JPH03154876A - Instrument for measuring solution resistivity - Google Patents

Abstract

PURPOSE:To maintain the accuracy of measurement with measuring liquids of a wide range by having an oscillation circuit which generates a pseudo sinusoidal wave AC signal formed by distorting a sinusoidal wave. CONSTITUTION:The oscillation circuit 1 is so operated that the oscillation amplitude is satd. The pseudo sinusoidal wave formed by distorting the sinusoidal wave is generated in the output thereof. An AC constant voltage circuit 2 is for setting the amplitude of the pseudo sinusoidal wave AC signal outputted by the oscillation circuit 1 at a prescribed stable value and is constituted of, for example, Zener diodes 2a and 2b which are connected in series in the directions opposite from each other. The Zener diodes 2a and 2b reverse connected in such a manner have the temp. characteristics reverse from each other and the fluctuation of the AC constant voltage circuit 2 by a temp. change is prevented in such a manner. The accuracy of the measurement is thus maintained.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention generally relates to a solution resistivity measuring device, and more particularly, to a solution resistivity measuring device for measuring the resistivity or conductivity of pure water or an electrolyte solution. This relates to a measuring device.

[Prior Art] When measuring the resistivity or conductivity of pure water or an electrolyte solution, a measuring electrode called a cell is used.

This measurement electrode has a cell constant k determined by its shape, size, distance between opposing electrodes, and frequency of the applied alternating current signal.
It has a unique value called (am-'). The equivalent electrical resistance between the measurement electrodes having such a cell constant k (co+-') is R (kΩ), and the resistivity of the solution is ρ (k
Ω·cm) and conductivity as σ (mS/cm), the following relationship holds true.

ρ=R/to, σ=to/R, ρ=1/σ...(1)
Here, if the cell constant k maintains a constant value, by measuring the equivalent electrical resistance R between the electrodes, the above formula (1
), resistivity ρ or conductivity σ can be easily determined.

When measuring the cell constant of the above-mentioned measurement electrode, a measuring device shown in FIG. 4 is used. In this measuring device, a measuring electrode C with an unknown cell constant is connected to one side of a bridge circuit to which a sinusoidal AC signal of a predetermined amplitude and frequency generated by a high-precision oscillation circuit a is applied. Immerse it in a solution whose resistivity ρ or conductivity σ is already known, and use a potentiometer e to check that the bridge circuit is balanced while adjusting the variable resistance d. Read the equivalent electrical resistance R between.

Note that the resistances f and g of the bridge circuit have the same value. As a result, the cell constant k can be calculated based on the above equation (1) using, for example, the electrical conductivity σ of the solution at that time and k−σ·R. At this time, it is assumed that the conductivity σ of the solution is measured in advance with a high-precision conductivity meter or a resistivity needle serving as a standard, and its value is known.

When measuring the resistivity ρ and conductivity σ of an arbitrary solution using the measurement electrode C having a known cell constant measured as described above, conventionally, an alternating current signal consisting of a sine wave or a rectangular wave is The interelectrode voltage of the measuring electrode at this time is measured and determined, and the resistivity ρ or conductivity σ of the solution is calculated based on this measured voltage. If the amplitude of the AC signal fluctuates, the measured value will change, making accurate measurement impossible.

Therefore, when using a sine wave AC signal, for example, an operational amplifier OP as shown in Fig. 5 is used as an AC generating circuit.
Two diodes D1 and D2 are connected to the output terminal of t with opposite polarities, these diodes D+ and D2, and the operational amplifier OP.

A Wien bridge type oscillation circuit in which the oscillation amplitude is stabilized by a fixed resistor R1 connected between the inverting input terminal of the operational amplifier OP2 and a diode D3 in the output circuit of the operational amplifier OP2 as shown in FIG. A quadrature oscillation circuit having components for stabilizing the oscillation amplitude consisting of ~Rh was used.

In the case of the above Wien bridge type oscillator circuit, as shown in Fig. 5, the following formula (2) is obtained using two resistors R2 and two capacitors CI of the RC block forming a positive feedback circuit.
It oscillates at the frequency r osc shown in .

rosc = 1/2 x ・Cl-R1・・(2
) In this AC generating circuit, the resistance value between the output of the operational amplifier OP and the inverting input is rI, and the resistance value between the inverting input and ground is r! If so, oscillation occurs with the voltage gain Aζ3 of the feedback circuit, and the following formula (3) holds true.

rt-TI/ (A I)...”
(3) In order to maintain the generation of a sine wave with a stable oscillation amplitude, the above-mentioned components are necessary.

In this respect, when using a rectangular wave alternating current signal, it is relatively easy to stabilize the oscillation amplitude, and the alternating current generating circuit is, for example, a circuit with an extremely simple configuration as shown in Figure 7. can be used.

[Problem to be solved by the invention]

However, when using a rectangular wave alternating current signal as described above, the value of the cell constant changes greatly when measuring over a wide range from small to large resistivity or conductivity. In other words, in FIG. For example, when investigating the unknown cell constant/number k of measurement electrode C, the frequency is 1kHz.
The data when using the rectangular wave alternating current signal is as shown in Table 1 below.

Table 1 As shown in the table ■ above, within the range where the value of conductivity σ changes by two orders of magnitude,
The value of the cell constant changes by -3% to +53% when k=0.100 cm-' is used as a reference. Therefore, it is not possible to maintain good measurement accuracy of the resistivity or conductivity of the solution, so the arithmetic circuit becomes complicated and expensive, such as requiring a linearization circuit, and it is difficult to obtain measurement accuracy.

On the other hand, when a sinusoidal AC signal is used, the cell constant value changes less. That is, the data when using a sine wave AC signal with a frequency of 1 kHz is as shown in Table 3 below.

Table■ As is clear from the above table■, within the range where the conductivity σΦ value changes by two orders of magnitude, the cell constant value changes only by -3% to +7% when k-0,100cm-' is the standard. Therefore, measurement accuracy can be obtained without any special modifications to the arithmetic circuit. However, as described above, the sine wave oscillation circuit requires measures to stabilize the oscillation amplitude, which has the drawback of making the circuit complex and expensive.

Therefore, in view of the above-mentioned problems of the conventional art, the present invention is designed to reduce the change in the value of the cell constant of the electrode even when measuring resistivity or conductivity over a wide range, maintain measurement accuracy, and simplify the circuit. , our objective is to provide an inexpensive solution resistivity measuring device.

[Means to solve the problem]

In order to solve the above problems, the solution resistivity measurement device according to the present invention includes an oscillation circuit that is operated with the oscillation amplitude saturated and generates a pseudo sine wave alternating current signal that is a distorted sine wave;
a fixed resistor to which a pseudo sine wave alternating current signal output from the oscillation circuit is supplied; a measuring electrode connected in series to the fixed resistor; and an alternating current voltage output from a connection point between the fixed resistor and the measuring electrode. A rectifying circuit that rectifies the voltage, an arithmetic circuit that calculates resistivity or conductivity based on the DC voltage output from the rectifying circuit, and a display that displays measurement results based on the calculation results of the arithmetic circuit. There is.

[Effect]

In the above configuration, as the AC signal applied to the measurement electrode via the fixed resistor, a pseudo sine wave AC signal is used, which is a distorted sine wave generated by an oscillation circuit that operates with the oscillation amplitude saturated. When measuring solutions with a range of conductivities, it is not as accurate as a sinusoidal AC signal, but
Compared to the case where a rectangular wave alternating current signal is used, since the waveform shape is close to a sine wave, it is possible to obtain measurement results with accuracy close to that of a sine wave alternating current signal. Moreover, since the pseudo sine wave AC signal is generated by operating the oscillation circuit in a saturated state, there is no fluctuation in its amplitude, and a circuit for stabilizing the oscillation amplitude, which is required for a sine wave oscillation circuit, is not required. No parts required.

〔Example〕

Embodiments of the present invention will be described below based on the drawings.

FIG. 1 is a diagram showing an embodiment of the solution resistivity measuring device according to the present invention. In the figure, 1 is an oscillation circuit as an AC generating circuit, and 2 is an oscillation circuit that keeps the amplitude of the AC wave outputted by the oscillation circuit l constant. 3 is the AC constant voltage circuit 2 which maintains the AC constant voltage circuit.
A fixed resistor with a known resistance value, one end of which is connected to the output of 4
is a measuring electrode connected between the other end of the fixed resistor 3 and the ground and immersed in the solution whose resistivity is to be measured; 5 is the fixed resistor 3;
An input is connected to the connection point X between the electrode 4 and the measurement electrode 4, and a rectifier circuit rectifies the AC voltage generated at the connection point Or an arithmetic circuit that calculates conductivity, 7 is an arithmetic circuit 6
8 is an A/D conversion circuit that converts the analog signal indicating the resistivity or conductivity outputted by the A/D conversion circuit 7 into a digital signal, and 8 displays the resistivity or conductivity as a digital value based on the digital signal from the A/D conversion circuit 7. It is a digital display.

The oscillation circuit 1 is operated so that the oscillation amplitude is saturated,
Its output generates a pseudo sine wave that is a distorted sine wave.
As the oscillation circuit l, for example, the voltage gain Aζ3 of the feedback circuit
A Wien bridge type oscillator circuit with a circuit configuration as shown in Fig. 2 is used, which oscillates with a gain A of 3 or more.
For example, by setting the oscillation amplitude to A-3°1 and saturating the oscillation amplitude, it is possible to generate a wi (JIJ sine wave alternating current signal) containing stable distortion in both the oscillation frequency and the oscillation amplitude.

In the circuit shown in FIG. 2, the oscillation frequency r osc is determined by the value R of the two resistors 12a and 12b constituting the positive feedback circuit 12 connected between the output of the operational amplifier 11 and the non-inverting input.
I! and the value C1 of the two capacitors 12c and 12d! It is determined by the following formula.

fosc ” 1/21t・Ctz” Rra
...'(4) Also, the value R of the resistors 13 and 14 connected between the output and the inverting input of the operational amplifier 11
I! The following equation holds true between RI4 and voltage gain A.

Rra-R1s/ (A l)
(5) Therefore, in order to set the voltage gain A to 3.1, the value R13 of the resistor 13 and the value R14 of the resistor 14 may be set in a relationship such that the following formula holds.

Rls=2. I Rra The AC constant voltage circuit 2 is for setting the amplitude of the pseudo sine wave AC signal outputted by the oscillation circuit 1 to a predetermined stable value. For example, as shown in FIG. It is composed of Zener diodes 2a and 2b connected in series. The Zener diodes 2a and 2b connected in opposite directions in this manner have opposite temperature characteristics, which can also prevent the output of the AC constant voltage circuit 2 from varying due to temperature changes.

AC voltage V outputted by the AC constant voltage circuit 2.

is applied to a resistor 3 having a resistance value R0 and a measuring electrode 4 connected in series to this resistor 3. As a result, the measurement electrode 4 exhibits an equivalent electrical resistance value R=k・ρ due to the relationship between the known cell constant shown in the above equation (1) and the unknown resistivity ρ of the solution to be measured. Become. Now, if the magnitude of the AC voltage generated at the connection point X is V, the following formula holds true.

■. = [k・ρ/(R0+k・ρ)] ・v! ...(
6) By modifying this equation, it becomes the following equation, and the resistivity ρ of the unknown solution to be measured can be obtained.

ρ= (R@ /k) [V, / (Vi-v.)
]...(7) Further, from the relationship between the conductivity σ and the resistivity ρ in the above formula (1), the conductivity σ can be determined by the following formula (8).

a= (k/R,) ・C(v, -Vl)/vo
].-(8) The alternating current voltage V generated at the connection point X is input to the rectifier circuit 5, where it is rectified and converted into a direct current voltage. This DC voltage V is supplied to the arithmetic circuit 6, and this DC voltage v
0, the resistivity ρ or conductivity σ is calculated based on the above equation ('r) or (8).

The calculation result in the calculation circuit 6 is output as an analog voltage signal, this analog voltage signal is converted into a digital signal in the A/D conversion circuit 7, and the measurement result is digitally displayed on the digital display 8 based on this digital signal. .

As described above, when the pseudo sine wave AC signal generated by the oscillation circuit 1 is used, the conductivity σ ranges over a wide range from small to large values, compared to the conventional method using a rectangular wave AC signal. The value of the cell constant does not change significantly even if the cell constant is measured.

That is, in FIG. 1, when a pseudo sine wave alternating current signal with a frequency of 1 kHz is used to investigate, for example, the unknown cell constant k of the measurement electrode, the data is as shown in Table 2 below.

In the range of approximately 7 u 37 cm to 700 u S/ctn, when a sine wave, pseudo sine wave, or square wave alternating current signal with a frequency of 1 kHz is applied, the cell constant of the measurement electrode is
When k ~0.100 cm-' is used as a reference, it changes as shown in Table 3 below.

Table ■ As shown in the table ■ above, within the range where the value of conductivity σ changes by two orders of magnitude,
Based on k-0,100cm-', the value of the cell constant changes by only 13% to +16%, which is extremely small compared to 13% to 53% in the case of a rectangular wave AC signal, which indicates good measurement. Accuracy can be maintained.

To summarize the above, as is clear from the above table, the conductivity of the liquid under test is significantly improved compared to the rectangular wave, and the change is similar to that of a sine wave. value, and it is possible to measure resistivity or conductivity with high accuracy.

Furthermore, since the oscillation circuit oscillates in a saturated state, the oscillation amplitude is stable, and there is no need to stabilize the amplitude of the oscillation circuit, and costs can be reduced by simplifying the circuit configuration.

In the embodiment, the AC constant voltage circuit 2 is connected to the output of the oscillation circuit 1, but it can be omitted if the oscillation circuit 1 generates an oscillation output at a desired level.

〔effect〕

As explained above, according to the present invention, since a pseudo sine wave AC signal is used, the cell constant can be maintained over a wide range of conductivity of the liquid to be measured, which is a major factor in measurement accuracy. Also, since the oscillation circuit 'that generates the pseudo sine wave AC signal is operated with the oscillation amplitude saturated, circuit components for stabilizing the oscillation amplitude, which are required by the sine wave oscillation circuit, are not required. However, the amplitude does not fluctuate, and even when measuring resistivity or conductivity over a wide range, there is little change in the cell constant of the electrode, and measurement accuracy can be maintained, and the circuit is simple and inexpensive. A solution resistivity measuring device can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the solution resistivity measuring device according to the present invention, FIG. 2 is a circuit diagram showing a specific example of a part of the circuit in FIG. 1, and FIG. A circuit diagram showing a specific example of the other part of the circuit, Figure 4 is a diagram showing a general measuring device for measuring cell constants, and Figures 5 to 7 are diagrams showing a conventional solution resistivity measuring device. FIG. 2 is a circuit diagram showing an example of an oscillation circuit used. l... Oscillator circuit, 3... Fixed resistance, 4... Measuring electrode, 5... Rectifier circuit, 6... Arithmetic circuit, 8... Digital display.

Claims (1)

[Claims] An oscillation circuit that is operated with the oscillation amplitude saturated and generates a pseudo sine wave alternating current signal that is a distorted sine wave, and a pseudo sine wave alternating current signal output from the oscillation circuit is supplied to one end. a fixed resistance, a measurement electrode connected in series to the fixed resistance, a rectifier circuit that rectifies the AC voltage output from the connection point of the fixed resistance and the measurement electrode, and a DC voltage output from the rectification circuit. A solution resistivity measurement device comprising: a calculation circuit that calculates resistivity or conductivity; and a display that displays measurement results based on the calculation results of the calculation circuit.