JPH0648436Y2 - Electrical conductivity measuring device - Google Patents

Description

[Detailed Description of the Invention] [Industrial field of application] The present invention relates to an electric conductivity measuring device used for physics and chemistry, industrial use, agricultural use and the like.

[Conventional technology]

The electric conductivity is a physical quantity mainly used as an index of the electrolyte concentration in water, and has been widely adopted for the water quality control of boiler water and the fertilizer concentration control in the agricultural field.

Further, as a method for measuring the electrical conductivity, a two-electrode method in which electrodes having a certain area are immersed in a solution at a certain distance from each other and an alternating voltage is applied to these electrodes to measure a current value flowing between the electrodes is used. It is commonly used.

However, since the two-electrode method is affected by the polarization of the solution, it has a drawback that the electrical conductivity cannot be measured with high accuracy, and at present, it is used only for simple measurement.

On the other hand, a four-electrode method has been devised as a measurement method that eliminates the drawbacks of the two-electrode method, and at present, the electrical conductivity is generally measured by this four-electrode method.

This four-electrode method is one in which four electrodes are immersed in a solution to measure the electrical conductivity, and the effects of the polarization of the solution and the effects on the resistance of the cable connecting the electrodes to the measurement circuit are avoided. Therefore, it is possible to measure the electric conductivity with higher accuracy than the two-electrode method.

[Problems to be solved by the device]

However, in the above-described conventional four-electrode method for measuring electrical conductivity, the error is increased by the length of the cable because it is affected by the reactance of the cable connected to the electrode. There were some problems such as restrictions on the installation location.

The present invention has been made for the purpose of improving the above-mentioned problems, and an object thereof is to provide an electric conductivity measuring device which does not increase the error due to the length of the cable and is not restricted in the installation place.

[Means and Actions for Solving the Problems]

In order to achieve the above-mentioned object, the present invention uses a lock-in amplifier for measuring voltage and current. Referring to the drawings, FIG. 1 is a block diagram showing the basic concept of the present invention. The measurement cell 2 indicates the liquid to be measured contained in the measurement cell 2.

The measuring cell 1 is provided with four electrodes 3, 4, 5 and 6 so that the voltage between the electrodes 4 and 5 is measured by the lock-in amplifier 8 and is connected to the electrode 3. By controlling the output voltage of the generated amplifier 7, the voltage between the electrodes 4 and 5 is kept constant.

The current flowing between the electrodes 3 and 6 is measured by the lock-in amplifier 9, and a signal E ₀ proportional to the electrical conductivity of the measured liquid 2 is output from the lock-in amplifier 9 and the lock-in amplifiers 8 and 9 are also output. From the oscillator 10 to the same frequency as the amplifier 7,
In-phase signals are applied as clock signals.

On the other hand, FIG. 2 shows an equivalent circuit of the electrodes 3 to 6 including the cables connected to the electrodes 3 to 6, and R _X1 ,
R _X2 and R _X3 are resistances of the measured liquid 2, and the electric conductivity of the measured liquid 2 to be measured is proportional to the reciprocal of R _X2 . Also
C _X1 and C _X2 represent capacitances caused by polarization.

R _S1 to R _S4 are the resistance component due to contamination of electrodes 3 to 6, Cl
Indicates the capacitance generated between the cables and Rl
₁ to Rl ₄ indicate the resistance of the cable, respectively.

Measurements of the solution conductivity of by the four electrode method is to control the voltage which the voltage between E ₂ -E ₃ shown in Figure 2 is applied between E ₁ -E ₂ to be constant, then E ₁ -E ₄ is measured by measuring the current flowing between R _X1,2 , C _X1,2 , R _S1 or R _S4 and Rl
A value proportional to 1 / R _X2 can be measured without being affected by ₁ to Rl ₄ , but the current flowing due to the capacitance Cl generated between the cable lines is also added to the measurement current, so there is an error in the measurement result. Occurs.

Therefore, in this device, the voltage applied between E _{1 and} E ₄ is measured by the lock-in amplifiers 8 and 9 that operate with the clock signals of the same phase.

As a result, the current flowing due to the capacitance Cl between the cable lines with a 90 ° phase advance is not added to the measured current, so only the current proportional to 1 / R _X2 can be measured, and the electrical conductivity without error can be measured. Like

〔Example〕

An embodiment of the present invention will be described below in detail with reference to the drawing shown in FIG.

In the figure, A ₁ to A ₃ are instrumentation amplifiers for amplifying AC voltage between E _{2 and} E ₃ , and voltage measurement is performed by a synchronous rectifier composed of a capacitor C ₄ , a resistor R ₁₀ and an analog switch S _1. It constitutes a lock-in amplifier for. A ₄ is an error amplifier that calculates the difference between the reference voltage E ₁ and the output of the lock-in amplifier, and an integrator is used.

The output of this error amplifier A ₄ is converted into an alternating current by the analog switch S ₂ and applied to the reference voltage E ₁ to form a feedback loop for the entire circuit described above, and a constant alternating voltage is applied between E _{2 and} E _3. Is operated to be applied.

On the other hand, A ₅ is a current amplifier, which converts the current flowing between E ₁ -E ₄ into a voltage.

The capacitor C ₇ , the resistor R ₁₃ , and the synchronous rectifier composed of the analog switch S ₃ form a lock-in amplifier for current measurement, and the analog switches
A clock signal fclk having the same frequency and the same phase is applied to S ₁ , S ₂ and S ₃ from the oscillator OSC, and the analog switches S ₁ and S
₂ and S ₃ are designed to be driven.

As a result, signals with different frequencies and phases of the clock signals fclk that drive the analog switches S ₁ , S ₂ , and S ₃ are not added to the measured value, and as a result, the measured value is changed by the current flowing due to the capacitance between the cable lines. No error will occur.

[Effect of device]

As described in detail above, this invention can eliminate the influence of the reactance component of the cable connected to each electrode, so that the measurement result does not have an error even if the length of the cable changes.

This makes it possible to measure electrical conductivity with high accuracy without being restricted by the length of the cable or the installation location of the measuring device, and by using the lock-in amplifier,
Since it is less likely to be affected by noise and the like, it is possible to further improve the noise resistance, which is an important factor as an industrial instrument.

[Brief description of drawings]

FIG. 1 is a basic conceptual diagram for explaining an electric conductivity measuring device of the present invention, FIG. 2 is an equivalent circuit of electrodes including cables, and FIG. 3 is a circuit diagram showing an embodiment of the present invention. . 1 is a measuring cell, 2 is a liquid to be measured, 3,4,5,6 are electrodes, 7 is an amplifier, 8 and 9 are lock-in amplifiers, and 10 is an oscillator.

Claims (1)

[Scope of utility model registration request] 1. An electric conductivity measuring device for measuring electric conductivity of a liquid to be measured using four electrodes, wherein at least one of a voltage between the electrodes or a current flowing between the electrodes is detected by a lock-in amplifier. An electrical conductivity measuring device characterized by: