JPH0658333B2 - Conductivity measuring method and measuring apparatus therefor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a method and a measuring apparatus for applying an AC voltage to a pair of electrodes immersed in a solution to be measured and measuring the electrical conductivity of the solution by the current flowing between them. [Related Art] [Prior Art] Conventionally, it is known to measure the electric conductivity and apply it to the detection of a liquid component such as water, and in such a case, a certain amount of electricity is applied between a pair of electrodes immersed in a solution to be measured. This is usually done by applying an AC voltage and measuring the interelectrode current.

[Problems to be solved by the invention]

However, the standard of water contained in hydrofluoric acid anhydride, for example, is gradually becoming stricter than 20ppm, so that the measuring range of conductivity meter is required to be much wider than the conventional one. In the constant voltage method, it was difficult to detect such a small amount of component and to measure in a wide range.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a conductivity measuring method and a measuring apparatus therefor capable of detecting a minute amount of a liquid component and widening a measuring range.

[Means for solving problems]

The present invention is a method of applying an AC voltage between a pair of electrodes immersed in a solution to be measured to measure the electrical conductivity of the solution by a current flowing between the electrodes, in which the power consumed between the electrodes is constant. As described above, the AC voltage applied between the pair of electrodes is controlled, and the specific measuring device is a voltage for applying the AC voltage between the pair of electrodes immersed in the solution to be measured. A control circuit, a current detection circuit that measures a current flowing between the electrodes, a logarithmic conversion circuit that converts the signal of the current detection circuit into a logarithm, a power calculation circuit that calculates the power consumed between the electrodes, and the calculation. It is characterized by including a comparison circuit for comparing the output of the circuit and the reference power and outputting a signal corresponding to the difference to the voltage control circuit.

[Action]

A description will be given by taking an example of electric conductivity measurement for detecting water in hydrofluoric anhydride.

Immersing a pair of electrodes in anhydrous hydrofluoric acid and measuring the resistance between them by R
(Ω), the conductivity K (μS / cm) in hydrofluoric acid is (C: cell constant), and the water content is proportional to the electrical conductivity, so the electrical conductivity K may be measured. That is, 1.5 μS / cm
0.1 ppm and 15000 μS / cm correspond to 1000 ppm.

Here, the detected current, that is, the interelectrode current i is usually several mA to several mA / 100, and the cell constant may be 0.3 to 0.5. Therefore, when the electric conductivity is 1.5 μS / cm, the resistance R = 200 kΩ,
When the interelectrode current i = 0.05 mA, the electric conductivity is 15000 μS / cm, the resistance R = 20 Ω, and the interelectrode current i = 5 mA, the power consumed between the electrodes (hereinafter abbreviated as interelectrode power) P (MW) is P = i ² R, so both are 0.5m
It becomes constant with W. This relationship is expressed by an alternating voltage (hereinafter abbreviated as interelectrode voltage) E applied between a pair of electrodes and an interelectrode current i.
It is as shown in Fig. 2 in terms of the relationship with, and the conductivity is K = 1.
E = 10 V when 5 μS / cm, E = 1 V when K = 150 μS / cm, E = 0.1 V and K when K = 15000 μS / cm
= Voltage150-2logE, the voltage between the electrodes is 0.05mA-5mA and the decimal value is 2
digit In the preferred range of water concentration from 0.1ppm to 1000ppm, 10
It can measure a very wide range of 4 digits.

On the other hand, with the conventional constant voltage method, for example, the water concentration of 0.1 ppm
If you try to measure (conductivity 1.5μS / cm), the constant voltage is 10V.
(i = 0.05 mA), so the water concentration
Electrode current i at 1000 ppm (conductivity 15000 μS / cm)
Is Since it is proportional to the electric conductivity, it becomes 500 mA and the polarization effect is high, and the calorific value (P = 5 W) also becomes large, so the error becomes large and not only the electrode is adversely affected, but 0.05
mA-500mA current, that is, 4 decimal digits (500 / 0.05 = 1
0 ⁴ ) must be measured in one range, which is practically impossible. If the constant voltage is lowered to lower the inter-electrode current i, the inter-electrode current i will be greatly reduced in places where the electrical conductivity is small, making detection difficult and increasing the error as well as measuring the decimal 4-digit current. Is necessary and practically impossible.

Further, according to the method of the present invention, Since P is constant, K = Ai ² (A: constant), that is, logK∝2logi, which can be displayed on a logarithmic scale.

〔Example〕

 Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit configuration diagram showing a preferred embodiment of the present invention, and FIG.
The figure shows the relationship between the interelectrode voltage E (V) and the interelectrode current i (mA) for explaining the operation of the present invention, and FIG. 3 shows the scale plate in the electric conductivity measuring apparatus of the present invention.

As shown in FIG. 1, the conductivity measuring apparatus of the present invention comprises a voltage control circuit 2 having a voltage adjusting function for applying an AC voltage to a pair of electrodes 1, and a current detection circuit 3 for measuring a current flowing between the electrodes 1. , A logarithmic conversion circuit 4 for converting the output signal of the current detection circuit into a logarithm, an interelectrode voltage E (V) and an interelectrode current i.
The electric power P (mW) (= E) consumed between the electrodes based on (mA)
i), a power calculation circuit 5 for calculating the reference power Po, a reference power setting circuit 6 for setting the reference power Po (mW), and a signal B (P-Po) corresponding to the difference between the interelectrode power P and the reference power Po. (B:
At least a comparison circuit 7 for outputting a constant) to the voltage control circuit 2 may be provided, and a current conversion circuit 8 for connecting to various instruments not shown may be provided.

In order to apply the electric conductivity measuring device having such a constitution to the case of measuring the water concentration in the anhydrous hydrofluoric acid as described above, the electric conductivity K1.5 μS / cm should be kept constant at 0.5 mW between the electrodes.
The interelectrode voltage E is varied in the range of 10 V to 0.1 V according to ˜15000 μS / cm, and the interelectrode current i at that time (0.05 mA to 5 m
It is possible to detect the water concentration from 0.1ppm to 1000ppm with the conductivity K by the scale plate as shown in Fig. 3 from A), and the accuracy is within ± 2% in the range of 0.1ppm to 500ppm. It was confirmed that it is highly accurate and can be put to practical use with an accuracy within ± 4% even within the range of 500 ppm to 1000 ppm.

Although the preferred embodiment has been described above, the present invention is not limited to this, and various applications are possible.

The solution to be measured is suitable when a wide range of measurement is required in addition to hydrofluoric acid, such as measurement of water quality in soil, but it is needless to say that it can be applied even when the range of measurement is narrow.

In the voltage control circuit, the waveform applied to the pair of electrodes may be an AC voltage such as a square wave, but an AC voltage having a sine waveform is preferable because it has less DC drift that causes an error. Further, the method of controlling the voltage is to change the amplitude of the oscillation circuit, the method of changing the resistance in the voltage drop method,
A method of controlling the firing angle of the (silicon-controlled rectifying element) can be used.

It is more preferable to attach a temperature compensating circuit for compensating a change in characteristics due to temperature to various circuits, particularly to a power operation circuit.

〔The invention's effect〕

In the present invention, since the voltage between the electrodes is controlled and applied so that the power consumed between the pair of electrodes is constant,
Not only is it possible to measure a wide range of four digits and microconductivity, but the calorific value is also constant, so the adverse effect on the electrodes etc. due to heat generation is eliminated and the display is performed on a logarithmic scale over a wide range. It is very effective as it can.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing a preferred embodiment of the present invention, and FIG.
The figure shows the relationship between the interelectrode voltage E (V) and the interelectrode current i (mA) for explaining the operation of the present invention, and FIG. 3 shows the scale plate in the electric conductivity measuring apparatus of the present invention. 1 ... Pair of electrodes, 2 ... Voltage control circuit 3 ... Current detection circuit, 4 ... Logarithmic conversion circuit 5 ... Power calculation circuit, 6 ... Reference power setting circuit 7 ... Comparison circuit

Claims (2)

[Claims] 1. A method of measuring an electric conductivity of a solution by applying an AC voltage between a pair of electrodes immersed in a solution to be measured and measuring a conductivity of the solution by a current flowing between the electrodes. So that the AC voltage applied between the pair of electrodes is controlled. 2. A voltage control circuit for applying an AC voltage between a pair of electrodes immersed in a solution to be measured, a current detection circuit for measuring a current flowing between the electrodes, and an output signal of the current detection circuit in logarithm. A logarithmic conversion circuit for converting, a power calculation circuit for calculating the power consumed between the electrodes, and a comparison circuit for comparing the output of the calculation circuit and the reference power and outputting a signal according to the difference to the voltage control circuit. An electric conductivity measuring device comprising: