board 2, the impedance of the connection wiring LC connecting the main board 1 and the sub-board 2 can be kept low. Therefore, in measuring the electrical conductivity, the impedance of the jumper wires J1 and J2 and the connection wiring LC can be ignored.

As shown in FIG. 2, an upper case 9 is attached to the upper portion of the lower case 4 so as to cover the opening 4D. The main substrate 1 is fixed inside the upper case 9, and has circuit units such as a detection circuit 11, an arithmetic processing circuit 12, a setting/display circuit 13, and a transmission circuit 14 mounted thereon. The connector CN2 of the sub-board 2 is connected to the connector CN1 of the main board 1 via the connection wiring LC.
A ground pattern connected to the ground voltage GND may be formed in a region other than the circuit parts and the wiring pattern on the sub-board 2 . As a result, noise entering the electrode T2 from the outside of the electrical conductivity meter 10 can be reduced, and measurement errors can be suppressed.

The sub-board 2 may be attached in any direction as long as it is attached to the outer peripheral surface 3A of the measurement tube 3. , the sub-board 2 is fixed to the measuring tube 3 .
As shown in FIGS. 6 and 7, in the sub-board 2, a tube hole 2H for press-fitting the measuring tube 3 is formed at a center position in the left-right direction Y, which is the left-right direction as viewed in the plane of the paper. . As a result, the sub-board 2 can be fixed to the measuring tube 3 with a very simple structure without using fixing members such as mounting screws.

The size of the tube hole 2H is set to be the same as or slightly smaller than the size of the outer peripheral portion of the measurement tube 3 . At this time, the tube hole 2H does not need to be perfectly circular in conformity with the outer peripheral shape of the measurement tube 3, and may be substantially polygonal, and in FIGS. 6 and 7, substantially octagonal. As a result, the end portion of the tube hole 2H is partially in contact with the outer peripheral surface 3A. 2, the influence of heat transmitted to the signal generation circuit 21 and the buffer amplifier 22 mounted on the circuit board 2 can be suppressed.

In addition, since the gaps 2S are discretely formed between the tube hole 2H and the measuring tube 3, the measuring tube 3 can be easily press-fitted into the tube hole 2H, and a jig dedicated to press-fitting is prepared. It is not necessary and can reduce the work load.
The shape of the tube hole 2H is not limited to a substantially polygonal shape, and the hole wall surface of the tube hole 2H may be provided with a plurality of projections, and the projections may contact the outer peripheral surface 3A. Alternatively, a part of the periphery of the tube hole 2H may be provided with a notch that opens directly toward the side end of the sub-board 2 or a slit that opens indirectly. As a result, the same effects as those described above can be obtained.

Further, in the present embodiment, the inner wall portion 4C of the side surface 4B of the lower case 4 is formed with a pair of guide portions 7X and 7Y made of convex or concave rails. By inserting the sub-board 2 through the opening 4D of the lower case 4 so that the side ends 2X and 2Y of the sub-board 2 are fitted into the guides 7X and 7Y, the measurement tube 3 is inserted through the sub-board 2 into the lower case. 4 is attached. As a result, the sub-board 2 and furthermore the measuring tube 3 can be mounted inside the lower case 4 with a very simple structure.

It should be noted that the guide portions 7X and 7Y do not need to be formed by extending the convex portion or the concave portion. It may be formed separately into a plurality of parts. Also, in FIG. 3, the case where the guide portions 7X and 7Y consist of two ridges is shown as an example, but grooves into which the side ends 2X and 2Y are inserted may be used instead of the ridges.
In addition, it is not necessary to fix the sub-board 2 with the guide portions 7X and 7Y. On the contrary, it is better to have a little play to reduce the mechanical stress applied to the measuring tube 3 or the sub-board 2 when the joints 5A and 5B are screwed. can be mitigated.

[Operation of the first embodiment]
Next, operation of the electrical conductivity meter 10 according to the present embodiment will be described with reference to FIG. FIG. 8 is a signal waveform diagram showing the operation of the electrical conductivity meter according to the first embodiment.
Here, an example in which the electrode T2 is a non-wetted electrode and the rectangular wave signal SG is a rectangular wave constant voltage signal will be described.

Based on the clock signal CLK0 from the arithmetic processing circuit 12, the clock generation circuit 11A generates a clock signal CLKs for generating the rectangular wave signal SG and clock signals CLKh and CLKl for sampling control. Here, a case is shown where the frequency of CLKs, that is, the signal frequency fg of the rectangular wave signal SG is 150 kHz.

The signal generation circuit 21 controls on/off of the rectangular wave current source IG based on CLKs. As a result, as shown in FIG. 8, the applied current Ig is switched between the preset set current Is and zero every half cycle of the signal frequency fg, and is applied to the electrode T2. . Therefore, the voltage generated by the liquid resistance of the liquid between the electrodes T1 and T2 due to the applied current Ig supplied from the signal generation circuit 21 becomes the voltage between the electrodes T1 and T2, that is, the detection voltage Vt.

The sample-and-hold circuit 11B is supplied with Is of the output voltage Vt′ obtained by stabilizing Vt (impedance conversion) by the buffer amplifier 22 based on CLKh from the clock generation circuit 11A. The detection voltage VH is sampled at TH (half cycle of SG). Also, the sample hold circuit 11B samples the detection voltage VL during the low level period TL (half cycle of SG) during which zero is supplied in Vt' based on CLK1 from the clock generation circuit 11A.

The A/D conversion circuit 11C A/D-converts the differential voltage ΔVt between VH and VL obtained by the sample-and-hold circuit 11B into amplitude data DA and outputs the amplitude data DA.
In general, a method of full-wave rectifying the detected AC voltage Vt, for example, a method of folding back the detected voltage Vt at TL at an intermediate level of Vt and adding it to Vt at TH can be considered. However, in such a method, if the Vts of TL and TH are not equal, a pulsating current remains even after full-wave rectification, and a stable DC voltage cannot be obtained, which causes a measurement error.

According to the present embodiment, the detected AC voltage Vt is not full-wave rectified, but separately sampled at TL and TH, and the obtained differential voltage between VH and VL is acquired as the amplitude data DA. Therefore, even if Vt includes fluctuations due to changes in the flow velocity of the liquid, etc., or if common mode noise is mixed in Vt from the outside through the liquid, the influence on the amplitude data DA is minimized. can be avoided, and stable measurement of electrical conductivity can be realized.

12 A of electrical conductivity calculation parts calculate the electrical conductivity of a liquid based on DA from 11 C of A/D conversion circuits.
Further, the empty state determination section 12B determines whether or not the inside of the measuring tube 3 is in an empty state by comparing the electrical conductivity obtained by the electrical conductivity calculation section 12A with the threshold conductivity.

FIG. 9 is a configuration example of a rectangular wave current source. As shown in FIG. 9, the rectangular wave current source IG comprises a switch SWi, an operational amplifier Ug, and a current detection circuit DET. SWi is an analog switch that switches and outputs Vs and GND based on CLKs. DET is a circuit that detects the current value of the applied current Ig output from the IG. Ug has the function of maintaining and controlling the current value of Ig at the set current Is based on the current detection output from DET, and controlling the on/off of the output of Ig based on the output of SWi.

FIG. 10 is an equivalent circuit on the electrode side according to the first embodiment. As described above, in this embodiment, a rectangular wave constant current signal is used as the rectangular wave signal SG. Therefore, as shown in FIG. 10, the equivalent circuit on the electrode side viewed from the sub-board 2 is the equivalent circuit on the side showing the impedance between the electrodes T1 and T2 with respect to the rectangular wave current source IG of the signal generating circuit 21. It becomes a form in which Zt is connected.

At this time, in Zt, when the electrodes T1 and T2 are in contact with the liquid, a polarization capacitance Cp and a polarization resistance Rp are generated between the electrodes and the liquid. An electrode capacitance Ct is generated at . Therefore, if the liquid resistance of the liquid between the electrodes T1 and T2 is Rl, Zt is represented by an equivalent circuit in which the parallel circuit of the polarization capacitance Cp and the polarization resistance Rp, the liquid resistance Rl, and the electrode capacitance Ct are connected in series. be done. Here, when the signal frequency of the rectangular wave signal SG is fg=150 kHz, although the impedance of Cp is relatively small, the impedance of Ct increases to some extent, so the voltage Vct across Ct and further Vt change transiently. become.

FIG. 11 is a signal waveform diagram showing the operation of an electrical conductivity meter using a rectangular wave constant voltage signal. When fg=150 kHz as in FIG. 8, the impedance of Cp is relatively small, but the impedance of Ct is somewhat large. Therefore, when an alternating rectangular wave voltage having a constant amplitude (set voltage Vs) is used as a rectangular wave signal SG, that is, a rectangular wave constant voltage signal, Vct, Vrl, and Vt are exponentially function with their respective time constants. , making it impossible to stably detect VH and VL.

In this way, when the Vt waveform is distorted, an error is likely to be included in the detection of the amplitude data DA, resulting in a decrease in the accuracy of measurement of electrical conductivity. Therefore, as fg, it is necessary to use a frequency so high that the impedances of Cp and Ct can be ignored. On the other hand, if fg is increased, the influence of the inter-line capacitance Cw of the electrode wiring increases as in the conventional equivalent circuit shown in FIG.

In contrast, in the present embodiment, since a rectangular wave constant current signal is used as the rectangular wave signal SG, even when fg=150 kHz, the slopes of Vct and Vt are linear, and VH and VL are stabilized. can be detected by

Let VH be the detection voltage Vt detected during the high level period TH when the applied current Ig is the set current Is, and let VrlH and VctH be Vrl and Vct at that time, then VH=VrlH+VctH. Also, if the detection voltage Vt detected during the low level period TL when Ig=0 is VL, and Vrl and Vct at that time are VrlL and VctL, then VL=VrlL+VctL.

At this time, although Vct is included in the detected VH and VL, since CLKh and CLKl indicate the central positions of TH and TL (half cycles of SG), VctH included in the sampled VH and VL VctL will be equal. As a result, VctH and VctL are offset by taking the differential voltage ΔVt between VH and VL, and amplitude data DA not including Vct is obtained.

That is, ΔVt=VH−VL=VrlH−VrlL. Accordingly, since Ig is constant, Rl can be obtained by the following formula (1).

In equation (1), Ig is known, and the differential voltage VH-VL is detected by the SH circuit 11B, converted to amplitude data DA by the A/D conversion circuit 11C, and input to the arithmetic processing circuit 12. FIG. Therefore, the electrical conductivity calculator 12A can easily calculate Rl based on these data.

FIG. 12 is a characteristic diagram showing the correspondence relationship between amplitude data and electrical conductivity, where the vertical axis represents amplitude data DA and the horizontal axis represents electrical conductivity. By performing calibration work using a plurality of standard fluids with known electrical conductivity, the correspondence relationship between such amplitude data DA and electrical conductivity is measured in advance, and the obtained characteristics are used as a lookup table, For example, it is set in a semiconductor memory (not shown), and based on the amplitude data DA from the detection circuit 11, the electrical conductivity calculator 12A refers to the lookup table to determine the electrical Conductivity may be derived.

[Effects of the first embodiment]
Thus, in the present embodiment, the signal generation circuit 21 generates an alternating rectangular wave current having a constant amplitude at a preset signal frequency fg as the rectangular wave signal SG, which is attached to the measuring tube 3. A detection circuit 11 detects the amplitude of the detection voltage Vt by sampling the detection voltage Vt detected from these T1 and T2.
As a result, even if fg is relatively low, the slope of the detection voltage Vt becomes linear, and the amplitude of the detection voltage Vt can be stably detected. Therefore, as fg, it is possible to use a frequency that can suppress the influence of the line-to-line capacitance of the electrode wiring connecting T1 and T2, making it possible to measure electrical conductivity with high accuracy.

In the present embodiment, T1 may be a liquid-contacting electrode that is in contact with the liquid, and T2 may be a non-contacting liquid-contacting electrode that is formed on the outer periphery of the measuring tube 3 and is not in contact with the liquid. .
As a result, it is possible to suppress the occurrence of measurement errors due to adhesion of dirt to the electrode surfaces and corrosion of the electrodes. In addition, there is no need to use an expensive wetted electrode such as platinum black, and a significant cost reduction can be achieved. Further, when a non-wetted electrode is used, although an electrode capacitance Ct is generated between the electrode and the liquid, since a rectangular wave constant current signal is used as the rectangular wave signal SG, the amplitude of the detected voltage Vt can be stabilized. can be detected.

Further, in the present embodiment, the detection circuit 11 may sample the detection voltage Vt at the central time position of the half cycle of the rectangular wave signal SG.
As a result, even when a non-wetted electrode is used as T2, the voltage VctH across the electrode capacitance Ct of T2 included in VH sampled during the high level period TH and the voltage Ct included in VL sampled during the low level period TL becomes equal to the both-ends voltage VctL. Therefore, by taking the differential voltage ΔVt between VH and VL, VctH and VctL are offset, and amplitude data DA not including Vct is obtained. Therefore, it is possible to measure the electrical conductivity with high accuracy.

In this embodiment, the rectangular wave current source IG of the signal generation circuit 21 is replaced by a current detection circuit DET for detecting the magnitude of the applied current Ig, which is the rectangular wave signal SG, and a clock signal CLKs indicating the signal frequency fg. It may be configured with an operational amplifier Ug that maintains the amplitude of Ig at the set current Is based on the detection result from the current detection circuit DET. As a result, highly accurate and stable Ig can be generated with a relatively simple configuration.

Further, in the present embodiment, the sub-board 2 is arranged near the electrodes T1 and T2 attached to the measuring tube 3, and the signal generating circuit 21 for generating the rectangular wave signal SG and the electrodes T1 and T2 At least one or both of the buffer amplifiers 22 that stabilize and output the detected detection voltage Vt may be mounted on the sub-board 2 .
As a result, the length of the electrode wiring connecting the signal generation circuit 21 and the buffer amplifier 22 to the electrodes T1 and T2, that is, the length of the jumper lines J1 and J2 can be significantly shortened, and the capacitance between the electrode wirings can be reduced. can do. Therefore, even if a relatively high signal frequency is used, the electrical conductivity can be measured with high accuracy.

Further, in the present embodiment, the sub-board 2 is provided with a tube hole 2H into which the measuring tube 3 is inserted, and the tube hole 2H and the outer peripheral surface 3A of the measuring tube 3 are brought into contact with each other so that the measuring tube 3 can be attached to the outer peripheral surface 3A. can be
As a result, the sub-board 2 can be fixed to the measuring tube 3 with a very simple structure without using fixing members such as mounting screws.

Moreover, with such a configuration, the sub-board 2 can be arranged between the electrodes T1 and T2 so as to be perpendicular to the longitudinal direction of the measuring tube 3 . Therefore, the electrode wiring from the sub-board 2 to the electrodes T1 and T2, that is, the jumper lines J1 and J2 can be arranged and connected in different positions and directions, and the inter-line capacitance between the electrode wirings can be extremely reduced. can. In addition, when a metal pipe is connected to the joint 5A, which is the electrode T1, the current applied to the liquid may go around the metal pipe and cause a measurement error. T2 can be easily placed by Therefore, it is possible to suppress the wraparound of the applied current to the metal pipe, and it is possible to measure the electrical conductivity with high accuracy.

In the present embodiment, at least one of pads (electrode connection terminals) for connecting electrode wiring to the electrodes T1 and T2, the signal generation circuit 21, and the buffer amplifier 22 is provided on the pattern surface of the sub-board 2. Either one or both of them may be formed with wiring patterns for connecting pads.
As a result, the signal generation circuit 21 and the buffer amplifier 22 mounted on the sub-board 2 can be very easily connected to the electrodes T1 and T2 by the jumper wires J1 and J2 without using connectors.

[Second embodiment]
Next, an electrical conductivity meter 10 according to a second embodiment of the present invention will be described with reference to FIGS. 13 to 16. FIG. FIG. 13 is a side view of the electrical conductivity meter according to the second embodiment; FIG. 14 is a top view of an electrical conductivity meter according to a second embodiment; FIG. 15 is a perspective view of an electrical conductivity meter according to a second embodiment; FIG. 16 is another perspective view of the electrical conductivity meter according to the second embodiment.

In the first embodiment, the case where the non-contact liquid electrode that does not contact the liquid is used as the electrode T2 has been described as an example. In the present embodiment, a case of using a liquid-contact electrode that is in contact with liquid as the electrode T2 will be described.

[Structure of electrical conductivity meter]
Next, the structure of the electrical conductivity meter 10 according to the present embodiment will be described with reference to FIGS. 13 to 16. FIG. Hereinafter, for convenience, the direction in which the measuring tube 3 extends will be referred to as a first direction X, the left-right direction of the measuring tube 3 orthogonal to the first direction X will be referred to as a second direction Y, and the first and second The vertical direction of the measuring tube 3 perpendicular to the two directions X and Y is called a third direction Z. As shown in FIG.

The measuring tube 3 is made of a material having excellent insulating and dielectric properties, such as a cylindrical ceramic or resin, and is accommodated inside the lower case 4 . The lower case 4 is made of a bottomed box-shaped resin or metal housing.

A pair of side surfaces 4A orthogonal to the first direction X of the side surfaces of the lower case 4 are provided with a metal plate capable of connecting a pipe (not shown) provided outside the electrical conductivity meter 10 and the measuring pipe 3. Tubular joints 5A and 5B made of material (eg, SUS) are provided. At this time, the measuring tube 3 is housed inside the lower case 4 along the longitudinal direction X, and a joint 5A and a joint 5B are connected to both ends of the measuring tube 3 with a pair of O-rings OR interposed therebetween. be.

At least one of the joints 5A and 5B functions as an electrode (first electrode) T1. For example, the joint 5A is connected to the ground voltage GND (common potential) to not only connect the external pipe and the measurement pipe 3 but also function as the electrode T1.
Thus, by realizing the electrode T1 with the joint 5A made of metal, the area where T1 contacts the liquid is increased. As a result, even if foreign matter adheres or corrodes on T1, the area of the portion where foreign matter adheres or corrodes is relatively small with respect to the total area of T1, so the change in polarization capacity Measurement errors can be suppressed.

On the other hand, a pair of side surfaces 4B of the side surfaces of the lower case 4 perpendicular to the second direction Y and the outer surface of the bottom surface 4E of the lower case 4 are attached with a shield 6 made of a metal plate having a U-shaped cross section. It is Thereby, the noise radiated outside from the electrical conductivity meter 10 can be reduced.

Further, on the side of the outer peripheral surface 3A of the measuring tube 3 opposite to the joint 5A across the sub-board 2, a metal rod is formed so as to penetrate the wall of the measuring tube 3 and protrude into the measuring tube 3. A wetted electrode (second electrode) T2 is attached. The portion protruding into the measuring tube 3 comes into contact with the liquid inside the measuring tube 3 .

As described above, at least one of the signal generation circuit 21 and the buffer amplifier 22 is mounted on the sub-board 2 mounted near the electrodes T1 and T2 on the outer peripheral surface 3A of the measurement tube 3, and a jumper The electrodes T1 and T2 are electrically connected to the sub-board 2 via lines J1 and J2. Specifically, J1 is soldered to the outer surfaces of P1 and T1, and J2 is soldered to P2 and T2.

[Operation of Second Embodiment]
Next, the operation of the electrical conductivity meter 10 according to this embodiment will be described.
When the electrode T2 is changed from the non-wetted electrode to the liquid-wetted electrode, the electrode capacitance Ct between T2 and the liquid in the case of the non-wetted electrode disappears. Therefore, the equivalent circuit Zt shown in FIG. 10 is represented by an equivalent circuit in which the parallel circuit of the polarization capacitance Cp and the polarization resistance Rp and the liquid resistance Rl are connected in series. Other electrical conductivity measurement operations according to the present embodiment are the same as those in the first embodiment, and detailed descriptions thereof are omitted here.

[Effects of Second Embodiment]
Thus, in this embodiment, the electrodes T1 and T2 are liquid-contact electrodes that come into contact with the liquid. As a result, it is possible to eliminate the influence of the capacitance Ct generated between the liquid and the electrode T2, which is peculiar to the use of a non-wetted electrode as T2, and the signal frequency of the rectangular wave signal SG is relatively low. can be used. Therefore, the influence of the inter-line capacitance of the electrode wiring, that is, the jumper wires J1 and J2 can be extremely reduced, and the electrical conductivity can be measured with extremely high accuracy.

[Expansion of Embodiment]
Although the present invention has been described with reference to the embodiments, the present invention is not limited to the above embodiments. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention. In addition, each embodiment can be implemented in any combination within a non-contradictory range.

DESCRIPTION OF SYMBOLS 10... Electrical conductivity meter 1... Main substrate 2... Sub substrate 2A, 2B... Substrate surface 2G... Circuit mounting area 2H... Tube hole 2S... Gap 2X, 2Y... Side end 3... Measurement Pipe, 3A... outer peripheral surface, 4... lower case, 4A, 4B... side surface, 4C... inner wall portion, 4D... opening portion, 4E... bottom surface, 5A, 5B... joint, 6... shield, 7X, 7Y... guide portion, 9 upper case 11 detection circuit 11A clock generation circuit 11B sample hold circuit (SH circuit) 11C A/D conversion circuit (ADC circuit) 12 arithmetic processing circuit 13 setting/display circuit , 14... transmission circuit, 21... signal generation circuit, 22... buffer amplifier, IG... rectangular wave current source, T1, T2... electrodes, P1, P2, P3... pads, J1, J2... jumper wires, LC... connection wiring, CN1, CN2... connectors, LP1, LP2... wiring patterns, SWg, SWh, SWl, SWi... switches, CLK0, CLKs, CLKh, CLKl... clock signals, Vs... reference voltages, GND... ground voltages, SG... square wave signals, Vg: Applied voltage, Ig: Applied current, Vt, VH, VL: Detection voltage, Vt': Output voltage, DA: Amplitude data.

Claims (5)

An electrical conductivity meter for measuring the electrical conductivity of a liquid in a measuring tube,
a signal generating circuit that generates an alternating rectangular wave current having a constant amplitude at a preset signal frequency as a rectangular wave signal;
first and second electrodes attached to the measurement tube for applying the square wave signal to the liquid;
a detection circuit that detects the amplitude of the detection voltage by sampling the detection voltage detected from the first and second electrodes;
an arithmetic processing circuit that calculates the electrical conductivity of the liquid based on the amplitude ,
The first electrode is a liquid-contact electrode connected to the measurement tube and composed of a metal tube whose inner peripheral surface is in contact with the liquid,
The second electrode is formed on the outer peripheral surface of the measuring tube and consists of a non-wetted electrode that is not in contact with the liquid.
An electrical conductivity meter characterized by: In the electrical conductivity meter according to claim 1,
An electrical conductivity meter, further comprising a printed wiring board disposed between the first and second electrodes and on which the signal generating circuit is mounted. In the electrical conductivity meter according to claim 1,
The electrical conductivity meter further comprising a printed wiring board mounted with a buffer amplifier arranged near the first and second electrodes to stabilize the detected voltage and output it to the detection circuit. In the electrical conductivity meter according to any one of claims 1 to 3 ,
The electrical conductivity meter, wherein the detection circuit samples the detection voltage at a central time position of a half cycle of the rectangular wave signal. In the electrical conductivity meter according to any one of claims 1 to 4 ,
The signal generation circuit is
a current detection circuit that detects the magnitude of the square wave signal;
an operational amplifier for maintaining the amplitude of the rectangular wave signal at a set current based on the clock signal indicating the signal frequency and the detection result from the current detection circuit.

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