JPH09281164A - Apparatus for measuring electric conductivity - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an apparatus for measuring electric conductivity with a small number of circuits and with high measuring accuracy. SOLUTION: An oscillator is formed from a material to be measured in contact with electrodes 1a, 1b, a capacitor C1 connected in series with the material, and comparator sets Q1, Q2, Q3, Q4 for charging the capacitor C1 via the material and having hysteresis to be input with the terminal voltage of the capacitor C1. Since the frequency of the oscillator is proportional to the electric conductivity of the material, the oscillating frequency is measured to measure the conductivity. The number of circuits is small to provide a simple constitution, and the manufacturing cost is reduced. A resistor R4 is connected in parallel with the material to facilitate the calibration of the circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric conductivity measuring device, and more specifically, it is preferably used for measuring the purity of various kinds of water such as factory water, tap water, pure water and distilled water. The present invention relates to an electric conductivity measuring device.

[0002]

2. Description of the Related Art Conventionally, the purity of water such as pure water and distilled water is
It is measured by a method of measuring the amount of ions dissolved in the water, that is, a method of measuring the electric conductivity of water.
The method for measuring the degree of purity by the electric conductivity of water is described in Japanese Industrial Standards (JIS0101) as a test method for industrial water.

The electrical conductivity of water such as industrial water is calculated by applying a voltage to a pair of electrodes immersed in water and measuring the current flowing between the electrodes. When a DC voltage is used as the reference voltage applied to the electrodes, a measurement error occurs due to gas or the like due to electrolysis. Therefore, an AC voltage is generally used as the reference voltage.

It is necessary to apply a reference AC voltage having an accurate amplitude between the electrodes, and a stabilizing circuit is used as a voltage source for the reference AC voltage. The signal current flowing between the electrodes is converted into a DC voltage by a rectifying / smoothing circuit (rectifying and smoothing circuit), and then input to an AD converter, and the output thereof is input to a computer to obtain electric conductivity. .

A conventional electrical conductivity measuring device will be described below with reference to the circuit block diagram of FIG. The electrodes 1a and 1b held by the electrode holder 1 are immersed in water, which is an object to be measured, and one electrode 1a is connected to the output of the reference AC voltage generating circuit 2 via a capacitor-C1. . The other electrode 1b is connected to the measured voltage input terminal of the AD converter 5 via the rectifying circuit 3 and the smoothing circuit 4.

A reference voltage from a DC voltage source E1 is applied to the other input terminal of the AD converter 5. The AD converter 5 measures the voltage to be measured with the input reference voltage as a reference, and converts it into a digital signal. The electrical conductivity can be obtained by reading the magnitude of the digital signal with a computer.

[0007]

SUMMARY OF THE INVENTION A conventional electrical conductivity measuring apparatus includes a reference AC voltage generating circuit, a rectifying circuit, a smoothing circuit,
It is composed of many circuits such as AD converter and DC voltage source,
There is a drawback that the cost is high because of the large number of circuits. Further, considering that the number of circuits is large, it is indispensable to employ circuit components with small error and high accuracy in order to keep the measurement error low, and these circuit components further increase the cost. Particularly, a high-precision AD converter having a fine resolution has a drawback that it is expensive and has a large number of output terminals and occupies a large number of input terminals of a computer that receives the output.

The present invention has been made to solve the above problems, and it is an object of the present invention to provide an electric conductivity measuring apparatus which can reduce the number of circuits and thereby reduce the size of the circuits and reduce the cost. To aim.

A further object of the present invention is to improve the accuracy of the above electrical conductivity measuring device after achieving the above object.

[0010]

In order to achieve the above object, the electrical conductivity measuring apparatus of the present invention is in contact with a device to be measured, and a comparator having a hysteresis characteristic at an input voltage threshold and having an inverting output. One pair of electrodes, one of which has the output voltage of the comparator applied to it, and the other end of which is connected to one end of the pair of electrodes to form a series circuit with the DUT and to output the comparator output. A capacitor that is charged via the DUT by means of, and a means for measuring the oscillation cycle or the oscillation frequency of the oscillation circuit by connecting the one end of the capacitor to the input of the comparator. Is characterized by.

In the electrical conductivity measuring apparatus of the present invention, the oscillation frequency of the oscillation circuit is determined by the resistance (or electrical conductivity) of the object to be measured that constitutes the RC oscillation circuit and the known capacitance of the capacitor. By measuring this oscillation period or oscillation frequency, the electrical conductivity of the measured object can be easily obtained based on the known capacitance. Here, in the electrical conductivity measuring apparatus of the present invention, the main circuit can be composed only of a comparator having a hysteresis characteristic, an oscillation period (frequency) measuring means and a DC voltage source, and the circuit configuration is significantly larger than the conventional one. To be simplified. This enables downsizing and cost reduction. Also, to measure the oscillation period etc. with a computer,
For example, since it is sufficient to take out the output of the comparator, the number of input terminals of the computer can be reduced.

The electrical conductivity measuring apparatus of the present invention preferably further comprises a parallel resistor connected between the electrode pairs to form a parallel circuit with the object to be measured. In this case, even if the conductivity of the DUT fluctuates suddenly or the electrode is separated from the DUT in an unstable state, the input voltage of the comparator becomes stable and stable electrical conductivity measurement is possible. Is possible. It is also possible to calibrate this electric conductivity measuring device based on this parallel resistance.

It is also preferable to insert the second capacitor at any position between the output of the comparator and one terminal of the capacitor. In this case, the minute DC leakage current flowing through the electrode is blocked by the second capacitor,
Electrolytic corrosion at the electrodes can be prevented. This makes it possible to manufacture electrodes without using expensive materials such as platinum.
Conventional materials conventionally used can be used.

Further, it is also a preferred embodiment of the present invention to insert a series resistor at any position between the output of the comparator and one terminal of the capacitor. In this case, the oscillation frequency of the oscillation circuit can be limited to a predetermined value or less regardless of the electric conductivity of the non-measurement object, so that stable measurement can be performed when measuring the oscillation frequency or the oscillation cycle by a computer or the like. Become.

The electric conductivity measuring apparatus of the present invention is, for example,
It is preferably used for measuring the electric conductivity of a conductive liquid such as pure water or seawater. Here, in order to obtain highly accurate electric conductivity, it is preferable to adopt a plastic film or the like having a small leakage current as the material of the capacitor for current storage. Further, the comparator preferably has a MOSFET having a large input resistance and a small leakage current as an input stage.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of an electric conductivity measuring apparatus according to an embodiment of the present invention. The electrical conductivity measuring device of the present embodiment is a direct current power supply device E that supplies a power supply voltage (E).
1, an integrated circuit 20 having an input terminal IN and an output terminal OUT, an electrode holder 10 holding a pair of electrodes 1a and 1b, one terminal connected to the electrode 1a and the other terminal connected to the ground, A capacitor C for accumulating charge, which is connected in series with an object to be measured (for example, water) in contact with 1a and 1b.
1, a second capacitor C2 inserted between one electrode 1b and the output terminal OUT of the integrated circuit, and a pair of electrodes 1
A parallel resistor R4 connected between a and 1b to form a parallel circuit with the DUT, and an output resistor R5 for transmitting the output of the oscillation circuit to the outside are provided.

The integrated circuit 20 includes a pair of comparators Q.
1, Q2 and a pair of NORs that form a flip-flop
Gates Q3 and Q4, two-stage inverters Q5 and Q6 for transmitting the output of the flip-flop, a transistor (FET) Q8 for transmitting the output of the flip-flop via the output resistor R5, and a DC power supply voltage are divided. Voltage dividing resistor R for applying a predetermined reference voltage to each of the comparators Q1 and Q2.
1 to R3. The pair of comparators Q1 and Q2 and the pair of NOR gates Q3 and Q4 form a comparator (group) having a hysteresis characteristic and having an inverted output.

The voltage dividing resistors R1 to R3 have, for example, resistance values equal to each other, and the reference voltage of 2E / 3 is applied to the inverting input terminal (-) of the first comparator Q1 and the second comparator Q2.
A reference voltage of E / 3 is applied to the non-inverting input terminal (+) of each. As a result, the upper threshold value in this comparator group becomes 2E / 3 and the lower threshold value becomes E / 3. The terminal voltage of the capacitor C1 is input to the non-inverting terminal (+) of the first comparator Q1 and the inverting terminal (−) of the second comparator Q2, and compared with the respective threshold values in the comparators Q1 and Q2.

In the electric conductivity measuring apparatus of this embodiment,
The integrated circuit 20 for detecting the voltage and controlling the charging / discharging for constituting the oscillator circuit is simplified by using a commercially available CMOS type 555 type integrated circuit. This type of integrated circuit is a general-purpose circuit often used in, for example, a timer circuit. Hereinafter, the operation of the electric conductivity measuring apparatus according to the present embodiment will be described first from the charge cycle and the discharge cycle of the charge storage capacitor C1.

(Charge Cycle) When the terminal voltage of the capacitor C1 is lower than the reference voltage (E / 3) of the second comparator Q2, the output of the first comparator becomes L level,
The output of the comparator Q2 becomes H level, and a pair of NO
The output of the flip-flop formed by the R gates Q3 and Q4 is set to the H level. The output of the flip-flop is 2
It is output from the integrated circuit 20 via the inverters Q5 and Q6 connected to the stage. The output current of the inverter Q6 passes through the second capacitor C2 having a large capacitance, the electrode 1b, the liquid to be measured, the electrode 1a, and the capacitor C.
Charge 1. As a result, the terminal voltage of the capacitor C1 rises with time. The reason why the inverters are arranged in two stages is to reduce the output resistance when the capacitor C1 is charged by the inverter Q6 having a large current drive capability.

(Discharge Cycle) After the charging time has passed,
When the terminal voltage of the capacitor C1 rises above the reference voltage (2E / 3) of the first comparator Q1, the first comparator Q1 becomes H level and the flip-flops Q3 and Q4.
The output of is inverted to L level. Therefore, the output of the inverter Q6 is also inverted to L level, and the charge charged in the capacitor C1 is stored in the electrode 1a, the DUT, the electrode 1
b, the second capacitor C2, and the inverter Q6 to discharge. In response to this, the terminal voltage of the capacitor C1 decreases with time.

When the terminal voltage of the capacitor C1 falls below 2E / 3, the output of the first comparator Q1 becomes L level, but the flip-flops Q3 and Q4 retain the previous state. When the terminal voltage of the capacitor C1 becomes lower than E / 3, the output of the second comparator Q2 becomes H level, the outputs of the flip-flops Q3 and Q4 become H level, and the charge cycle of the capacitor C1 starts again. . This operation is repeated and the circuit oscillates.

The oscillation of the circuit is taken out as a signal voltage at the external terminal TP depending on whether the transistor Q8 is on or off. FIG. 3 shows the terminal voltage of the capacitor C1 and the output voltage waveform at the external terminal TP. Capacitor C
By selecting an appropriate capacitance for 1, the capacitor C1
The terminal voltage is substantially sawtooth as shown in the figure, and the output voltage waveform is a pulse train of period T. This pulse train is input to a computer (not shown), the computer measures the oscillation period, and the oscillation frequency is obtained.

The time required to charge and discharge the capacitor C1 is
Since it is almost proportional to the resistance value between the electrodes, that is, to the value obtained by multiplying the reciprocal of electric conductivity by the electrode constant, the oscillation frequency, which is the reciprocal of the oscillation period T, is eventually proportional to electric conductivity. Therefore,
In the electric conductivity measuring apparatus of this embodiment, the oscillation frequency proportional to the electric conductivity is measured, and the electric conductivity is obtained from the known capacitance.

The second capacitor C2 prevents a direct current component from flowing between the electrodes, thereby preventing electrolytic corrosion of the electrodes. In general, each of the comparators Q1 and Q2 has a high input impedance, but a very small amount of leakage current flows through its input terminal. Therefore,
If the capacitor C2 is not provided, this leakage current escapes to the ground without being stored in the capacitor-C1. This causes a difference between the charged charge and the discharged charge passing between the electrodes, and unless a metal such as platinum that is difficult to corrode is used as the electrode material, the difference in the passing charges causes the electrodes to corrode. is there.

The parallel resistor R4 is connected to the comparators Q1 and Q2.
Has the effect of stabilizing the input of. If the parallel resistor R4 is not provided, the circuit portion including the terminal T1 on the electrode 1a side is galvanically separated from the other circuit portion by the capacitors C1 and C2. In this case, since the comparators Q1 and Q2 have high input inspiration, the terminal T1 is in a floating state, and its potential is easily influenced by the surrounding potential and becomes unstable. By providing the parallel resistor R4 having an appropriate resistance value, the potential of the terminal T1 is stabilized.

The measuring circuit can be calibrated by adopting a highly accurate resistor as the parallel resistor R4. When the terminals T1 and T2 are opened by not connecting the electrodes or not bringing the electrodes into contact with the non-measurement object, the oscillation frequency in the measurement device is determined by the parallel resistor R4. For example, a parallel resistance R4 with a non-measurement object of 1 μS
If a resistor having a resistance value corresponding to the electric conductivity of (micro-Siemens) is used, a reference frequency corresponding to 1 μS can be obtained. After the measurement device is manufactured, the reference frequency is measured and stored in the computer instead of the calibration in the normal measurement device. The electrical conductivity of the measured object can be obtained by dividing the oscillation frequency obtained when measuring the measured object by this reference frequency and subtracting 1 μS from the obtained value. Generally, a highly accurate resistor is available at a low price, and a highly accurate capacitor is expensive. In the present embodiment, by adopting the parallel resistor R4 with high accuracy, an accurate reference frequency can be obtained without requiring special calibration, and an inexpensive and highly accurate electric conductivity measuring device can be obtained.

FIG. 2 is a circuit diagram showing an electric conductivity measuring apparatus according to the second embodiment of the present invention. The electrical conductivity measuring apparatus of this embodiment is different from that of the first embodiment in that a series resistor R6 for limiting oscillation frequency is added between the terminal T1 and one terminal of the capacitor C1 in the circuit of FIG. different. The action of the series resistor R6 is as follows.

The electrical conductivity measuring apparatus of the present invention utilizes the principle that the oscillation frequency changes in proportion to the electrical conductivity of the liquid to be measured. For example, the electrical conductivity of water is extremely wide when considering the electrical conductivity of pure water to dirty seawater. However, the processing capability of a computer that receives a signal from an oscillator circuit and measures the frequency has a limit determined by the clock frequency, and if a signal of a high frequency exceeding that limit is received, an incorrect frequency is output. there is a possibility. In the present embodiment, in order to prevent this, the upper limit of the oscillation frequency is limited. That is, the series resistor R6 has a function of limiting the oscillation frequency within the range of the processing capacity of the computer. The provision of the parallel resistor R6 impairs the proportional relationship between the electrical conductivity and the oscillating frequency to some extent, especially in the high frequency range, which can be easily corrected in the computer from the resistance value of R6.

In both the first and second embodiments, when the power supply voltage (E) changes, the reference voltage input to the comparator also changes in proportion to it.
The current that charges the capacitor also changes in proportion to the change in the power supply voltage. Therefore, even if the power supply voltage fluctuates, the charging time of the capacitor does not fluctuate and the frequency of the oscillation circuit does not change, which does not affect the measurement accuracy of the electric conductivity. In other words, an inexpensive DC power supply device can be used for the conductivity measuring device that requires high accuracy.

Although the present invention has been described above based on its preferred embodiments, the electrical conductivity measuring apparatus of the present invention is not limited to the configuration of the above-mentioned embodiments, and the above-mentioned embodiments are not limited thereto. The electric conductivity measuring device in which various modifications and changes are made from the above configuration is also included in the scope of the present invention.

[0032]

As described above, according to the electric conductivity measuring device of the present invention, the number of circuits of the electric conductivity measuring device is reduced and the structure is simplified, so that it is possible to reduce the size and cost. Become.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an electric conductivity measuring apparatus according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram of an electric conductivity measuring device according to a second embodiment of the present invention.

FIG. 3 is a signal waveform diagram in the embodiment example of FIG.

FIG. 4 is a circuit diagram of a conventional electric conductivity measuring device.

[Explanation of symbols]

 10 electrode holder 20 integrated circuit 1a, 1b electrode Q1 first comparator Q2 second comparator Q3, Q4 NOR gate Q5, Q6, Q7 inverter Q8 transistors (FET) R1, R2, R3, voltage dividing resistance R4 parallel resistance R5 output resistance R6 Series resistance C1, C2 Capacitor TP output terminal E1 DC power supply T1, T2 terminal

Claims (4)

[Claims] 1. A comparator having a hysteresis characteristic at an input voltage threshold and having an inverted output, and a pair of electrodes in contact with an object to be measured, one electrode of which is applied with the output voltage of the comparator. A pair of capacitors, one end of which is connected to the other of the pair of electrodes and which forms a series circuit with the device under test and which is charged by the output of the comparator through the device under test. An electric conductivity measuring apparatus comprising means for connecting to the input of the above to construct an oscillation circuit and measuring an oscillation period or an oscillation frequency of the oscillation circuit. 2. The electrical conductivity measuring device according to claim 1, further comprising a parallel resistor connected between the electrode pairs to form a parallel circuit with the DUT. 3. The electrical conductivity measuring device according to claim 1, further comprising a second capacitor inserted between the output of the comparator and the one end of the capacitor. 4. The electrical conductivity measuring device according to claim 1, further comprising a series resistor inserted between the output of the comparator and the one end of the capacitor.