JPH06201634A - Method and apparatus for measuring moisture - Google Patents

Abstract

PURPOSE:To provide a method and an apparatus for accurately and continuously measuring moisture content (water content) even in the case of substance mixed with reducing substance or polar solvent having a high electric conductivity. CONSTITUTION:Substance 14 to be measured is provided between two parallel plate electrodes 10A and 10B, the capacitance Cx between the electrodes is detected by utilizing a transient phenomenon due to charging/discharging, and water content of the substance to be measured is measured by utilizing the relationship between the capacitance and the water content of the substance in which the water content is previously known.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a moisture measuring method and a moisture measuring device for detecting moisture contained in a substance as a moisture content.

[0002]

2. Description of the Related Art Conventionally, the Karl Fischer method has been known as a general method for measuring the water content of a substance. However, according to the Karl Fischer method, when the substance to be measured contains a reducing substance, the water content cannot be measured, and continuous measurement is difficult. By the way, as a method for enabling continuous measurement, there is a method disclosed in JP-A-60-259943. This method utilizes a change in the impedance of the sensor unit due to the adsorption of water on the sensor unit.

[0003]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The method disclosed in Japanese Patent Publication No. 9943 can be applied to a solvent in which water is relatively insoluble, such as Henry's law, but cannot be applied to polar solvents having high electric conductivity such as ethanol. . This is because the impedance change of the sensor portion is more likely to be affected by the adsorption of the solvent than the adsorption of water. Therefore, in the above method, the reaction becomes dull with respect to the change of the water concentration in the polar solvent.

Therefore, the present invention solves the above conventional problems,
Moisture measurement that can measure moisture continuously with good reproducibility even for samples that are difficult to measure with conventional methods such as substances to be measured that contain reducing substances or polar solvents with high electrical conductivity An object is to provide a method and a moisture measuring device.

[0005]

In order to achieve the above object, in the present invention, the moisture content (also called water content) of a substance to be measured is detected by utilizing a transient phenomenon.
I am trying to measure. That is, according to the present invention, 2
The substance to be measured is filled between the parallel plate electrodes, and the capacitance between the two parallel plate electrodes is detected by utilizing a transient phenomenon due to charge / discharge, and the static content of the substance whose moisture content is known in advance is detected. A moisture measuring method for measuring the moisture content of the substance to be measured by using the relationship between the capacitance and the moisture content is provided.

Further, according to the present invention, as an apparatus for realizing the above method, two parallel plate electrodes arranged to form a capacitor when the substance to be measured is filled between the parallel plate electrode and the capacitor are connected in series. Known resistor, a switch connected in series with the capacitor and the resistor, a DC power source for applying a voltage to a series circuit of the capacitor, the resistor and the switch, and between both ends of the resistor. There is provided a moisture measuring device having means for measuring a voltage and means for measuring a voltage across the capacitor.

[0007]

Since the present invention has the above-described structure, the relative permittivity of the substance is detected by detecting the capacitance of the capacitor formed by the two parallel plate electrodes and the substance to be measured filled between them. However, it becomes possible to easily measure the water content continuously.

[0008]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a perspective view showing a sensor unit of a measuring device for measuring the water content in a substance as a water content according to the present invention. The substance that can be measured in the present invention is not particularly limited as long as it has a relative dielectric constant different from that of water. In the case of a liquid, it can be applied to a liquid containing an electrolyte such as a reducing substance, or a polar solvent such as ethanol having a relatively high electric conductivity. In the case of a solid, it is applicable as long as it does not have voids, and an operation for filling a measurement sample between the electrodes is required at the time of measurement. The sensor unit 10 is formed by a pair of copper parallel plates 10A and 10B, the distance between electrodes is 3 mm, and the size of the electrodes is 7 × 20 mm.
To form a parallel plate capacitor. The sensor unit 10 is attached to the insulating support member 16 and is located inside the container 12.
Therefore, when the substance to be measured 14 is placed in the container 12, the substance to be measured 1 is placed between the two opposing parallel plate electrodes 10A and 10B.
Filled at 4. Reference numeral 18 is a cable containing two lead wires for connecting to the electrodes 10A and 10B. In the case of a parallel plate capacitor, its capacitance is given by the following equation.

[0009]

[Equation 1] C = εS / d C: capacitance ε: permittivity d: distance between electrodes S: electrode area

In order to increase the change in capacitance with respect to the change in dielectric constant, the distance d between the electrodes should be narrowed and the electrode area S should be large, and even substances with a small difference in dielectric constant from water can be measured. Becomes However, when the measurement sample contains an electrolyte, or when the electrical conductivity is relatively high, such as with a polar solvent, the resistance between the electrodes decreases, so the sampling time is reduced in the method for detecting capacitance changes described below. Need to be short.

FIG. 2 is a circuit diagram showing a circuit of a moisture measuring apparatus for measuring the water content of a substance to be measured by using the sensor section 10 shown in FIG. Resistance R _x surrounded by a dotted line in FIG.
The parallel circuit of the capacitor C _x and the capacitor C _x is an equivalent circuit of the sensor unit 10 with the substance 14 to be measured filled in between. That is, in the case where the sensor unit 10 is the one shown in FIG. 1, the substance to be measured 14 is placed in the container 12 and the sensor 1
It is assumed that the space between the zero electrodes 10A and 10B is filled with the substance to be measured 14. R ₁ is a resistor connected in series with a parallel circuit of a resistor R _x and a capacitor C _x , and SW is an open / close type switch connected in series with the parallel circuit and the resistor R ₁ . A DC power source E is connected to the parallel circuit and a series circuit of a resistor R ₁ and a switch SW which are connected in series with the parallel circuit.

In this embodiment, the electromotive force of the DC power source E is 4.0V and the resistor R ₁ is 500kΩ. A voltmeter VM ₁ is connected in parallel to the resistor R ₁ , and the resistor R 1
_The voltage between both terminals of ₁ can be measured. Similarly, another voltmeter VM ₂ is connected in parallel to the sensor unit 10, that is, the parallel circuit of the resistor R _x and the capacitor C _x , and the voltage between both terminals of the resistor R _x can be measured.

Next, the measurement procedure will be described. (1) Before measurement: Switch SW is off (2) Inter-electrode resistance R _x is measured: Switch SW is turned on, the voltage values V ₁ and V ₂ of the _two voltmeters VM _{1 and} VM ₂ are read, and from the following equation Ask. R _x = (V ₂ / V ₁ ) × R ₁ However, dV ₂ / dt = 0 (3) Measurement of capacitance C _x of the sensor unit 10: switch S
W is turned off from the on state, and is calculated from the voltage value V _2t of the voltmeter VM ₂ at the time when a predetermined time Δt has elapsed and the voltage value V ₂₀ (the value immediately before the switch SW is turned off) by the following equation. _{C x = △ t / R x} / l n (V 20 / V 2t)

To further explain the measurement of the capacitance C _x , after the switch SW is turned off, the voltage value V ₂ of the voltmeter VM ₂ decreases exponentially as shown in the waveform diagram of FIG. The predetermined time Δt is the time from the start of discharge until the voltage V ₂ between the electrodes of the sensor unit 10 is read, and this is called the sampling time. This sampling time △
If t is too large, the voltage value V ₂ is sufficiently attenuated as shown in FIG.
It becomes difficult to detect the change of ₂ . In the above embodiment, Δt was 0.18 seconds. That is, when the waveform changes gently, it is difficult to detect a voltage change due to a slight capacitance change when continuously measuring,
In the waveform diagram of FIG. 3, it is necessary to set the sampling time Δt where the voltage draws an appropriate curve after the start of discharge. Therefore, before starting the measurement of the substance to be measured, it is necessary to charge and discharge it under the same conditions as the measurement, and to set the sampling time Δt which seems to be appropriate in advance.

The capacitance C _x can be known by the above method. Next, a method of finding the moisture content of the substance 14 to be measured from the capacitance will be described. The capacitance and the moisture content have a linear function, and calibration is performed in advance to know the relation between the moisture content and the capacitance by measuring by a titration method such as the Karl Fischer method. By using the relationship thus obtained, the moisture content can be known by knowing the capacitance.

A measurement example according to the present invention will be described below. (Measurement Example 1) Tertiary butyl alcohol (46.2
FIG. 4 shows the results obtained by adding pure water with a microsyringe to a sample in which ascorbic acid (0.23 g) was dissolved in g) and measuring the water content different. As can be seen from FIG. 4, when the sampling time Δt is lengthened, it becomes impossible to detect the change in capacitance due to the change in water content. Also, it can be seen that the capacitance increases as the water content increases.

(Measurement Example 2) In Measurement Example 1, a sample prepared by dissolving ascorbic acid (0.10 g) in ethanol (40.4 g), which is one of polar solvents instead of tertiary butyl alcohol, was used. The measurement result is shown in FIG. It can be seen from FIG. 5 that the linearity of the measured values is high even with the polar solvent containing the electrolyte, and the measurement can be performed with good reproducibility and accuracy.

(Measurement Example 3) FIG. 6 shows the measurement results when an acetic acid aqueous solution having a conductivity about 10 times larger than that of ethanol was used as a sample instead of ethanol. However, no electrolyte was mixed. From this figure, it can be seen that even if the electric conductivity is high, the measurement can be performed as in the measurement example 2.

(Measurement Example 4) Samples were sampled in the process of removing water from ethanol using a molecular sieve and the results of the water content measured by the Karl Fischer method and the measured values of the capacitance measured by the method of the present invention Is shown in FIG. It can be seen from FIG. 7 that the measured values of the Karl Fischer method correspond well.

In the present invention, the capacitance of the capacitor is detected by utilizing the transient phenomenon by charging / discharging the capacitor of the sensor unit 10 by the circuit shown in FIG. 2, and thus the moisture content is measured. By automating each step of this measurement, continuous measurement can be easily realized.

FIG. 8 is a block diagram showing another embodiment of the present invention for realizing such automatic measurement. In FIG. 8, the personal computer 20 has a CPU (central processing unit), necessary memories such as RAM and ROM, and an interface. Personal computer 20
Is connected to the relay circuit 24 via the D / A converter 22, and the relay circuit 24 is connected to the sensor circuit 26. The sensor unit 10 shown in FIG. 1 is connected to the sensor circuit 26, and the output of the sensor circuit 26 is connected to an A / D converter 30 via a low pass filter (LPF) 28. The A / D converter 30 is connected to the personal computer 20.

FIG. 9 is a circuit diagram showing a specific structure of the sensor circuit 26 shown in FIG.

In FIG. 9, those parts which are substantially the same as those corresponding parts in FIG. 2 are designated by the same reference numerals. The connection point between the resistor R ₁ and the sensor unit 10 is connected to the + input terminal of the operational amplifier 32, and the − input terminal thereof is connected to the connection point of the resistor R ₂ and the amplification degree adjusting variable resistor R ₃ . The other end of the resistor R ₂ is grounded, and the other end of the variable resistor R ₃ is connected to the output terminal of the operational amplifier 32. The series circuit of the resistor R ₄ and the capacitor C ₁ constitutes the low pass filter (LPF) 28 shown in FIG. 8, and is connected between the output terminal of the operational amplifier 32 and the other end of the sensor unit 10. The voltage across the capacitor C ₁ is supplied to the A / D converter 30, converted into digital data, and input to the personal computer 20. The switch S
W is an on / off operation in the relay circuit 24 of FIG.

FIG. 10 is a flow chart showing the operation for measurement in the personal computer 20. After the switch SW is turned off in step S4, if a measurement start command is given, step S5 becomes YES, and the switch SW is turned on in step S6. The switch SW is turned on and off by the relay circuit 24,
The relay circuit 24 is controlled by sending a necessary signal to the D / A converter 22. A semiconductor switching element may be used instead of the relay circuit 24.
In step 7, V ₂ is sampled (V ₂₀ is calculated), and in step S8, dv ₂ / dt <ε (ε = 1
It is confirmed that the steady state is reached by judging 0 ^-5 ), and if YES, _Rx is calculated in step S9, and then the switch SW is turned off in step S10. The timer measures the time from the time of turning off, and when Δt has elapsed, V ₂ is sampled again in step S11 (V ₂ t is obtained). Using this value, step S1
Calculate C _x at 2. In the calculation of C _x , the above V
₁ is obtained by calculation with V ₁ = E−V ₂ .

It is assumed that Δt is set in advance to the desired value determined as described above in step S2.
The data files of step S3 and step S14 are for recording the measurement results, and are for recording the measurement date and time, the measurement object, the list of measurers in advance, and also recording and storing the actual measurement data. . The measurement result is step S
At 13, it can be displayed by a display or a printer.

[0026]

As described above, in the moisture measuring method and the moisture measuring apparatus of the present invention, the substance to be measured is filled between the two parallel plate electrodes, and the two parallel plate electrodes and the substance to be measured are used. By charging and discharging the configured capacitor, a transient phenomenon is produced, and the water content is measured by detecting the capacitance, so high conductivity, which is difficult to measure by the titration method such as Karl Fischer method, is measured. It is possible to accurately measure the water content of a wide range of substances to be measured including the above samples. Also, because it is a method of detecting capacitance rather than titration method, even if the water content changes moment by moment,
It is possible to measure continuously.

[Brief description of drawings]

FIG. 1 is a perspective view showing an example of a parallel plate electrode and a container for a substance to be measured, which are used in a moisture measuring method and a moisture measuring apparatus of the present invention.

FIG. 2 is a circuit diagram showing one embodiment of the moisture measuring device of the present invention.

FIG. 3 is a waveform diagram showing a change in inter-electrode voltage during discharge of the sensor unit.

FIG. 4 is a graph showing an example in which a change in capacitance with respect to a change in water content of an organic solvent mixed with a reducing substance is measured by the method of the present invention.

FIG. 5 is a graph showing an example in which a change in capacitance with respect to a change in water content of a polar organic solvent mixed with a reducing substance is measured by the method of the present invention.

FIG. 6 is a graph showing an example in which a change in capacitance with respect to a change in water content of a polar organic solvent was measured by the method of the present invention.

FIG. 7 is a graph showing the correspondence between the water content measurement result by the Karl Fischer method and the capacitance measurement result by the present invention.

FIG. 8 is a block diagram showing another embodiment of the moisture measuring apparatus of the present invention.

9 is a circuit diagram showing a specific configuration of the sensor circuit shown in FIG.

10 is a flow chart for explaining the operation of the personal computer shown in FIG.

[Explanation of symbols]

10 sensor part 10A, 10B electrode 12 container 14 substance to be measured (sample) 20 personal computer 22 D / A converter 24 relay circuit 26 sensor circuit 28 low-pass filter (LPF) 30 A / D converter C ₁ capacitor C _x static of sensor part Capacitance E DC power supply R ₁ , R ₂ , R ₃ , R ₄ Resistor and its resistance value R _x Equivalent DC resistance of sensor SW switch VM ₁ , VM ₂ Voltmeter

Claims (4)

[Claims] 1. A substance to be measured is filled between two parallel plate electrodes, and the electrostatic capacitance between the two parallel plate electrodes is detected by utilizing a transient phenomenon due to charge and discharge, and the moisture content is measured in advance. A moisture measuring method for measuring the water content of the substance to be measured, using the relationship between the capacitance and the water content of a known substance. 2. A parallel plate electrode and a capacitor made of the substance to be measured, which is filled between the parallel plate electrode and a resistor, are connected in series, and a direct current voltage is applied to the series circuit via a switch to make a steady state. The moisture measuring method according to claim 1, wherein the capacitance is detected by reading a voltage across the capacitor at a time point when a predetermined time has elapsed after the switch is turned off to cause a transient phenomenon and the switch is turned off. 3. The resistance value of the resistor is R ₁ , the DC resistance value across the capacitor is R _x , the predetermined time is Δt, and the voltage across the capacitor in the steady state is V _20. and then, when the voltage between said capacitor ends at the time has elapsed the predetermined time △ t and V _2t, the capacitance _{C x, C x = △ t} / R x / l n (V 20 / V _2t )
The water content measuring method according to claim 2, which is obtained from 4. Two parallel plate electrodes arranged to form a capacitor when filled with a substance to be measured, a known resistor connected in series to the capacitor, the capacitor and the resistor. Switch connected in series to the container,
Moisture measurement having a DC power supply for applying a voltage to a series circuit of the capacitor, the resistor and the switch, means for measuring the voltage across the resistor, and means for measuring the voltage across the capacitor. apparatus.