JP2631786B2 - Conductivity measuring method and its device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for measuring the conductivity or concentration of various solutions.

[0002]

2. Description of the Related Art The basic principle of all measuring methods is a displacement method in which measurement is performed using the displacement of a spring (object) like a spring balance,
It has an equilibrium mechanism equivalent to a lever with the center as a fulcrum like a balance, and has a zero-point method (normally equipped with an automatic equilibrium mechanism) for measuring by balancing with a reference amount equivalent to a weight (the same as the measured amount) Therefore, the former has a feature that the structure is simple but the accuracy and stability are low, and the latter has a feature that the structure is complicated but high accuracy and stability. In addition, there are an electrode method using an electrode and an electromagnetic method without an electrode as a method for measuring the conductivity of a solution, and the method applicable to the present invention relates to an electromagnetic method, and two methods based on the above measurement principle include: Exists. A conventional technique is disclosed in Japanese Utility Model Publication No. 54-34632, Japanese Patent Laid-Open Publication No. Sho 54-34632. First, when an example of a deflection method conductivity measuring apparatus is shown, an excitation transformer in which an AC power supply is connected to a primary side is described. Vessel secondary coil,
A liquid path filled with the test liquid in a single-turn insulated path that shares the primary coil of the detection transformer with a signal measuring device that measures the signal corresponding to the conductivity of the test liquid on the secondary side. When a constant AC voltage is applied to the primary coil of the exciting transformer, a constant AC voltage is induced in the secondary coil of the exciting transformer and the (fluid path coil). As a result that an alternating current that is directly proportional to the conductivity flows through the primary coil (fluid path coil) of the detection transformer, a non-linear voltage corresponding to the conductivity of the test liquid is induced in the secondary coil of the detection transformer. There is known a method of measuring a non-linear voltage induced in a secondary coil of a detection transformer by the signal measuring device to measure the electric conductivity of a test liquid, and the like. Next, an example of a conventional zero-position (automatic equilibrium method) conductivity measuring apparatus will be described. An AC power supply, an excitation transformer, a liquid path coil, a detection transformer, and the like used in the above-described deflection method conductivity measuring apparatus are shown. In the same way,
Further, an error signal amplifier connected to the secondary side of the detection transformer and a phase comparator connected to the error signal amplifier and obtaining a control output based on the phase relationship between the error signal and the reference signal; and Is a compensating coil provided in such a direction as to cancel the magnetic flux generated by the current flowing through the liquid path coil, and a winding type variable resistor and a servo motor which are connected to the compensating coil and the phase comparator and are also components of the recorder. And a signal measuring device (recorder) that visually measures the position and the scale of the movable contact piece of the winding type variable resistor, and the measuring method is the above-described deflection method conductivity measuring method. As described above, since a non-linear voltage corresponding to the conductivity of the test liquid is similarly induced in the secondary coil of the detection transformer, the error signal connected to the secondary coil of the detection transformer is increased. The current supplied from the AC power supply or the like is adjusted to the compensation coil by the current regulator so that the output of the recorder becomes zero (the automatic balance mechanism of the recorder is shared), and the current flows during the equilibrium. There is known a method of visually measuring the conductivity of a test liquid from the position and scale of the movable contact piece of the wire wound variable resistor. However, the disadvantage of the conventional measuring apparatus and measuring method described above is that the displacement method uses a non-linear magnetic characteristic of the iron core of the detecting transformer (hereinafter referred to as a core), so that the output of the signal measuring device is not sufficient. Since the characteristic curve that saturates on the high conductivity side without being directly proportional to the conductivity of the test solution is drawn, the linearity is poor at ± 1%, and is directly affected by the core temperature characteristics, external stress characteristics, etc. Therefore, the stability is poor at ± 1%. In particular, the external stress characteristics of the core are very sensitive, and the permalloy core, which is commonly used and has a high magnetic permeability, varies the output of the signal measuring instrument by several percent to several tens of percent even if it is sandwiched by a fingertip with a force of several tens of grams. The mounting method of (1) could not be put into practical use unless sufficient measures were taken against external stress, and the accuracy and stability of about ± 1% were obtained at most. Further, in the conventional null method, although the adverse effects due to the above-mentioned characteristics of the core described in the above-described displacement method can be extremely reduced, a current regulator for adjusting the compensation current is expensive due to mechanical moving parts. Since the wound type variable resistor and the servomotor are used in combination, an expensive high-output amplifier was separately required to drive the servomotor. Regarding accuracy and stability, the winding type variable resistor changes the resistance by the contact between the movable contact piece and the winding. Due to poor linearity and stepwise change in resistance value, the resolution is poor, and reading accuracy is poor because the signal measuring device is a recorder. Only the accuracy and stability of the

[0003]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and [1] has extremely high measurement accuracy and stability. [2] has a structure having no mechanical moving parts. [3] Being inexpensive to manufacture [4] To develop a conductivity measurement method and a measurement device that satisfy the four small tasks of obtaining a current or voltage output that is directly proportional to conductivity at one time. Research progressed.

[0004]

Means for solving the problem [1] that the measurement accuracy and stability of the small problem [1] are extremely high are as follows, as indicated by the characteristics of the measurement principle and the above-described conventional technology. Discard the displacement method with low accuracy and stability,
We decided to use the null method with high accuracy and stability. The accuracy and stability of the zero-level conductivity measuring device are the constituent elements of the measuring device described above: (1) AC power supply, (2) excitation transformer, (3) detection transformer, (4) current regulation. Tableware (5)
Since the accuracy and the stability of the error signal amplifier, (6) the phase comparator, and (7) the signal measuring device are established, it is necessary to increase the accuracy and stability of the conductivity measuring device by using individual components of the measuring device. It is necessary to make the accuracy and stability of the elements extremely high in a well-balanced manner. Hereinafter, the target characteristics for each element and the means for enhancing them will be described in order. (1) The characteristic of interest for an AC power supply is an AC voltage which is an output of the AC power supply, and a means for making the accuracy and stability of the AC voltage extremely high is a reference voltage (for example, a semiconductor reference) having extremely high accuracy and stability. 0.3p for voltage generator
pm / ° C., 150 ppm / year) or by forming an AC power supply by a switching method based on the reference voltage. (2) The characteristics of the exciting transformer are the degree of coupling between the coils, the magnetic permeability of the core, the magnetostriction and the leakage flux rate, and wire winding is performed so that the degree of coupling between the primary, secondary and coil is extremely high. Or forming a liquid path coil, and having a sufficient effective magnetic permeability at the frequency of the AC power supply, having a small magnetostriction, and a permalloy core commonly used in which the core is annular and the leakage flux is reduced. This is achieved by using an amorphous core. (3) The detection transformer is the same as the excitation transformer of (2). Here, the influence of the distortion characteristic of the current regulator and the distortion removal characteristic of the error signal amplifier on the measurement accuracy of the conductivity around the detection transformer will be described in some detail. When all the elements except for the current regulator and the error signal amplifier have high accuracy, and when the distortion factor of the liquid path current that is completely directly proportional to the conductivity of the test liquid is zero, the conductivity is Η (p
pm), the distortion rate of the current regulator is α (ppm), the distortion removal rate of the error signal amplifier is β, (0 ≦ β ≦ 1)
Since η = α × (1−β), from this equation (which can be easily derived from the current balance around the detection transformer),
It can be seen that in order to decrease η, α can be decreased, β can be increased, and β can be increased while decreasing α. In other words, when high accuracy is realized only with the current regulator, the distortion factor of the current regulator is reduced as much as possible, and α →
When high accuracy is realized only by the 0 error signal amplifier, the distortion removal rate of the error signal amplifier is increased, and β → 1, when high accuracy is realized by both the current regulator and the error signal amplifier, By making the distortion factor of the current regulator small, α → 0, and at the same time, increasing the distortion removal rate of the error signal amplifier, β → 1, the accuracy of measuring the conductivity can be increased. Therefore, (4) the characteristics that are the object of the current regulator are the distortion factor and the resolution. As described above, when high accuracy is realized only by the current regulator or in combination with the error signal amplifier, the distortion factor is high. Must be reduced, and the resolution must be as high as possible. The specific means is achieved by using a semiconductor variable current device. What is a semiconductor variable current device?
Among those having a function of continuously changing an AC voltage or an AC current or an electric resistance by an external control signal,
The mechanism for changing the AC voltage or the AC current or the electric resistance has a function of adjusting an AC current formed using a semiconductor, and specific examples of the structure are (a) a multiplier,
It has a function of adjusting an alternating current composed of a divider, (b) a variable amplification type amplifier, (c) a semiconductor variable resistor, etc. For example, a more specific and simple example is shown. In the case where the multiplier is used alone, the relationship Z = X × Y exists between X and Y as inputs and the output Z, and the AC output voltage Z Is proportional to the product of the AC input voltage X and the control voltage Y. If an output of the multiplier is connected to a passive element (coil, capacitor, resistor) having an appropriate value, an AC current flows through this circuit. A semiconductor variable current device having an adjusting function can be configured. A variable transconductance type four quadrant (possible positive and negative polarities for both X and Y inputs) multiplier has a distortion rate of 2000 ppm and a resolution of several ppm. (B) In the case where the variable amplification type amplifier is used alone, the relationship of Z = A × X is established when the input is X and the output is Z, where the amplification factor A of the amplifier is variable. The AC output voltage Z is proportional to the product of the AC input voltage X and the amplification factor A corresponding to the control voltage, and a semiconductor variable current device can be configured in the same manner as the multiplier. Voltage controlled variable amplification type amplifier (generally called VCA)
Has a distortion rate of 40 ppm and a resolution of several ppm. (C) An example in which a semiconductor variable resistor is used alone is an example in which a cadmium sulfide optical variable resistor (hereinafter, referred to as CdS) is used, and a light amount corresponding to a control voltage is emitted using a light emitting element. A method in which one end of CdS whose electric resistance changes according to the light amount is connected to an AC power supply so that an AC current corresponding to the resistance value flows, or the CdS is combined with an amplifier to obtain a variable amplification factor of (b) There are a method of configuring a type amplifier, a method of directly adjusting an alternating current by the same combination, and the like. All of them can constitute a semiconductor variable current device having a function of adjusting an alternating current. Cd
S has a distortion rate of 20 ppm and a resolution of several ppm. In addition to the method configured to be used alone as in the above specific examples, a method configured by using a plurality or a combination thereof is also included. (5) The target characteristics of the error signal amplifier are the distortion removal rate and the amplification rate (resolution). As described above, when high accuracy is realized only with the error signal amplifier or in combination with the current regulator. Needs to increase the distortion removal rate (β → 1). This can be achieved by providing a filter or a waveform processor for removing waves other than the waveform of the AC power supply in the error signal amplifier. Since the gain determines the resolution, it needs to be increased, and can be achieved by using an amplifier having a high S / N ratio. (6) The target characteristic of the phase comparator is the controllability, and in order to enhance the controllability, use a method that can obtain a control output that is as linear as possible with respect to the phase relationship between the error signal and the reference signal. Is achieved by: (7) The characteristic of interest for the signal measuring instrument is the method of measuring what is measured as a signal corresponding to the electrical conductivity. The characteristic of the wound variable resistor at the time of equilibrium as shown in the example of the conventional null method is shown. This is achieved not by a method of visually measuring the position and the scale of the movable contact piece but by a method of measuring a current flowing through a compensation coil that is directly proportional to the electric conductivity of the test solution at the time of equilibrium. A common characteristic of interest for each component is the zero drift and the stability of the amplification factor of the individual amplifier. For the zero drift, an amplifier having extremely high zero drift stability (for example, 0.2 p
pm / ° C., 3 ppm / year), use an amplifier having a sufficient amplification factor at the frequency of the AC power supply, and use an extremely stable resistor (for example, 1 ppm) for determining the amplification factor. / ° C, 25 ppm / year). High stabilization of the zero-point drift of the amplifiers used in AC power supplies, error signal amplifiers, and current regulators can be achieved by connecting capacitors of appropriate values in series, utilizing the fact that the signals to be handled are AC. You. Next, means for solving the above-mentioned problem [2], ie, the structure having no mechanical movable parts, and the above-mentioned problem [3], which can be manufactured at a low cost, include the current described in the above [1]. This is achieved by using a semiconductor variable current device for the regulator. Means for solving the problem [4] that a current or voltage output directly proportional to the electric conductivity is obtained is a method of measuring the current flowing through the compensation coil described in (7) of [1]. Achieved by using As described above, all means for solving the above problems have been described.
Among these means, those which can be solved by the conventional technology and those which are new can be summarized and the current regulator (4) and the current regulator (4) among the seven elements constituting the zero-point conductivity measuring device can be summarized. Since the means for increasing the accuracy and stability of the individual elements except for the signal measuring instrument (7) is a conventionally used technique, the accuracy and stability of these individual elements are
Technically, it can be considered that it has already reached a very high level. Therefore, the problem here is that the conventional zero-conductivity conductivity measuring device requires the current regulator (4) and the signal measuring device (7) to have extremely poor accuracy and stability in the above-mentioned mechanical moving parts. And a method of measuring visually. On the other hand, the zero-point conductivity measuring apparatus according to the present invention includes the above-mentioned semiconductor variable current device in the current regulator (4).
It has been found that the electric conductivity measuring method is characterized in that the electric conductivity of the test liquid is measured using a method of measuring the current flowing through the compensation coil in the signal measuring instrument (7).

[0056]

The structure using the semiconductor variable current device for the current regulator (4) operates as follows. (A) The semiconductor variable current device has a resolution as described in (4) of [1] of the means for solving the above-mentioned problem (hereinafter referred to as means), based on the characteristics of the components constituting the current variable device and the configuration thereof. It has the characteristics of being very high and having a very small distortion rate. The very high resolution also means that the distortion factor of the current regulator (4), the distortion rejection factor of the error signal amplifier (5), and the conductivity measurement accuracy described after (3) in [1] of the above means. As can be seen from the relationship between the two, the fact that the distortion rate is very small acts to make the measurement accuracy of the conductivity measuring device extremely high. (B) The semiconductor variable current device has a low operation speed (for example, 1
Second) Since there is no mechanical moving part and the signal is transmitted by light or current, the response speed to the control signal is very fast (for example, 0.001 second). Speed) has the effect of significantly increasing. (C) Since the semiconductor variable current device does not include any expensive mechanical moving parts, it has an effect that it can be manufactured at low cost. When a method of measuring the current flowing through the compensation coil is used in the signal measuring device (7), an electric signal that is accurately proportional to the electric conductivity of the test liquid is provided. (D) The measurement accuracy of the electric conductivity measuring device is extremely high. It works to raise it. (E) It acts so that a current or voltage output directly proportional to the conductivity is obtained immediately.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings. FIG. 1 is a principle explanatory view showing an embodiment of the present invention.
A secondary coil L ₂ of the coil L ₁ of the primary-side amplitude stable AC power source (1) is connected to the excitation transformer (2), the secondary side of the detection coil L ₃ to the error signal amplifier (5) a liquid path coil L ₂ satisfying the test liquid in an insulating passage formed between the first turn covalently primary coil L ₂ of the connected detector transformer (3),
A phase comparator (6), which is connected to the error signal amplifier and obtains a control output based on a phase relationship between the error signal and a reference signal from an AC power supply, and a current flowing through the fluid path coil is provided to the detection transformer. a compensation coil L ₄ provided in the direction of canceling the magnetic flux generated by, and is connected to the compensating coil and the phase comparator and an AC power source, the semiconductor variable current coils and compensation current regulator (4) with L
_4. A signal measuring device (7) for measuring a current flowing through ₄ ,
Among these elements, the excitation transformer (2) and the liquid path coil L
_The detection transformer ( ₂₎ and the detection transformer (3) housed in a container (not shown) are immersed in an aqueous solution of, for example, hydrofluoric acid as the detection unit (8). The remaining elements are housed in a box (not shown) and installed as a measuring unit (9). Next, the operation principle of the electric conductivity measuring device configured as described above will be described. Assuming that the output voltage of the AC power supply (1) is e ₁ , the conductivity of the test liquid is K (the reciprocal of the liquid path resistance r ₀ is multiplied by the container constant γ), and the number of turns of the coil L ₁ is n ₁ current flowing through the road coil _{L 2} i
₀ is i ₀ = (e ₁ / (γ × n ₁ )) × K −−
(I) and is directly proportional to the conductivity K of the test solution.
On the other hand than the current balance in the detection transformer, the current flowing through the liquid passage coil L _2, the number of turns of the i ₀ compensation coil L ₄ n
_{4 The} current flowing therethrough is i ₄ , and the number of turns of the detection coil L ₃ is n
₃ Assuming that the unbalanced current flowing through this is i ₃ , the following holds: n ₃ × i ₃ = i ₀ −n ₄ × i ₄ − (II) In equilibrium, the left unbalance current _{i 3} is the fact that the zero of formula (II) is made zero n ₄ × _{i 4} = _{i 0}
- (III) by substituting enacted this equation (I) is, and solving for _{i 4 i 4 = (e 1} / (γ × n 1 ×
_{n 4)) × K - (} IV) , and the current _{i 4} flowing through the compensation coil _{L 4} are, since the exact directly proportional to the conductivity K of the test solution, a signal measuring the current _{i 4} flowing through the compensation coil If the measurement is performed by the device (7), the conductivity of the test liquid can be accurately obtained. Conversely suddenly changes conductivity K of the test solution, if it becomes unbalanced _{state, i 3 = (i 0 -n} 4 × i 4) / n 3 that solving for _{i 3} the formula (II) - (V) becomes unbalanced current i ₃ is flows through the detection coil L _3, with respect to a reference signal an error signal obtained by amplifying is obtained from an AC power source (1) in the error signal amplifier the signal (5) A phase comparator (6) that provides a control output that increases or decreases in positive or negative phase, and a current regulator (4) using a semiconductor variable resistor is controlled by the control output to generate a compensation coil. adjusting the current _{i 4} flowing through the L _4. This round to step is repeated in quick succession to the unbalanced current i ₃ becomes zero. In this way, a new equilibrium point corresponding to the conductivity of the test solution is quickly reached (this is called an automatic equilibrium mechanism), and the change in conductivity is automatically followed quickly.
Next, the current regulator (4) is a generic name including two variable current devices (semiconductor variable current devices are singly or plurally or in combination, and therefore, those having a function of adjusting one AC current are hereinafter referred to as variable current devices. constituted by called), it is one of the variable current circuit of the current i ₄ flowing through the compensation coil L _4, so as to automatically balanced with respect to the effective current, also the other variable current devices is self-balancing with respect to the reactive current In this way, the electric conductivity measuring device and the current controller (4) constituted by using the double automatic balancing mechanism (hereinafter referred to as the double balancing method) are constituted by one variable current device, and the compensation coil L ₄ was set to be automatically balanced with respect to the effective current of the current i ₄ flowing through (hereinafter referred to as single equilibrium method)
An example of conducting a performance test of a conductivity measurement device

Embodiment 1 ~

Fifth Embodiment An example in which a concentration measurement was carried out using the above-described measuring device is described below.

Embodiment 6 is shown.

Embodiment 1 In the double balance method, when the above-described VCA, which is a kind of a variable gain type amplifier, is used as a variable current device for adjusting an effective current and a reactive current, a linearity of ± 10 ppm and a resolution of 5 ppm, A stability of 3 ppm / ° C. was obtained.

Embodiment 2 In the double balance method, the above-described VCA is used as a variable current device for adjusting an effective current, and the above-described C is used as a type of semiconductor variable resistor as a variable current device for adjusting a reactive current.
When dS was used, linearity of ± 20 ppm, resolution of 5 ppm, and stability of 3 ppm / ° C. were obtained.

Embodiment 3 In the double balance method, when the above-mentioned CdS is used as a variable current device for adjusting the effective current and the reactive current, the linearity of ± 20 ppm, the resolution of 5 ppm, and the 3 pp
A stability of m / ° C. was obtained.

Embodiment 4 In the single balance method, when the above-mentioned VCA is used as a variable current device for adjusting the effective current, ± 150 p
A linearity of pm, a resolution of 20 ppm and a stability of 20 ppm / ° C. were obtained.

Embodiment 5 In the single balance method, when the above-described CdS is used for a variable current device for adjusting an effective current, ± 150 p
A linearity of pm, a resolution of 20 ppm and a stability of 20 ppm / ° C. were obtained.

Example 6 The concentration of a 55% hydrofluoric acid aqueous solution was measured continuously for about one year by using the single equilibrium method. As a result, the measured concentration and the wet analysis (using a 1N sodium hydroxide solution) were used. All were determined by titration method), all matched with an accuracy of ± 0.1% or less. When the double equilibrium method is used, the accuracy of the measuring instrument exceeds the accuracy of the wet analysis, so if it is intentionally expressed, it is lower than the accuracy of the wet analysis. The response speed of the measuring instrument was 0.01 seconds or less in both the double equilibrium method and the single equilibrium method. Although the preferred embodiments have been described above, the present invention is not limited to these, and various applications are possible. As for the phase comparison method in the phase comparator (6), in addition to the commonly used synchronous rectification method, a method of extracting and comparing a part of an input waveform by switching, a method of converting an input into a square wave and performing logic processing, an analog method A method using a multiplier can also be used. As a method of obtaining a control output from the phase comparison signal, a proportional unit, an integral unit, a derivative unit, or an arithmetic unit (including a unit configured by software) configured by combining these can be used. The signal source of the current regulator (4) is a signal source which processes a signal from an AC power source in addition to the AC power source (1) shown in the embodiment, a coil separately wound around an exciting transformer,
Etc. may be used. For use, it is possible to measure not only the conductivity of various solutions but also the concentration.

[0007]

【The invention's effect】

[A] The measurement accuracy and stability were extremely high. The environmental conditions to be measured are 2
Assuming that the temperature is 3 ° C. ± 30 ° C., as described in the related art, the conventional displacement method is about ± 10000 ppm (± 1%) and the conventional null method is about ± 30000 ppm (± 3%). On the other hand, the double equilibrium method of the present invention described in Examples
/ ° C × (± 30 ° C.) = ± 90 ppm
The measurement accuracy and stability of 0 ppm / ° C. × (± 30 ° C.) = ± 600 ppm were obtained.
17 times higher. [B] The linearity between the conductivity and the measured output was extremely high. As described in the prior art, the conventional displacement method is about ± 10
000 ppm and about ± 30000 ppm in the conventional null method, ± 10 ± 20 ppm in the double equilibrium method of the present invention described in Examples, and ± 150 p in the single equilibrium method.
pm linearity was obtained, so it was 3000 times to 6
7 times higher. [C] The response speed of the conductivity was extremely high. As described in the operation (b) and the sixth embodiment, the conventional null method is about 1 unit.
In the present invention, the response speed was about 0.01 second, which was 100 times faster than the conventional null method. [D] Individualization (simplification) of the automatic balance mechanism was completed. The present invention using an inexpensive semiconductor variable current device does not use any expensive mechanical moving parts as in the conventional null method, and can be completely individualized. And 30% less expensive. [E] A current or voltage output that is exactly directly proportional to the conductivity is obtained. [F] If a compensating coil, an error signal amplifier, a phase comparator, and a current regulator are newly added to the conventional displacement measuring apparatus, a significantly higher-performance conductivity measuring apparatus according to the present invention can be manufactured. , Its migration is good.

[Brief description of the drawings]

FIG. 1 is a principle explanatory view showing an embodiment of the present invention.

[Explanation of symbols]

 (1) AC power supply (2) Excitation transformer (3) Detection transformer (4) Current regulator (5) Error signal amplifier (6) Phase comparator (7) Signal measuring instrument (8) ) Is the detection unit (9) is the measurement unit

Claims (2)

(57) [Claims] An exciting transformer having an AC power supply connected to a primary side, a detecting transformer having an error signal amplifier for removing a distortion wave other than an AC waveform connected to a secondary side, and an exciting transformer connected to the exciting transformer. Conductivity measurement of a solution using a liquid path coil filled with a test liquid in a one-turn insulating path connected to the primary side and the primary side of the detection transformer, and a compensation coil provided in the detection transformer Outputting a control signal based on a phase relationship between an output signal of the error signal amplifier and a signal from the AC power supply, and adjusting a current flowing through the compensation coil by a semiconductor variable current device based on the control signal. Measuring the electric current flowing through the compensating coil to measure the electric conductivity of the solution. 2. An excitation transformer having an AC power supply connected to a primary side, a detection transformer having an error signal amplifier for removing a distorted wave other than an AC waveform connected to a secondary side, and an excitation transformer connected to the excitation transformer. A liquid path coil filled with a test liquid in a one-turn insulating path connected to the primary side and the primary side of the detection transformer, a compensation coil provided in the detection transformer, and an output signal of the error signal amplifier. And a phase comparator that obtains a control output based on the phase relationship between the signal from the AC power supply and a current regulator using a semiconductor variable current device that adjusts a current flowing through the compensation coil based on the control output. And a signal measuring device for measuring a current flowing through the compensating coil.