CN110780122B

CN110780122B - Conductivity meter

Info

Publication number: CN110780122B
Application number: CN201910660602.2A
Authority: CN
Inventors: 间间田浩一; 百濑修
Original assignee: Azbil Corp
Current assignee: Azbil Corp
Priority date: 2018-07-24
Filing date: 2019-07-22
Publication date: 2022-02-25
Anticipated expiration: 2039-07-22
Also published as: JP2020016498A; JP7132014B2; KR102270974B1; CN110780122A; KR20200011370A

Abstract

The invention provides a conductivity meter, which can inhibit the influence caused by the line-to-line capacitance of the electrode circuit connected with the electrode and measure the conductivity with high precision. A signal generation circuit (21) generates an alternating rectangular wave current having a fixed amplitude (set current Is) as a rectangular wave Signal (SG) at a preset signal frequency (fg), applies the rectangular wave signal to electrodes (T1, T2) attached to a measurement tube (3), and a detection circuit (11) samples a detection voltage detected from the electrodes (T1, T2) to detect the amplitude of a detection voltage (Vt).

Description

Conductivity meter

Technical Field

The present invention relates to a conductivity measurement technique for measuring the conductivity of a liquid.

Background

As a device for measuring the conductivity (electric conductivity) of a liquid, a two-electrode conductivity meter is known. A two-electrode conductivity meter is a measuring instrument that applies an alternating current signal such as a sine wave or a square wave between 2 electrodes and detects an electric signal generated between the electrodes to determine the conductivity of a liquid. Conventional techniques of a two-electrode conductivity meter are disclosed in patent documents 1 to 3.

For example, patent document 1 discloses a two-electrode conductivity meter that measures conductivity from the resistance of a liquid to be measured by detecting a current flowing to one electrode when an ac voltage is applied to the other electrode in a state where 2 electrodes are immersed in the liquid to be measured.

Patent documents

2 and 3 disclose a two-electrode conductivity meter in which 2 electrodes are formed in a rod shape.

Documents of the prior art

Patent document

Patent document 1: japanese patent publication No. Hei 7-15490

Patent document 2: japanese patent laid-open No. 2005-148007

Patent document 3: japanese patent laid-open No. 2002-296312

Disclosure of Invention

[ problem to be solved by the invention ]

In such a two-electrode conductivity meter, it is necessary to connect 2 electrodes to a circuit for deriving conductivity by using a pair of lines, but the length of these lines may increase the impedance of the lines and may not be ignored in the measurement of conductivity. Therefore, there is a problem that the signal waveform detected by the electrode is distorted due to the influence of the line impedance, and the measurement accuracy of the electrical conductivity is lowered.

This is studied in patent document 1. Fig. 17 is a circuit diagram showing a signal processing circuit of a conventional conductivity meter. Fig. 18 is an equivalent circuit associated with the electrodes and cables of fig. 17.

In the signal processing circuit 50 shown in fig. 17, the applied voltage Vg of the ac rectangular wave generated by the signal generation circuit 51 is stabilized to the applied voltage Vg' by the buffer amplifier U1, and then applied to the electrode T1 via the terminal N1 and the electrode line LT 1. The detection current It generated at the electrode T2 is input to the operational amplifier U2 and the feedback resistor Rf via the electrode line LT2 and the terminal N2, converted into a detection voltage Vt, rectified by the synchronous rectifier circuit 52, and converted into a dc voltage Et.

In the equivalent circuit shown in fig. 18, Cp and Rp are polarization capacitance and polarization resistance generated between the electrodes T1 and T2 and the liquid when the electrodes are in contact with the liquid, and Rl is liquid resistance associated with the liquid between the electrodes T1 and T2. Cw is a line-to-line capacitance generated between the electrode lines LT1 and LT2, and Rw is a line resistance of LT1 and LT 2.

As shown in fig. 18, when the electrodes T1 and T2 are viewed from the signal processing circuit 50 side through the terminals N1 and N2, a series circuit of the line capacitance Cw and the line resistance Rw is parallel to a series circuit of a parallel circuit of the polarization capacitance Cp and the polarization resistance Rp and the liquid resistance Rl.

In patent document 1, at a timing when the influence of the distortion of the detected voltage Vt by Cp and Cw is small, for example, in a period after a differentiation noise generated immediately after the start of the output of the applied voltage Vg 'and before the stop of the output of the applied voltage Vg', Vt is sampled a plurality of times, and Vt which is not influenced by Cp and Cw is obtained from the obtained plurality of sample voltages by the calculation formula.

However, since the above calculation formula assumes that the line-to-line capacitance Cw can be ignored, there is a problem that the conductivity cannot be measured with high accuracy when the electrode lines LT1 and LT2 are long and Cw cannot be ignored.

In general, to suppress the influence of the polarization capacitance Cp, the frequency of the applied voltage Vg applied between T1 and T2 is increased. However, when the frequency of the applied voltage Vg is increased, the influence of the line-to-line capacitance Cw increases, and the distortion of the detection voltage Vt obtained from T1 and T2 increases, thereby causing a decrease in the measurement accuracy of the conductivity. Further, the longer the electrode lines LT1, LT2 are, the larger Cw becomes and the larger distortion of the detection voltage Vt becomes, resulting in a decrease in the measurement accuracy of the conductivity.

The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a conductivity measurement technique capable of measuring conductivity with high accuracy while suppressing the influence of line-to-line capacitance of electrode lines connected to electrodes.

[ MEANS FOR SOLVING PROBLEMS ] to solve the problems

To achieve the above object, the conductivity meter of the present invention measures a conductivity associated with a liquid in a measurement tube, the conductivity meter including: a signal generation circuit that generates an alternating rectangular wave current having a fixed amplitude as a rectangular wave signal at a predetermined signal frequency; a 1 st electrode and a 2 nd electrode which are mounted on the measuring tube and apply the rectangular wave signal to the liquid; a detection circuit that detects the amplitude of the detection voltage by sampling the detection voltage detected from the 1 st electrode and the 2 nd electrode; and an arithmetic processing circuit for calculating the conductivity of the liquid by arithmetic processing based on the amplitude.

In one embodiment of the conductivity meter according to the present invention, the conductivity meter further includes a printed circuit board disposed in the vicinity of the 1 st electrode and the 2 nd electrode, and on which the signal generating circuit is mounted.

In one configuration example of the conductivity meter according to the present invention, the conductivity meter further includes a printed circuit board disposed in the vicinity of the 1 st electrode and the 2 nd electrode, and having a buffer amplifier mounted thereon for stabilizing the detection voltage and outputting the stabilized detection voltage to the detection circuit.

In one embodiment of the conductivity meter according to the present invention, the 1 st electrode is a liquid contact electrode that is in contact with the liquid, and the 2 nd electrode is a non-liquid contact electrode that is formed on an outer peripheral portion of the measuring tube and is not in contact with the liquid.

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

In one configuration example of the conductivity meter according to the present invention, the signal generating circuit includes: a current detection circuit that detects a magnitude of the rectangular wave signal; and an operational amplifier for maintaining the amplitude of the rectangular wave signal at a set current based on a clock signal indicating the signal frequency and a detection result from the current detection circuit.

[ Effect of the invention ]

According to the present invention, even if the signal frequency is relatively low, the inclination of the detection voltage is linear, and the amplitude of the detection voltage can be detected while being stabilized. Therefore, a frequency of a degree that can suppress the influence of the line-to-line capacitance of the electrode line connecting the electrodes can be used as the signal frequency, and the conductivity can be measured with high accuracy.

Drawings

Fig. 1 is a block diagram showing a circuit configuration of a conductivity meter according to embodiment 1.

Fig. 2 is a side view of the conductivity meter of embodiment 1.

Fig. 3 is a plan view of the conductivity meter according to embodiment 1.

Fig. 4 is a perspective view of the conductivity meter according to embodiment 1.

Fig. 5 is another perspective view of the conductivity meter according to embodiment 1.

Fig. 6 is a front view showing the sub-board.

Fig. 7 is a rear view showing the sub-board.

Fig. 8 is a signal waveform diagram showing the operation of the conductivity meter according to embodiment 1.

Fig. 9 shows an example of the configuration of the rectangular wave current source.

Fig. 10 is an equivalent circuit of the electrode side of embodiment 1.

Fig. 11 is a signal waveform diagram showing the operation of the conductivity meter using a rectangular wave constant voltage signal.

Fig. 12 is a characteristic diagram showing a correspondence relationship between amplitude data and conductivity.

Fig. 13 is a side view of the conductivity meter according to embodiment 2.

Fig. 14 is a plan view of the conductivity meter according to embodiment 2.

Fig. 15 is a perspective view of the conductivity meter according to embodiment 2.

Fig. 16 is another perspective view of the conductivity meter according to embodiment 2.

Fig. 17 is a circuit diagram showing a signal processing circuit of a conventional conductivity meter.

Fig. 18 is an equivalent circuit associated with the electrodes and cables of fig. 17.

Detailed Description

Next, an embodiment of the present invention will be described with reference to the drawings.

[ 1 st embodiment ]

First, a conductivity meter 10 according to embodiment 1 of the present invention will be described with reference to fig. 1 to 5. Fig. 1 is a block diagram showing a circuit configuration of a conductivity meter according to embodiment 1. Fig. 2 is a side view of the conductivity meter of embodiment 1. Fig. 3 is a plan view of the conductivity meter according to embodiment 1. Fig. 4 is a perspective view of the conductivity meter according to embodiment 1. Fig. 5 is another perspective view of the conductivity meter according to embodiment 1.

As shown in fig. 1 to 5, the conductivity meter 10 of the present invention has the following functions: an alternating rectangular wave signal SG is applied between 2 electrodes T1 and T2 attached to the measurement tube 3, and the electrical conductivity of the liquid in the measurement tube 3 is determined from the amplitude of the detection voltage Vt detected between T1 and T2.

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

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

The detection circuit 11 has a function of applying an alternating rectangular wave current having a fixed amplitude (set current Is) as a rectangular wave signal SG, that Is, a rectangular wave constant current signal, to the electrodes T1 and T2 at a predetermined signal frequency fg by the control signal generation circuit 21, and a function of detecting the amplitude of the detection voltage Vt generated in the electrodes T1 and T2 and outputting the amplitude to the arithmetic processing circuit 12.

In the modification of the conductivity meter 10 of the present invention, a non-liquid contact electrode that does not contact liquid may be used as the electrode T2, and a liquid contact electrode that contacts liquid may be used as the electrode T2. In the present embodiment, a case where the electrode T2 is a non-liquid contact electrode will be described as an example. The case where the counter electrode T2 is a liquid contact electrode will be described later in embodiment 2.

The detection circuit 11 includes a clock generation circuit 11A, a sample-and-hold circuit (SH circuit) 11B, and an a/D conversion circuit (ADC circuit) 11C as main circuit units.

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 from the clock signal CLK0 from the arithmetic processing circuit 12.

The sample hold circuit 11B has a function of sample-holding 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, and outputting the obtained detection voltages VH and VL to the a/D conversion circuit 11C.

The a/D converter circuit 11C has a function of a/D converting an amplitude voltage of Vt, which is a differential voltage between VH and VL from the sample hold circuit 11B, and outputting the obtained amplitude data DA to the arithmetic processing circuit 12.

The connector CN1 attached to the main board 1 is connected to the connector CN2 attached to the sub board 2 via a 4-core connection line LC. Thereby, the main board 1 and the sub board 2 are electrically connected together. Specifically, the clock signal CLKs is supplied from the terminal T11 of CN1 to the terminal T21 of CN2 via LC. Further, the reference voltage Vs is supplied from the terminal T12 of CN1 to the terminal T22 of CN2 via LC. Further, the ground voltage GND is supplied from the terminal T13 of CN1 to the terminal T23 of CN2 via LC. Further, 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.

In addition, the sub-substrate 2 is electrically connected to the 1 st electrode T1 and the 2 nd electrode T2 via jumpers J1 and J2. Specifically, the pad (electrode connection terminal) P1 formed on the sub-substrate 2 is connected to the 1 st electrode T1 via the jumper J1, and the pad (electrode connection terminal) P2 formed on the sub-substrate 2 is connected to the 2 nd electrode T2 via the jumper J2. P1 is connected to the ground voltage GND on the sub board 2 via the wiring pattern LP1 formed on the sub board 2, and P2 is connected to the signal generating circuit 21 and the buffer amplifier 22 on the sub board 2 via the wiring pattern LP2 formed on the sub board 2.

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

The buffer amplifier 22 is composed of, for example, an operational amplifier and a buffer circuit, and has a function of stabilizing the detection voltage Vt detected from the electrodes T1 and T2 and outputting the stabilized detection voltage Vt 'as the 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 electrical conductivity related to the liquid in the measurement tube 3 by arithmetic processing based on the amplitude data DA obtained by the detection circuit 11 by causing the CPU to cooperate with a program. The arithmetic processing circuit 12 includes a conductivity calculating unit 12A and an empty state determining unit 12B as main processing units.

The conductivity calculating unit 12A has a function of calculating the conductivity of the liquid in the measuring tube 3 based on the amplitude data DA obtained by the detection circuit 11. Specifically, the conductivity corresponding to the amplitude data DA from the detection circuit 11 may be calculated using a preset conductivity calculation formula, but the conductivity related to the liquid in the measurement pipe 3 may be derived by previously measuring the correspondence between the amplitude data DA and the conductivity, setting the obtained characteristics as a lookup table, and referring to the lookup table based on the amplitude data DA from the detection circuit 11.

The empty state determination unit 12B has a function of determining the presence or absence of the liquid in the measurement tube 3 based on the electrical conductivity calculated by the electrical conductivity calculation unit 12A. Specifically, the empty state determination unit 12B compares the electrical conductivity calculated by the electrical conductivity calculation unit 12A with a predetermined threshold electrical conductivity, and determines that the liquid is not present in the measurement tube 3, that is, the empty state, when the calculated electrical conductivity is smaller than the threshold electrical conductivity.

The setting and display circuit 13 includes a display device such as an operation button, an LED, and an LCD, and has a function of detecting a setting operation input by an operator and outputting the input to the arithmetic processing circuit 12, and a function of displaying various data from the arithmetic processing circuit 12.

The transmission circuit 14 has a function of transmitting data to and from a higher-level device (not shown) such as a controller via a transmission line LT, and a function of transmitting the conductivity and empty state determination result obtained by the arithmetic processing circuit 12 to the higher-level device.

[ Structure of conductivity meter ]

Next, the structure of the conductivity meter 10 according to the present embodiment will be described with reference to fig. 2 to 5. For convenience, hereinafter, the direction in which the measurement tube 3 extends is referred to as a 1 st direction X, the left-right direction of the measurement tube 3 perpendicular to the 1 st direction X is referred to as a 2 nd direction Y, and the up-down direction of the measurement tube 3 perpendicular to the 1 st direction X and the 2 nd direction Y is referred to as a 3 rd direction Z.

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

Tubular joints

5A and 5B are disposed on a pair of side surfaces 4A orthogonal to the 1 st direction X among the side surfaces of the lower case 4, and the

joints

5A and 5B are made of a metal material (e.g., SUS) so as to connect a pipe (not shown) provided outside the conductivity meter 10 and the measurement tube 3. The measurement tube 3 is housed inside the lower case 4 in the 1 st direction X, and a joint 5A and a joint 5B are connected to both ends of the measurement tube 3 via a pair of O-rings OR.

Here, at least one of the

tabs

5A and 5B functions as an electrode (1 st electrode) T1. For example, the joint 5A is connected to the ground voltage GND (common potential), thereby connecting the external pipe to the measurement tube 3 and functioning as the electrode T1.

By implementing the electrode T1 with the tab 5A made of metal in this manner, the area of contact of the T1 with the liquid increases.

Thus, even when the adhesion and corrosion of foreign matter have occurred at T1, the area of the portion where the adhesion and corrosion of foreign matter have occurred is relatively small with respect to the total area of T1, and therefore, the measurement error due to the change in polarization capacitance can be suppressed. Further, since ground voltage GND is applied to connector 5A, even if the external pipe connected to connector 5A is made of metal, the external pipe does not act as an antenna and radiates electromagnetic wave noise. Further, since the tab 5A also serves as the electrode T1, it is not necessary to separately provide the T1, and the conductivity meter 10 can be miniaturized.

On the other hand, a shield 6 made of a metal plate having a Contraband-shaped cross section is attached to the outer surface of the pair of side surfaces 4B orthogonal to the 2 nd direction Y among the side surfaces of the lower case 4 and the bottom surface 4E of the lower case 4. This can reduce noise emitted from the conductivity meter 10 to the outside.

Further, a surface electrode (2 nd electrode) T2 made of a thin film conductor is patterned as a non-liquid contact electrode on the outer peripheral surface 3A of the measurement tube 3 on the side opposite to the joint 5A via the sub-substrate 2 so as to extend over the entire circumference of the measurement tube 3. Further, at T2, a land P3 is formed so that the side end portion on the sub board 2 side protrudes toward the sub board 2.

As described above, at least one of the signal generating circuit 21 and the buffer amplifier 22 is mounted on the sub-board (printed wiring board) 2 attached to 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 the jumpers J1 and J2.

Fig. 6 is a front view showing the sub-board. Fig. 7 is a rear view showing the sub-board.

As shown in fig. 6, a land P1 is patterned in the 2 nd direction Y on the substrate surface 2A on the electrode T1 side of the sub-substrate 2 constituted by the tab 5A in the lateral direction of the tube hole 2H, and the P1 and T1 are connected together via J1. J1 was welded to the outer surface of P1 and T1.

Further, as shown in fig. 7, a land P2 is patterned in the 3 rd direction Z on the substrate surface 2B on the electrode T2 side in the sub substrate 2 at a position above the tube hole 2H, and the P2 and the P3 are connected together via J2. J2 was welded to P2 and P3.

Further, on the upper side of the tube hole 2H including the pad P2 in the substrate surface 2B, a circuit mounting region 2G is provided to which the signal generation circuit 21, the buffer amplifier 22, and the connector CN2 are mounted, and P1 and P2 are connected via the wiring patterns LP1 and LP2 (not shown).

This can extremely shorten the lengths of the jumper wires J1 and J2, which are electrode lines connecting the sub-board 2 and the electrodes T1 and T2, and can suppress the impedances of J1 and J2 to a very low level. Further, since the signal generating circuit 21 or the buffer amplifier 22 is mounted on the sub board 2, the impedance of the connection line LC connecting the main board 1 and the sub board 2 can be suppressed to be low. Therefore, the impedances of the jumpers J1, J2 and also of the connection lines LC can be neglected in the measurement of the conductivity.

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 board 1 is fixed in the upper case 9, and various circuit portions such as a detection circuit 11, an arithmetic processing circuit 12, a setting and display circuit 13, and a transmission circuit 14 are mounted thereon. The connector CN2 of the sub board 2 is connected to the connector CN1 of the main board 1 via the connection line LC.

In the sub-board 2, a ground pattern connected to the ground voltage GND may be formed in a region other than the circuit components and the circuit pattern. This reduces noise mixed into the electrode T2 from outside the conductivity meter 10, thereby suppressing measurement errors.

The sub-substrate 2 can be attached to the outer peripheral surface 3A of the measurement tube 3 in any direction, and in the present embodiment, the measurement tube 3 is pressed into the tube hole 2H provided in the sub-substrate 2, whereby the sub-substrate 2 is fixed to the measurement tube 3.

As shown in fig. 6 and 7, a tube hole 2H into which the measurement tube 3 is pressed is formed in the sub-board 2 at a center position in the left-right direction Y, which is a left-right direction toward the paper surface. This makes it possible to fix the sub-board 2 to the measurement tube 3 with an extremely simple configuration without using a fixing member such as a mounting screw.

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. In this case, the pipe hole 2H may be formed in a substantially polygonal shape, or in a substantially octagonal shape in fig. 6 and 7, without being formed in a perfect circle shape in accordance with the outer peripheral shape of the measurement pipe 3. Thus, the end of the tube hole 2H is partially in contact with the outer peripheral surface 3A, and the influence of heat transmitted from the measurement tube 3 to the signal generation circuit 21 and the buffer amplifier 22 attached to the sub-substrate 2 can be suppressed compared to a configuration in which the entire circumference of the end of the tube hole 2H is in contact with the outer peripheral surface 3A.

Further, since the gap 2S is discretely formed between the pipe hole 2H and the measurement pipe 3, the measurement pipe 3 can be easily press-fitted into the pipe hole 2H without preparing a jig dedicated for press-fitting, and the work load can be reduced.

The shape of the pipe hole 2H is not limited to a substantially polygonal shape, and a plurality of convex portions may be provided on the hole wall surface of the pipe hole 2H, and the convex portions may abut against the outer peripheral surface 3A. Alternatively, a notch or a slit may be provided in which a part of the peripheral portion of the tube hole 2H is directly opened to the side end portion of the sub-board 2. Thereby, the same effects as described above can be obtained.

In the present embodiment, a pair of

guide portions

7X and 7Y each formed of a convex or concave rail is formed on the inner wall portion 4C of the side surface 4B of the lower case 4. The measurement tube 3 is mounted on the lower case 4 via the sub board 2 by inserting the sub board 2 from the opening 4D of the lower case 4 so that the

side end portions

2X and 2Y of the sub board 2 are fitted to the

guide portions

7X and 7Y. This allows the sub-board 2 and the measuring tube 3 to be mounted inside the lower case 4 with a very simple configuration.

The

guide portions

7X and 7Y do not need to be formed so as to extend in the convex portions or the concave portions, and the convex portions or the concave portions may be formed so as to be separated into a plurality of portions at intervals into which the

side end portions

2X and 2Y are smoothly inserted. In fig. 3, the case where the

guide portions

7X and 7Y are formed of 2 ribs is shown as an example, but grooves into which the

side end portions

2X and 2Y are inserted may be used instead of the ribs.

Further, the sub board 2 does not need to be fixed by the

guide portions

7X and 7Y, and on the contrary, in the case of some play, the mechanical stress applied to the measurement tube 3 or the sub board 2 can be relaxed at the time of screwing the

joints

5A and 5B.

[ operation of embodiment 1 ]

Next, the operation of the conductivity meter 10 according to the present embodiment will be described with reference to fig. 8. Fig. 8 is a signal waveform diagram showing the operation of the conductivity meter according to embodiment 1.

Here, a case where the electrode T2 is a non-liquid contact electrode and the rectangular wave signal SG is a rectangular wave constant voltage signal will be described as an example.

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 from the clock signal CLK0 from the arithmetic processing circuit 12. Here, the frequency of CLKs, that is, the signal frequency fg of the rectangular wave signal SG is shown as 150 kHz.

The signal generation circuit 21 controls on/off of the rectangular wave current source IG based on CLKs. Thus, 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 applied to the electrode T2. Therefore, the voltage generated by the liquid resistance of the liquid between the electrodes T1 and T2 by the applied current Ig supplied from the signal generating circuit 21 becomes the voltage between the electrodes T1 and T2, that is, the detection voltage Vt.

The sample hold circuit 11B samples the detection voltage VH in the high level period TH (half period of SG) during which Is supplied, among the output voltages Vt' obtained by stabilizing (impedance converting) Vt in the buffer amplifier 22, based on CLKh from the clock generation circuit 11A. Further, the sample-and-hold circuit 11B samples the detection voltage VL in the low level period TL (half period of SG) in which zero is supplied among Vt', based on CLKl from the clock generation circuit 11A.

The a/D conversion circuit 11C converts the difference voltage Δ Vt a/D between VH and VL obtained by the sample hold circuit 11B into amplitude data DA and outputs the amplitude data DA.

A method of full-wave rectifying the ac detection voltage Vt is generally considered, and for example, a method of folding back the detection voltage Vt in TL at an intermediate level of Vt and adding the resultant to Vt of TH. However, in this method, if TL and TH are not equal to Vt, a ripple current remains even if full-wave rectification is performed, and a stable dc voltage is not obtained, which causes a measurement error.

According to the present embodiment, the detection voltage Vt of the alternating current is not full-wave rectified, and sampling is performed individually in TL and TH, and the obtained difference voltage between VH and VL is acquired as the amplitude data DA. Therefore, even when the Vt includes a fluctuation due to a change in the flow rate of the liquid or when common mode noise is mixed into the Vt from the outside via the liquid, the influence on the amplitude data DA can be avoided, and stable measurement of the conductivity can be achieved.

The conductivity calculating unit 12A calculates the conductivity of the liquid from the DA from the a/D conversion circuit 11C.

The empty state determination unit 12B determines whether or not the inside of the measurement tube 3 is empty by comparing the electrical conductivity obtained by the electrical conductivity calculation unit 12A with a threshold electrical conductivity.

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

Fig. 10 is an equivalent circuit of the electrode side of embodiment 1. As described above, in the present 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 a rectangular wave current source IG in which the equivalent circuit Zt on the side showing the impedance between the electrodes T1 and T2 is connected to the signal generating circuit 21.

At this time, in Zt, when the electrodes T1 and T2 come into contact with the liquid, a polarization capacitance Cp and a polarization resistance Rp are generated between the electrodes and the liquid, and since T2 is a non-liquid contact electrode, an electrode capacitance Ct is generated between the liquid and the electrode T2. Therefore, let Rl be the liquid resistance relating to the liquid between the electrodes T1 and T2, Zt is represented by a parallel circuit of the polarization capacitance Cp and the polarization resistance Rp, and an equivalent circuit in which the liquid resistance Rl and the electrode capacitance Ct are connected in series. Here, when the signal frequency of the rectangular wave signal SG is fg equal to 150kHz, the impedance of Cp is relatively small, but the impedance of Ct is increased to some extent, and therefore the voltage Vct and Vt between both ends of Ct change transiently.

Fig. 11 is a signal waveform diagram showing the operation of the conductivity meter using a rectangular wave constant voltage signal. When fg is 150kHz as in fig. 8, the impedance of Cp is relatively small, but the impedance of Ct is increased to some extent. Therefore, when an ac rectangular wave voltage having a fixed amplitude (set voltage Vs) is used as the rectangular wave signal SG, that is, the rectangular wave constant voltage signal, Vct, Vrl, and Vt change exponentially with their respective time constants, and VH and VL cannot be detected while being stabilized.

When the waveform of Vt is distorted in this way, errors are likely to be included in the detection of the amplitude data DA, and as a result, the measurement accuracy relating to the conductivity is lowered. Therefore, a higher frequency to a negligible degree of the impedances of Cp and Ct is used as fg. On the other hand, if fg is increased, the influence of the line-to-line capacitance Cw of the electrode lines increases as in the conventional equivalent circuit shown in fig. 18, and signal leakage occurs in the electrode lines, resulting in distortion of the waveform of Vt.

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

When VH Is defined as the detection voltage Vt detected in the high level period TH during which the applied current Ig Is the set current Is, and Vrl and Vct at this time are VrlH and VctH, VH Is VrlH + VctH. When VL is a detection voltage Vt detected in the low level period TL in which Ig is 0 and Vrl and Vct at this time are VrlL and VctL, VL is VrlL + VctL.

At this time, although Vct is included in detected VH and VL, since CLKh and CLKl indicate the central positions of TH and TL (half period of SG), VctH and VctL included in the sampled VH and VL are equal to each other. Thus, by taking the differential voltage Δ Vt between VH and VL, VctH and VctL cancel each other out, and amplitude data DA not including Vct can be obtained.

That is, Δ Vt — VL — VrlH-VrlL. Since Ig is fixed, Rl can be obtained by the following formula (1).

[ formula 1 ]

In the formula (1), Ig is known, and the differential voltage VH-VL is detected by the SH circuit 11B, converted into amplitude data DA by the a/D conversion circuit 11C, and input to the arithmetic processing circuit 12. Therefore, the conductivity calculating unit 12A can easily calculate Rl from these data.

Fig. 12 is a characteristic diagram showing a correspondence relationship between amplitude data and conductivity, and the vertical axis represents amplitude data DA and the horizontal axis represents conductivity. The correlation between the amplitude data DA and the conductivity may be measured in advance by performing calibration using a plurality of types of standard fluids having known conductivities, the obtained characteristics may be set in a lookup table in, for example, a semiconductor memory (not shown), and the conductivity calculating unit 12A may refer to the lookup table based on the amplitude data DA from the detection circuit 11 to derive the conductivity related to the liquid in the measurement pipe 3.

[ Effect of embodiment 1 ]

In this way, in the present embodiment, the signal generating circuit 21 generates an alternating rectangular wave current having a fixed amplitude as a rectangular wave signal SG at a preset signal frequency fg, and applies the rectangular wave signal SG to the electrodes T1 and T2 attached to the measuring tube 3, and the detection circuit 11 samples the detection voltage Vt detected from these T1 and T2, thereby detecting the amplitude of the detection voltage Vt.

Thus, even if fg is relatively low, the inclination of the detection voltage Vt is linear, and the amplitude of the detection voltage Vt can be detected with stability. Therefore, a frequency of a degree that can suppress the influence of the line-to-line capacitance of the electrode lines connected to T1 and T2 can be used as fg, and the conductivity can be measured with high accuracy.

In the present embodiment, a liquid-contact electrode that is in contact with the liquid may be used as T1, and a liquid-non-contact electrode that is formed on the outer peripheral portion of the measurement tube 3 and does not come into contact with the liquid may be used as T2.

This can suppress the adhesion of dirt on the electrode surface and the occurrence of measurement errors due to corrosion of the electrode. Further, it is not necessary to use an expensive liquid contact electrode such as platinum black, and a significant cost reduction is achieved. In addition, in the case of using the non-liquid contact electrode, although the electrode capacitance Ct is generated between the electrode and the liquid, since the rectangular wave constant current signal is used as the rectangular wave signal SG, the amplitude of the detection voltage Vt can be detected while being stabilized.

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.

Thus, even when the non-contact electrode is used as T2, the voltage VctH across the electrode capacitance Ct of T2 included in VH sampled in the high-level period TH is equal to the voltage VctL across the electrode capacitance Ct included in VL sampled in the low-level period TL. Therefore, by taking the differential voltage Δ Vt between VH and VL, VctH and VctL cancel each other out, and amplitude data DA without Vct can be obtained. Therefore, the conductivity can be measured with high accuracy.

In the present embodiment, the rectangular-wave current source IG of the signal generation circuit 21 may be configured by a current detection circuit DET that detects the magnitude of the applied current IG as the rectangular-wave signal SG, and an operational amplifier Ug that maintains the amplitude of the applied current IG at the set current Is based on the clock signal CLKs indicating the signal frequency fg and the detection result from the current detection circuit DET. Thus, a stable Ig with high accuracy can be generated with a relatively simple configuration.

In the present embodiment, the sub-substrate 2 may be disposed in a position near the electrodes T1 and T2 mounted on the measurement tube 3, and at least one or both of the signal generation circuit 21 that generates the rectangular wave signal SG and the buffer amplifier 22 that stabilizes and outputs the detection voltage Vt detected by the electrodes T1 and T2 may be mounted on the sub-substrate 2.

This can significantly shorten the lengths of the electrode lines, i.e., the jumpers J1 and J2, connecting the signal generating circuit 21, the buffer amplifier 22, and the electrodes T1 and T2, and can reduce the line-to-line capacitance between the electrode lines. Therefore, even if a relatively high signal frequency is used, the conductivity can be measured with high accuracy.

In the present embodiment, a tube hole 2H into which the measurement tube 3 is inserted may be provided in the sub-substrate 2, and the sub-substrate 2 may be attached to the outer peripheral surface 3A of the measurement tube 3 by the tube hole 2H coming into contact with the outer peripheral surface 3A.

This makes it possible to fix the sub-board 2 to the measurement tube 3 with an extremely simple configuration without using a fixing member such as a mounting screw.

With this configuration, the sub-substrate 2 can be disposed between the electrodes T1 and T2 so as to be orthogonal to the longitudinal direction of the measurement tube 3. Therefore, the electrode lines from the sub-substrate 2 to the electrodes T1 and T2, that is, the jumpers J1 and J2, can be arranged and connected in different positions and directions, and the line-to-line capacitance between the electrode lines can be extremely reduced. In addition, in the case where a metal pipe is connected to the joint 5A as the electrode T1, there is a possibility that an applied current to the liquid is transmitted to the metal pipe to cause a measurement error, and with the above configuration, T2 can be easily arranged so as to be spaced apart from T1 to some extent. Therefore, the conductivity can be measured with high accuracy while suppressing the transfer of an applied current to the metal pipe.

In the present embodiment, a land (electrode connection terminal) for connecting electrode lines to the electrodes T1 and T2 and a line pattern for connecting the land and at least one or both of the signal generation circuit 21 and the buffer amplifier 22 may be formed on the pattern surface of the sub board 2.

Thus, the signal generation circuit 21, the buffer amplifier 22, and the electrodes T1 and T2 mounted on the sub-substrate 2 can be connected very easily by the jumpers J1 and J2 without using a connector.

[ 2 nd embodiment ]

Next, the conductivity meter 10 according to embodiment 2 of the present invention will be described with reference to fig. 13 to 16. Fig. 13 is a side view of the conductivity meter according to embodiment 2. Fig. 14 is a plan view of the conductivity meter according to embodiment 2. Fig. 15 is a perspective view of the conductivity meter according to embodiment 2. Fig. 16 is another perspective view of the conductivity meter according to embodiment 2.

In embodiment 1, a case where a non-liquid contact electrode that does not contact liquid is used as the electrode T2 is described as an example. In the present embodiment, a case where a liquid contact electrode that contacts a liquid is used as the electrode T2 will be described.

[ Structure of conductivity meter ]

Next, the structure of the conductivity meter 10 according to the present embodiment will be described with reference to fig. 13 to 16. For convenience, hereinafter, the direction in which the measurement tube 3 extends is referred to as a 1 st direction X, the left-right direction of the measurement tube 3 perpendicular to the 1 st direction X is referred to as a 2 nd direction Y, and the up-down direction of the measurement tube 3 perpendicular to the 1 st direction X and the 2 nd direction Y is referred to as a 3 rd direction Z.

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

Tubular joints

joints

5A and 5B are made of a metal material (e.g., SUS) so as to connect a pipe (not shown) provided outside the conductivity meter 10 and the measurement tube 3. At this time, the measurement tube 3 is housed inside the lower case 4 along the longitudinal direction X, and the joint 5A and the joint 5B are connected to both ends of the measurement tube 3 via a pair of O-rings OR, respectively.

Here, at least one of the

tabs

By realizing the electrode T1 with the tab 5A made of metal in this manner, the area of contact of T1 with the liquid increases. Thus, even when the adhesion and corrosion of foreign matter occur at T1, the area of the portion where the adhesion and corrosion of foreign matter occur is relatively small with respect to the total area of T1, and therefore, the measurement error due to the change in polarization capacitance can be suppressed.

A liquid contact electrode (2 nd electrode) T2 made of a metal rod is attached to the outer peripheral surface 3A of the measurement tube 3 on the side opposite to the joint 5A via the sub board 2 so as to penetrate the wall of the measurement tube 3 and protrude into the measurement tube 3. The portion protruding into the measuring tube 3 is in contact with the liquid in the measuring tube 3.

As described above, at least one of the signal generating circuit 21 and the buffer amplifier 22 is mounted on the sub-board 2 attached to the outer peripheral surface 3A of the measurement 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 the jumpers J1 and J2. Specifically, J1 was welded to the outer surfaces of P1 and T1, and J2 was welded to P2 and T2.

[ operation of embodiment 2 ]

Next, the operation of the conductivity meter 10 of the present embodiment will be described.

When the electrode T2 is changed from the non-liquid contact electrode to the liquid contact electrode, the electrode capacitance Ct between T2 and the liquid in the case of the non-liquid contact electrode disappears. Therefore, the equivalent circuit Zt shown in fig. 10 is an equivalent circuit in which a parallel circuit of the polarization capacitor Cp and the polarization resistor Rp is connected in series with the liquid resistor Rl. The operation of measuring the electrical conductivity is the same as that of embodiment 1 except for this embodiment, and the detailed description thereof will be omitted.

[ Effect of embodiment 2 ]

In this manner, in the present embodiment, the electrodes T1 and T2 are constituted by liquid contact electrodes that are in contact with the liquid. This eliminates the influence of the capacitance Ct generated between the liquid and the electrode T2, which is unique to the case where the non-liquid contact electrode is used as T2, and thus a relatively low frequency can be used as the signal frequency of the square wave signal SG. Therefore, the influence of the line-to-line capacitance of the electrode lines, i.e., the jumper wires J1, J2 can be extremely reduced, so that the conductivity can be measured with extremely high accuracy.

[ expansion of embodiment ]

The present invention has been described above with reference to the embodiments, but the present invention is not limited to the above embodiments. Various modifications which can be understood by those skilled in the art can be made in the constitution and details of the present invention within the scope of the present invention. In addition, the embodiments can be arbitrarily combined and implemented within a range where contradictions do not occur.

Description of the symbols

10 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 measuring tube, 3A outer peripheral surface, 4 lower side case, 4A, 4B side surface, 4C inner wall portion, 4D opening portion, 4E bottom surface, 5A, 5B joint, 6 shield, 7X, 7Y leading portion, 9 upper side case, 11 detecting circuit, 11A clock generating circuit, 11B sample holding circuit (SH circuit), 11C A/D converting circuit (ADC circuit), 12 operation processing circuit, 13 setting and display circuit, 14 transmission circuit, 21 signal generating circuit, 22 buffer amplifier, IG rectangular wave current source, T electrode, P pad, j1, J2 … jumpers, LC … connection lines, CN1, CN2 … connectors, LP1, LP2 … line patterns, SWg, SWh, SWl, SWi … switches, CLK0, CLKs, CLKh, CLKl … clock signals, Vs … reference voltage, GND … ground voltage, SG … rectangular wave signals, Vg … applied voltage, Ig … applied current, Vt, VH, VL … detection voltages, Vt' … output voltage, DA … amplitude data.

Claims

1. A conductivity meter for measuring conductivity of a liquid in a measurement pipe, the conductivity meter comprising:

a signal generation circuit that generates an alternating rectangular wave current having a fixed amplitude as a rectangular wave signal at a predetermined signal frequency;

a 1 st electrode and a 2 nd electrode which are mounted on the measuring tube and apply the rectangular wave signal to the liquid;

a detection circuit that detects the amplitude of the detection voltage by sampling the detection voltage detected from the 1 st electrode and the 2 nd electrode; and

an arithmetic processing circuit for calculating the conductivity of the liquid by arithmetic processing based on the amplitude,

the 1 st electrode is made of a tubular metal connected to the measuring tube and having an inner portion in contact with the liquid,

the 2 nd electrode is a non-contact electrode formed on an outer peripheral portion of the measurement tube so as not to contact the liquid.

2. The conductivity meter according to claim 1,

the device further comprises a printed wiring board which is disposed in the vicinity of the 1 st electrode and the 2 nd electrode and on which the signal generating circuit is mounted.

3. The conductivity meter according to claim 1,

the detection circuit further includes a printed wiring board which is disposed in the vicinity of the 1 st electrode and the 2 nd electrode and which is provided with a buffer amplifier for stabilizing the detection voltage and outputting the stabilized detection voltage to the detection circuit.

4. The conductivity meter according to any one of claims 1 to 3,

the detection circuit samples the detection voltage at a central time position of a half cycle of the rectangular wave signal.

5. The conductivity meter according to any one of claims 1 to 3,

the signal generation circuit includes:

a current detection circuit that detects a magnitude of the rectangular wave signal; and

and an operational amplifier for maintaining the amplitude of the rectangular wave signal at a set current based on a clock signal indicating the signal frequency and a detection result from the current detection circuit.