JPS6285852A - Salinometer for liquid - Google Patents

Abstract

PURPOSE:To make measurement of a liquid to be measured even if said temp. is other than 25 deg.C reference temp. by dividing the conductivity detection voltage relating to the current of a measuring cell by a temp. compensation voltage. CONSTITUTION:The measuring cell having a pair of electrodes 3, 4 is immersed into the liquid and when an excitation voltage ei is impressed to the electrodes 3, 4, the current flows between the electrodes 3 and 4 in proportion to the conductivity of the liquid. The conductivity detection voltage ed outputted from a synchronous rectifier circuit 6 depends on the construction of the measuring cell, the voltage ei of an AC power source, the set condition of a variable resistor R1, the conductivity of the material to be measured and the gain of an amplifier circuit 9 and the conductivity in this stage depends on the temp. On the other hand, the temp. compensation voltage er outputted from a temp. compensation circuit 9 is a linear function with the temp. detected by a temp. sensor 7 as a variable and the gradient thereof coincides preferably with the temp. coefft. alpha of the conductivity of the material to be measured but the gradient which is substantially of no problem can be set even with a salinometer for multiple purposes. The factors including the temp. (t) and coefft. alpha of the voltage ed and the same factors of the voltage er are divided in a divider 10, by which the temp. (t) of the measuring cell is erased.

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 the liquid by immersing it in the liquid to be measured is disclosed in, for example, Japanese Utility Model Application Publication No. 56-65478. Disclosed. In order to accurately correct the cell constant J, the conductivity meter disclosed in this document has a reference resistance Rk provided in parallel with the measurement cell, and a reference resistance Rk between the measurement cell and the reference resistance Rk.
The feature is that it is equipped with a selector switch.

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. Next, conductivity must be measured in the same manner while keeping the temperature of the saline solution to be measured 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°C.

In addition, it is not recommended to always keep a standard test solution and adjust the span each time a measurement is made because other liquids may enter the measurement cell or the conductivity of the standard test solution may change due to natural evaporation each time the measurement cell is immersed. It is inconvenient to use because the test solution must be replaced with a new one.

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

A particularly important issue is that the actual test liquid is Nacl or K.
It is rather rare that the solution is an aqueous solution containing only Cl; it contains various compositions such as seawater and miso soup. Therefore, the exact value of the temperature correction coefficient is unknown, and the test solution of the standard concentration is not used. This cannot be made 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.

Yet another object of the invention is 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 connects a set of electrodes immersed in the liquid to be measured, and a variable resistor (R1) from an AC power source. an excitation voltage applying means for applying a voltage to one of the electrodes of the set; a conductivity detection means for amplifying the current of the other of the electrodes at a predetermined amplification factor to obtain a conductivity detection voltage (ed); a temperature sensor provided on one side or in the vicinity of the electrode; and a temperature compensation means for outputting a temperature compensation voltage (er) that is a function of the temperature coefficient (α) of the conductivity of the liquid to be measured based on the output signal of the temperature sensor. and the quotient obtained by dividing the conduction detection voltage (ed) by the temperature compensation voltage (er) (
ed/er), and a signal related to the current of the set of electrodes and a signal related to the output voltage of the temperature sensor, which are provided in each of the conductivity detection means and the temperature compensation means, and are interlocked with each other. The variable resistor is characterized by having a changeover switch that cuts off both of the signals, and an output means that displays an output that depends only on the setting state of the variable resistor when the changeover switch cuts off both of the signals.

<Operation> When a measurement cell having a set of electrodes is immersed in a liquid and an excitation voltage (ei) is applied to the electrodes, a current flows between the electrodes in proportion to the conductivity of the liquid. For example, it is known that the conductivity of pure saline at a temperature of 25°C is 67.2 Ms/cm, and its temperature coefficient α 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, as shown in Fig. 3, the temperature compensation voltage (e') is a linear function with the temperature detected by the temperature sensor as a variable. However, for substances containing impurities such as miso soup and seawater, the salt content is approximately within the range of 2 to 3%, so even with a multi-purpose salt needle, it is practically useless. Can be set to no slope.

The calculation means calculates a factor (1+α(t-25)1) including temperature (1) and temperature coefficient (α) of the conduction detection voltage (ed), and the same factor (I+α(t-25) of the temperature compensation voltage (er)). 25))).

The changeover switch is a manual switch for switching between the measurement state (a) and the calibration state (b). When switched to the calibration state (b), the introduction of the conduction detection voltage (ed) and temperature compensation voltage (er) to the calculation means is cut off, and the voltage that depends only on the setting state of the variable resistor (R1) is outputted to the output means. will be introduced in As a result, the display value of the output means and the setting position of the variable resistor (R1) have a first-order correspondence relationship, and therefore, if the variable resistor is set so as to indicate the pre-calibrated display value, the measurement cell structure It is possible to correct variations in this and 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 a standard solution.

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

Oscillator l is an alternating current oscillator with constant amplitude. A series circuit of a capacitor C1 and a variable resistor R1 is connected to its output terminal, and its variable terminal is connected to the power line of the buffer amplifier 20, and the output line of the buffer amplifier 2 is connected to one electrode 3 of the measurement cell.
It is connected to the.

Therefore, a square wave AC voltage having an amplitude ei as shown in FIG. 2 is applied as an excitation voltage between the electrode 3 and the ground.

The measurement cell is electrically composed of two electrodes, but structurally it has three electrode plates 1 in a cylinder 13, as shown in FIG.
4, 15, and 16 are arranged, and those at both ends 14 and 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 S1.
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 ei, and outputs a detected voltage eO. The other terminal of the switch Sl is connected to the electrode 3 of the measuring cell through a resistor Rs. This resistance R9 is a pseudo resistance of the interelectrode resistance Rc of the measurement cell. amplifier 5
The output eo is input to the synchronous rectifier circuit 6 and rectified into a DC analog signal. This synchronous rectifier circuit 6 operates in synchronization with the square wave output of the oscillator 1 to switch nine elements s3. S4 alternately turns on and off, charging capacitor C3. The output voltage ed of this synchronous rectifier circuit 6 is determined by α, the temperature coefficient of conductivity of the liquid to be measured, RC25, the resistance between the electrodes when the measuring cell is immersed in the liquid to be measured at a temperature of 25°C, and the amplitude of the excitation voltage. is expressed as ei, and the feedback resistance of the amplifier 5 is Rf. This voltage ed is defined as a conduction 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 et proportional to the temperature of the measuring cell. Changeover switch S2 is switch Sl
It is linked with A contact a of the switch S2 is connected to the output terminal of the temperature detection voltage. A reference voltage R2+R3 obtained by dividing the reference voltage Es by two resistors R2 and R3 is introduced into the contact point of the switch S2. 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 R2 and R3 is as follows, if the resistance Rs corresponds 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 er is at a temperature of 25"C
The temperature detection voltage et25 is set to a value corresponding to the temperature detection voltage et25.

The temperature compensation circuit 9 has an output voltage er at a standard temperature of 25° C. as R25, and a temperature compensation voltage er=E2q (1
+α(t-25) l -+31 is output. This (3
) formula is shown in Figure 3.

The analog divider lO divides to find the salinity concentration e d
Run /er. ill (From Equation 31, the division quotient m is R25(1+α(t-25)).As is clear from Equation (4), the temperature t of the measurement cell has been eliminated.

Display 1) displays the salt percentage. Analog divider 1
0, use an A/D converter, set the voltage e' as the reference voltage,
If the voltage ed is used as the 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, if there is variation in the interelectrode resistance Rc, and its coefficient is the cell constant J, and the variable coefficient of the voltage ei is j, then (
Equation 4) is the same as Equation (4), and in order to obtain the salinity concentration m, it becomes j-J. That is, assuming that equation (4) is in the standard state, if there is a variation in RC25 by J, the voltage ei is also variably set by an amount corresponding to J to obtain a predetermined value m.

Furthermore, in formula (4) 7, if switches sl and s2 are turned to the b side, ET will be set instead of R25, and R will be set instead of JRC25.
s is inserted, the output display density m' becomes TR3, and the value m' becomes proportional to the cell constant J. That is,
Jei set to correct variations in cell constant J
Therefore, m' obtains a value proportional to J.

Therefore, for example, after adjusting ei so that the salinity display m displays 5% in a standard test solution of 5% saline and setting Jei, the value of m' when switches Sl and S2 are switched to the b side is This is 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.

For example, the manufacturer of this salt needle maintains pure saline solution with a concentration of 5% at a temperature of 25°C as a standard test solution, dips the electrode 3.4 of the measuring cell in this solution, and displays the indicator 1) indicating that the concentration is 5%. Adjust variable resistor R1 as indicated. Leave the setting state of the variable resistor R1 as it is and switch the changeover switches Sl and S2.
is switched from the a contact to the b contact side, and the displayed value at that time (%
) and record this as the calibration value. This calibration value is a unique value used when this measurement cell measures the salt concentration of saline water, and is recorded in the measurement cell. In other words, this calibration value means a set target value.

The user first switches the changeover switches Sl and S2 to the b contact side and adjusts the variable resistor R1 to the displayed value (%) recorded in the measurement cell. This completes the calibration process. Thereafter, by measuring the concentration of the saline solution while keeping the setting state of the variable resistor R1 fixed, the measurement can be performed without being affected by the temperature at the time of measurement. In the above embodiment, the resistor Rs was connected to the b-contact circuit of the switch S1, and the voltage dividing circuit of resistors R2 and R3 was connected to the b-contact circuit of the switch S2. For example, the feedback resistance Rf of the amplifier 5
It can also be implemented by alternative means, such as switching, selectively inserting an attenuator into the output circuit of the synchronous rectifier circuit 6, selectively inserting an attenuator into the output circuit of the temperature compensation circuit 10, etc.

FIG. 4 shows a circuit diagram of a modified embodiment of the present invention.

This embodiment differs from the one shown in FIG. 1 in that a semi-fixed resistor R4 is provided in the temperature sensor 8 in order to compensate for variations in the temperature sensor 7.

That is, when the equivalent circuit of the temperature detection circuit is expressed as shown in FIG. 5, the temperature detection voltage et becomes Rt +R4+R5. Here, if the variation coefficient of the sensor resistance Rt is X, then R1 in the above equation can be replaced with
). 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. This embodiment differs from the one in FIG. 1 by eliminating variations in the calibration values recorded in the measurement cells, so that each measurement cell has a fixed calibration value of, for example, 10%, that is, a set target value. , the pseudo-resistance Rs is moved to the probe side of the measurement cell.

That is, a contact point of the switch Sl is connected to a terminal on the main body side of the connector 12, and a pseudo resistor Rs is connected to the corresponding probe side, and this resistor Rs is calibrated by a semi-fixed resistor.

Therefore, the user must press the selector switch Sl before use.

The calibration work is completed by simply switching S2 to the b contact side and adjusting the variable resistor R1 so that the displayed value is, for example, 10% in any measurement cell.

<Effects of the Invention> According to the present invention, by dividing the conductivity detection voltage related to the current of the measurement cell by the temperature compensation voltage, 1) the temperature coefficient α of the conductivity of a constant object is eliminated; Standard temperature 25°C
Measurements can be made even at other times. In addition, the calibration work can be completed by simply switching the switch/notch to the calibration side and setting the variable resistor so that the value on the display becomes the predetermined value, so there is no need to keep standard test solutions on hand. , it is very 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 compatibility of the probes is also convenient for the user.

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.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a waveform diagram of the excitation voltage ei in FIG. 1, FIG. 3 is a characteristic diagram of the temperature compensation circuit 9 in 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 invention. FIG. 7 is a sectional view showing an example of the measurement cell of the present invention. Patent applicant: Hidetoshi Sugimori Representative of Merhub Trading Co., Ltd. Patent attorney Nishi 1) Novelty, -6 pillars temperature

Claims (5)

[Claims] (1) A set of electrodes immersed in the liquid to be measured, an excitation voltage applying means for applying voltage from an AC power source to one of the set of electrodes through a variable resistor (R_1), and A conductivity detection means for obtaining a conductivity detection voltage (e_d) by amplifying the current of the other electrode at a predetermined amplification factor, a temperature sensor provided on one side of the set of electrodes or in the vicinity thereof, and an output signal of the temperature sensor. temperature compensation means for outputting a temperature compensation voltage (e_r) that is a function of the temperature coefficient (α) of the conductivity of the liquid to be measured based on the temperature coefficient (α) of the conductivity of the liquid to be measured; Quotient (e_d/e_r
), and a switching device which is provided in each of the conductivity detecting means and the temperature compensating means and cuts off a signal related to the current of the set of electrodes and a signal related to the output voltage of the temperature sensor, which are provided in each of the conductivity detection means and the temperature compensation means A liquid salinity meter comprising a switch and an output means for displaying an output that depends only on the setting state of the variable resistor when the changeover switch cuts off both of the signals. (2) One switching contact (a) of one of the changeover switches (S_1) is connected to the other of the set of electrodes, and fixed between the other switching contact (b) and one of the set of electrodes. Resistance (R
2. The liquid salinity meter according to claim 1, wherein the changeover switch (S_1) has a common terminal connected to an input line of an amplifier. (3) The measuring instrument body and the measuring probe including the above-mentioned set of electrodes are configured to be detachable, and the above-mentioned fixed resistance (Rs)
The liquid salinity meter according to claim 2, wherein is a semi-fixed resistor provided on the measuring probe side. (4) A liquid salinity meter according to claim 1, wherein the calculation means for calculating the quotient (e_d/e_r) is an A/D converter. (5) When setting the variable resistor (R_1) with the measurement probe connected to the main body and the changeover switch set to cut off both signals, the output corresponding to the target position is set. 5. A liquid salinity meter according to claim 1, wherein the displayed value of the means is recorded on the measuring probe.