JPH052185B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a liquid salinity meter, and more particularly to a salinity meter that determines salinity based on the measurement of electrical conductivity between a set of electrodes.

<Prior art> A device equipped with a set of electrodes called a so-called measurement cell and measuring the electrical conductivity K of a liquid by immersing it in the liquid to be measured is disclosed, for example, in U.S. Pat.
It is disclosed in Publication No. 65478. The conductivity meter disclosed in this document has a reference resistor installed in parallel with the measurement cell in order to accurately correct the cell constant J.
Rk, a measurement cell, and a changeover switch for selecting this reference resistance Rk. Therefore, there is an advantage that a measurement value can be obtained without depending on fluctuations in the AC power supply voltage applied to the measurement cell.

When using this device to measure the concentration of, for example, saline water, first perform span calibration by measuring the conductivity of a saline solution with a known concentration, such as 5%, held at a standard temperature of 25°C, and then perform span calibration. , the temperature of the saline solution to be measured is
Conductivity must be measured in the same way while keeping the temperature at 25°C.

<Problems to be solved by the invention> When measuring actual salinity, for example, the salinity of seawater, the salinity of an aquarium tank, the salinity of hot miso soup, the salinity of a flowing liquid during a process in the food industry, etc. It is impossible to maintain the temperature at 25℃.

In addition, keeping a standard test solution on hand and adjusting the span each time the measurement cell is immersed is a good idea.
Because the conductivity of the standard test liquid changes due to intrusion of other liquids or natural evaporation, the test liquid must be replaced frequently with a new one, which is inconvenient in use.

Furthermore, if you carelessly turn the span adjustment volume, subsequent measurements will be meaningless.
You will have to start the calibration process again.

A particularly important problem is that the actual test liquid is rarely an aqueous solution of only Nacl or Kcl, but contains various compositions such as seawater and miso soup, and therefore it is difficult to accurately determine the temperature correction coefficient. This value is unknown and it is not possible to prepare test solutions of standard concentrations in advance.

Therefore, the main object of the present invention is to provide a salinity meter that can be used even when the temperature of the liquid to be measured is other than the standard temperature of 25° C., and does not require the constant provision of a standard test liquid.

Another object of the present invention is that the measurement probe including the measurement cell and the main body are detachable and compatible, and that the cell constant can be changed for each measurement probe once the type of liquid to be measured is specified. The purpose is to provide a salinity meter that is accredited.

Still another object of the present invention is to provide concentration characteristics of conductivity;
Considering that the temperature characteristic is an analog quantity and is almost linear in the practical range, it does not use digital calculation processing or digital memory, and therefore does not include errors caused by digitization, and the salinity concentration is completely continuous. Our goal is to provide the desired salinity meter.

<Means for Solving the Problems> The liquid salinity meter of the present invention includes a measuring cell equipped with a set of electrodes and a temperature sensor immersed in the liquid to be measured, and a variable resistor R connected from the oscillator output. excitation voltage applying means for applying a voltage to one of the set of electrodes through ₁ ; conductivity detection means for amplifying the current of the other of the set of electrodes to obtain a conductivity detection voltage ed;
Temperature compensation means for obtaining a predetermined temperature compensation voltage er based on the output signal of the temperature sensor, calculation means for calculating the quotient of dividing the conductivity detection voltage ed by the temperature compensation voltage er, and an output for displaying the calculation result. and two-circuit changeover switches S ₁ and S ₂ that interlock with each other, the first changeover switch S ₁ has its common terminal C connected to the input end of the conductivity detection means, and its one changeover contact a. is connected to the other 4 of the set of electrodes, another switching contact b is connected to one 3 of the set of electrodes via a fixed resistor Rs having a predetermined resistance value, and the second Changeover switch S ₂
is one in which the common terminal C is connected to the input terminal of the temperature compensating means, one switching contact a is connected to the temperature sensor, and the other switching contact b is connected to a predetermined voltage source. Calibration means.

<Operation> When a measurement cell with a set of electrodes is immersed in a liquid and an excitation voltage e _i is applied to the electrodes, a current flows between the electrodes in proportion to the conductivity of the liquid. For example, the conductivity of pure saline at a temperature of 25℃ is 67.2mS/cm,
It is known that the temperature coefficient α of this is 2.17%/°C.

The conductivity detection voltage _ed depends on the structure of the measurement cell, the voltage of the AC power supply, the setting state of the variable resistor _R1 , the conductivity of the object to be measured, and the gain of the amplifier. The conductivity at this time depends on the temperature. On the other hand, the temperature compensation voltage e _r is a linear function whose variable is the temperature detected by the temperature sensor, as shown in FIG. It is preferable that this slope corresponds to the temperature coefficient α of the conductivity of the object to be measured, but if it contains impurities such as miso soup or seawater, it is approximately 2 to 3%.
Since the slope is within the range of , even a multi-purpose salinity meter can be set to a slope that is acceptable for practical purposes.

The calculation means eliminates a factor {1+α(t-25)} including the temperature t and temperature coefficient α of the conduction detection voltage e _d and the same factor {1+α(t-25)} of the temperature compensation voltage e _r .

The changeover switch is a manual switch for switching between measurement state a and calibration state b. When switched to the calibration state b, the conduction detection voltage e _d and the temperature compensation voltage _er are not introduced into the calculation means, and a voltage that depends only on the setting state of the variable resistor R ₁ is introduced into the output means.
As a result, the displayed value of the output means and the setting position of the variable resistor R ₁ have a first-order correspondence, and therefore, if the variable resistor is set to indicate a pre-calibrated displayed value, it will depend on the structure of the measuring cell. It is possible to correct this variation and the variation of other factors, and if the display value is determined for each type of object to be measured,
Calibration can be easily performed without performing absolute calibration using standard solutions.

<Example> FIG. 1 shows a circuit diagram of an embodiment of the present invention.

Oscillator 1 is an AC oscillator with constant amplitude. A series circuit of capacitor C ₁ and variable resistor R ₁ is connected to the output terminal, and the variable terminal is connected to buffer amplifier 2.
The output line of the buffer amplifier 2 is connected to one electrode 3 of the measurement cell. Therefore, a square wave AC voltage having an amplitude e _i as shown in FIG. 2 is applied as an excitation voltage between the electrode 3 and the ground.

The measurement cell has two electrodes electrically, but in terms of structure, for example, as shown in FIG. , 16 are commonly connected to form one set of electrodes.

The other electrode 4 of the measuring cell is connected to the terminal a of the changeover switch S ₁ , and the common terminal c is input to the amplifier 5. This amplifier 5 has a feedback resistor Rf, detects the current generated by the excitation voltage e _i , and outputs a detected voltage e ₀ . The other terminal b of switch _S1 is connected to electrode 3 of the measuring cell through resistor Rs.
It is connected to the. This resistance Rs is a pseudo resistance of the interelectrode resistance Rc of the measurement cell. Amplifier 5 output
e ₀ is input to the synchronous rectifier circuit 6 and rectified into a DC analog signal. This synchronous rectifier circuit 6 synchronizes with the square wave output of the oscillator 1 and switches the switching elements S ₃ ,
S ₄ alternately turns on and off, charging capacitor C ₃ . The output voltage e _d of this synchronous rectifier circuit 6 is determined by α, the temperature coefficient of conductivity of the liquid to be measured, and the resistance between the electrodes when the measuring cell is immersed in the liquid to be measured at a temperature of 25°C.
When R _c25 is the amplitude of the excitation voltage, e _i is the feedback resistance of the amplifier 5, and Rf is the feedback resistance of the amplifier 5, it is expressed as _ed = Rf·e _i /R _c25 {1+α(t-25)} (1). This voltage _ed is defined as the conductivity detection voltage.

A temperature sensor 7 is attached to the electrode 4 of the measuring cell, and the terminal of this sensor 7 is connected to a temperature detector 8. This temperature detector 8 outputs an analog voltage e _t proportional to the temperature of the measuring cell. Changeover switch _S2 is interlocked with switch _S1 . switch
Contact a of S ₂ is connected to the output terminal of temperature detector 8. The reference voltage Es is applied to contact b of switch _S2 .
A reference voltage e _T = R ₃ /R ₂ + R ₃ · Es ...(2) is introduced, which is obtained by dividing the voltage by two resistors R ₂ and R ₃ . This reference voltage Es may be a predetermined constant voltage source. The relationship between the resistance Rs and the partial pressure ratio of the resistances R ₂ and R ₃ is such that the resistance Rs is a value corresponding to the electrical resistance between the electrodes when the measurement cell is immersed in pure saline solution with a concentration of 5% at a temperature of 25°C. For example, the reference voltage e _T is set to a value corresponding to the temperature detection voltage e _t25 at a temperature of 25°C.

The temperature compensation circuit 9 outputs a temperature compensation voltage e _r =E ₂₅ {1+α(t−25)} (3) where the output voltage er at a standard temperature of 25° C. is E ₂₅ . This equation (3) is shown in Figure 3.

The analog divider 10 performs the division e _d / _er to determine the salinity concentration. (1) (3) From both equations, the division quotient m is m=e _d / e _r = Rf・e _i /R _c25 {1+α(t+25
)}/E ₂₅ {1+α(t-25)}=Rf・e _i /E ₂₅・R _c25 …
…(4) becomes. As is clear from this equation (4), the temperature t of the measurement cell is eliminated.

The display 11 displays the salt percentage. If an A/D converter is used as the analog divider 10, the voltage _er is used as a reference voltage, and the voltage _ed is used as an input voltage (voltage to be converted), a digital output can be obtained immediately.
Note that the measuring device main body side and the measuring probe side are detachably connected by a connector 12.

Here, the coefficient of variation in the interelectrode resistance Rc is taken as the cell constant J, and the variable coefficient of the voltage e _i is j
Then, equation (4) is m=Rf・j・e _i /E ₂₅・JR _c25 ……(4)′ In order to obtain the same salinity concentration m as in equation (4), j=J. In other words, if equation (4) is in the standard state, R _c25 becomes J
If there is a variation of J, the voltage e _i is also variably set by an amount corresponding to J to obtain a predetermined value m. Furthermore, in equation (4)', if switches S ₁ and S ₂ are moved to side b,
E _T is inserted in place of E ₂₅ and Rs is inserted in place of JRC ₂₅ , so that the output display density m' becomes m'=Rf·Je _i /E _T R _S. Here, Rf/ _ETRc is a constant, and the displayed value m' is proportional to _the cell constant J. In other words, it was set to correct the variation in cell constant J.
Depending on Je _i, m' obtains a value proportional to J.

Therefore, for example, adjust e _i so that the salinity display m displays 5% in a standard test solution of 5% saline.
The value of m' when the switches S ₁ and S ₂ are switched to the b side after setting Je _i becomes the calibration value.

If the above calibration is performed using a standard test solution with a predetermined electrode, the calibration work will no longer be required every time a measurement is made by setting the above-mentioned calibration value.

Next, how to use the above embodiment will be explained.

The manufacturer of this salinity meter, for example, maintains pure salt water with a concentration of 5% at a temperature of 25°C as a standard test liquid, immerses the electrodes 3 and 4 of the measurement cell in this, and then displays the indicator 11 at 5%.
Adjust variable resistor R ₁ to indicate %.
Leave the setting of variable resistor R ₁ as it is, change the changeover switches S ₁ and S ₂ from the A contact to the B contact side, read the displayed value (%) at that time, and record this as the calibration value. . This calibration value is a unique numerical value used when this measuring cell measures the salt concentration of saline water, and is recorded in the measuring cell. In other words, this calibration value means a set target value.

The user first switches the changeover switches S ₁ and S ₂ to the b contact side and adjusts the variable resistor R ₁ to the displayed value (%) written on the measurement cell. This completes the calibration process. After that, the variable resistor R ₁
If the concentration of the saline solution is measured while the setting state of is fixed, the measurement can be performed without being affected by the temperature at the time of measurement. In the above embodiment, a resistor Rs is connected to the b contact circuit of switch _S1 ,
Although a voltage divider circuit of resistors R ₂ and R ₃ is connected to the b-contact circuit of switch S ₂ , the built-in calibration means of the present invention is not limited to the above embodiment, and can be used, for example, by switching the feedback resistor Rf of the amplifier 5, It can also be implemented by alternative means, such as selectively inserting an attenuator into the output circuit of the synchronous rectifier circuit 6 or selectively inserting an attenuator into the output circuit of the temperature compensation circuit 10.

FIG. 4 shows a circuit diagram of a modified embodiment of the present invention. The difference between this embodiment and the one in FIG.
In order to compensate for variations in the temperature sensor 7, the temperature detector 8 is provided with a semi-fixed resistor _R4 . That is, when the equivalent circuit of the temperature detection circuit is expressed as shown in FIG. 5, the temperature detection voltage e _t is e _t =Rt/Rt+R ₄ +R ₅ ·Es (5). Here _, if the variation coefficient of sensor resistance Rt is x, then Rt in the above equation can be replaced by xRt, but in order to keep e _t unchanged, (R ₄ + R ₅ ) can also be replaced by
+R ₅ ). This replacement is realized by adjusting the semi-fixed resistor _R4 .

FIG. 6 shows a circuit diagram of another modified embodiment of the present invention. The difference between this embodiment and the one in FIG.
In order to eliminate variations in the calibration values recorded on the measurement cells and to ensure that every measurement cell has a fixed calibration value of, for example, 10%, that is, the set target value, the pseudo resistance is used.
The reason is that Rs has been moved to the probe side of the measurement cell.

In other words, contact b of switch _S1 is connected to connector 12.
is connected to the terminal on the main body side, and a pseudo resistor Rs is connected to the corresponding probe side.
is calibrated by a semi-fixed resistor.

Therefore, the user must turn on the selector switch before use.
The calibration work is completed by simply switching S ₁ and S ₂ to the b contact side and adjusting the variable resistor R ₁ so that the displayed value is, for example, 10% in any measurement cell.

<Effects of the Invention> According to the present invention, the temperature coefficient α of the conductivity of the object to be measured is eliminated by dividing the conductivity detection voltage related to the current of the measurement cell by the temperature compensation voltage.
Measurements can be taken even at temperatures other than the standard temperature of 25°C. In addition, the calibration work can be completed by simply switching the switch to the calibration side and setting the variable resistor so that the value on the display becomes the predetermined value.
There is no need to keep standard test solutions on hand, making it extremely convenient to use.

Further, according to the present invention, the measuring meter main body and the measuring probe can be configured to be detachable, and the main body and the measuring probe can be manufactured separately, making it easier to manage the manufacturing process. The interchangeability of the probes is very convenient for users.

Furthermore, according to the present invention, the conductivity detection means, temperature compensation means, and calculation means are constructed from analog circuits, which simplifies the circuit configuration and allows for continuous and highly accurate measurement without the errors associated with digitization. value is obtained.

Furthermore, according to the present invention, the calibration operation can be easily performed for any liquid to be measured without requiring an A/D converter, a digital computer, or computer software. It has become possible to provide a general-purpose, high-precision salinity meter that can be used for various purposes at a low cost.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a waveform diagram of the excitation voltage e _i in FIG. 1, FIG. 3 is a characteristic diagram of the temperature compensation circuit 9 in FIG. 1, and FIG. FIG. 5 is an equivalent circuit diagram of the temperature detection circuit of FIG. 4, and FIG. 6 is a circuit diagram showing another modified embodiment of the present invention. FIG. 7 is a sectional view showing an example of the measurement cell of the present invention.

Claims (1)

[Claims] 1. A measuring cell with a set of electrodes and a temperature sensor immersed in the liquid to be measured, and one of the set of electrodes 3 from the oscillator output through a variable resistor _R1 .
an excitation voltage applying means for applying a voltage to the electrodes, and a conduction detection voltage by amplifying the current of the other one of the pair of electrodes
conductivity detection means for obtaining a predetermined temperature compensation voltage er based on the output signal of the temperature sensor; and calculation means for computing a quotient obtained by dividing the conductivity detection voltage ed by the temperature compensation voltage er. ,
It is equipped with an output means for displaying the calculation result, and two-circuit changeover switches S ₁ and S ₂ that interlock with each other, and the first changeover switch S ₁ has its common terminal C connected to the input terminal of the conductivity detection means. , one switching contact a
is connected to the other one of the pair of electrodes 4, and another switching contact b is a fixed resistor Rs having a predetermined resistance value.
The second changeover switch _S2 has its common terminal C connected to the input end of the temperature compensation means, and its one changeover contact a and a calibration means connected to the temperature sensor, the other switching contact b being connected to a predetermined voltage source. 2. A patent in which the temperature compensation voltage er is a function of {1+α(t-25)}, where α is the temperature coefficient of conductivity of the liquid to be measured, and t (Celsius) is the temperature of the liquid at the time of measurement. A liquid salinity meter according to claim 1. 3. Claim 1, wherein the measuring instrument body and the measuring probe including the set of electrodes are configured to be detachable, and the fixed resistor Rs is a semi-fixed resistor provided on the measuring probe side. Salinity meter for the liquid described. 4 When setting the variable resistor R ₁ with the first and second changeover switches S ₁ and S ₂ of the calibration means being switched to the other switching contact b side,
A liquid salinity meter according to claim 1, 2 or 3, wherein the display value of the output means corresponding to the set target position is recorded on the measurement probe.