JP6412679B2 - Conductivity meter and calibration method thereof - Google Patents

Description

  The present invention relates to a conductivity meter for measuring the conductivity of a measurement object from an induced current generated in a secondary coil via the measurement object when a predetermined AC voltage is applied to the primary coil, and a calibration method thereof. is there.

  The conductivity is measured in various fields, for example, to determine the concentration of the salt or acid or alkali to be measured.

  For example, as shown in FIG. 1, the conductivity meter for this purpose includes an annular tube member into which a liquid sample to be measured is introduced, and a primary coil and a secondary coil attached to the annular tube member via a toroidal core. And a power supply unit that applies a predetermined AC voltage to the primary coil, and a measurement circuit that measures the induced current of the secondary coil caused by the application of the AC voltage and calculates the conductivity of the liquid sample based on the measured induced current Those having the following are known.

  By the way, in this type of conductivity meter, the measurement sensitivity changes due to changes in characteristics of the toroidal core, fluctuations in the amplification factor of the measurement circuit, fluctuations in the power supply voltage, etc. Or after measurement), it is necessary to calibrate the measurement sensitivity, that is, the ratio of the change in measured value to the unit change in true conductivity.

  Therefore, conventionally, for example, a calibration annular circuit having a known resistance value (that is, a known conductivity) is arranged in parallel with the annular tube member so as to penetrate each toroidal core. Then, the measurement sensitivity is measured by measuring the conductivity with the calibration annular circuit mounted, and comparing the measured conductivity with the true conductivity (that is, the known conductivity of the calibration annular circuit). I am trying to calibrate.

  For example, the conductivity measuring instrument described in Patent Document 1 has two resistance elements with known values different from each other in the calibration ring circuit, and this calibration is performed by selecting one of the resistance elements with a switch. The resistance (that is, conductivity) of the annular circuit is configured to be switched. At the time of calibration, the conductivity when each resistance element is selected is measured, and the amplification factor of the measurement circuit or the like is set so that the difference in the measured conductivity matches the difference in true conductivity due to each resistance element. Adjust the voltage applied by the power supply.

  Further, the conductivity meter described in Patent Document 2 includes an open / close switch that switches the calibration annular circuit between an open state and a closed state. At the time of calibration, as in Patent Document 1, the measurement circuit is such that the difference between the measured conductivity in the open state and the measured conductivity in the closed state matches the true conductivity of the calibration loop circuit. The amplification factor and the voltage applied by the power supply unit are adjusted. The reason for measuring the conductivity in the open state and taking the difference from that in the closed state is to cancel the influence of the offset. With this configuration, the liquid sample is calibrated even when the annular tube member is still put. It becomes possible.

  However, in the conductivity meter, when a range switching mechanism for switching a plurality of measurement ranges is provided in order to increase measurement accuracy or increase the measurement range, one measurement is performed with the conventional configuration described above. There is a problem that only the range can be calibrated, and measurement accuracy in other measurement ranges cannot be guaranteed. In addition, the measurement sensitivity does not always change linearly, and in such a case, there is a problem that the conventional configuration cannot be accurately calibrated.

Japanese Utility Model Publication No. 38-5072 JP-A-4-361168

  In order to solve the above problems, the main object of the present invention is to ensure measurement accuracy in a conductivity meter while increasing measurement accuracy or widening the measurement range.

The conductivity meter according to the present invention includes a primary coil that forms a primary flux ring, a secondary coil that forms a secondary flux ring, a power supply unit that applies a predetermined AC voltage to the primary coil, and the primary When the coil and the secondary coil are attached so that the measurement object forming a closed loop passes through the magnetic flux ring, the measurement unit that measures the conductivity of the measurement object from the induced current generated in the secondary coil, and A conductivity measuring meter having a range switching unit for switching a measurement range by a measurement unit, and having a plurality of impedance elements provided in parallel and an open / close switch provided corresponding to each impedance element, and in a closed state said primary side magnetic flux ring and the secondary side as a circuit having a combined impedance obtained by synthesizing the impedance of said impedance element corresponding to the open-close switch on the And sensitivity calibration circuit through the Tabawa, comprising the said range sensitivity calibration circuit switching section that outputs a closing signal to the opening and closing switch based on the range signal obtained from the switching unit, closing the sensitivity calibration circuit When the electrical conductivity of the measurement unit when in the circuit state exceeds the measurement upper limit value of the measurement range indicated by the range signal, the sensitivity calibration circuit switching unit outputs an open / close signal to the open / close switch, and The impedance of the sensitivity calibration circuit is increased .

  Note that the measurement sensitivity here refers to the change in measured conductivity related values such as current value and voltage value for calculating the measured conductivity or measured conductivity with respect to the change in true conductivity of the measurement object. Is the ratio.

  In such a case, a plurality of sensitivity calibration circuits for calibrating the measurement sensitivity by the measurement unit are provided so that they can be switched corresponding to each measurement range. Calibration can be performed, and the measurement accuracy can be ensured while increasing the measurement accuracy or increasing the measurement range.

  To automate calibration of measurement sensitivity in each measurement range and improve workability, conductivity or conductivity related by the measurement unit when the sensitivity calibration circuit is switched between closed circuit state and open circuit state A measurement sensitivity calculation unit that calculates measurement sensitivity based on a change in value, a reference sensitivity storage unit that stores a reference sensitivity that is a reference measurement sensitivity for each measurement range, and the measurement sensitivity calculation unit It is preferable to provide a calibration unit that calibrates the obtained measurement sensitivity of each measurement range so as to be the corresponding reference sensitivity stored in the reference sensitivity storage unit.

Even when the measurement sensitivity changes nonlinearly, in order to enable accurate calibration of the sensitivity, it is desirable that the plurality of open / close switches be closed at the same time.

If configured in this way, it can be regarded as a sensitivity calibration circuit having a combined impedance that combines the impedances of the impedance elements corresponding to a plurality of open / close switches in the closed state, and corresponds to this sensitivity calibration circuit and each measurement range. By performing sensitivity calibration using a sensitivity calibration circuit that can be switched in this manner, accurate sensitivity calibration can be performed even when the measurement sensitivity changes nonlinearly.

In addition, by simultaneously closing multiple open / close switches in a closed circuit state, it is possible to use a sensitivity calibration circuit with combined impedance during calibration, so sensitivity calibration in each measurement range is possible with fewer impedance circuits than the number of measurement ranges. It becomes.

  In order to allow the user to recognize that the sensitivity calibration cannot be performed by setting the sensitivity calibration circuit in a closed circuit state, the conductivity by the measurement unit exceeds the measurement upper limit of the set measurement range. It is preferable to further include a warning unit that outputs a warning signal when the electrical conductivity of the measurement unit exceeds the measurement upper limit value of the measurement range when the circuit is in a closed circuit state.

It is preferable that the impedance of the sensitivity calibration circuit when the sensitivity calibration circuit switching unit outputs an open / close signal to the open / close switch based on the range signal acquired from the range switching unit is different for each measurement range. .

A specific configuration of the sensitivity calibration circuit includes a configuration in which the sensitivity calibration circuit includes an impedance element having a predetermined impedance for each measurement range.

  A specific example of the impedance element is a resistor having a known resistance value.

  According to the present invention configured as described above, the sensitivity calibration circuit for calibrating the measurement sensitivity by the measurement unit is provided so as to be switched corresponding to each measurement range. Calibration can be performed with high accuracy, and measurement accuracy can be ensured while increasing measurement accuracy and widening the measurement range.

The schematic diagram which shows the conventional conductivity measuring meter. The whole schematic diagram of the conductivity meter in a 1st embodiment of the present invention. The hardware block diagram of the information processing control apparatus in the embodiment. The functional block diagram of the information processing control apparatus in the embodiment. The graph which shows the sensitivity calibration method in the embodiment. The whole schematic diagram of the conductivity measuring meter in a 2nd embodiment of the present invention. The whole schematic diagram of the conductivity meter in other embodiments.

  A first embodiment of the present invention will be described below with reference to FIGS.

  As shown in FIG. 2, the conductivity meter 100 according to the present embodiment is used when measuring the conductivity of a measurement object such as a material liquid or a cleaning liquid used in a semiconductor process, for example. Generated in the annular tube member 10 into which the object is introduced, the primary coil 11 and the secondary coil 12 attached to the annular tube member 10, the power supply unit 13 that applies a predetermined alternating voltage to the primary coil 11, and the secondary coil 12 The information processing control device 14 measures the conductivity of the measurement object from the induced current and the sensitivity calibration circuit 2 for calibrating the measurement sensitivity.

  The annular tube member 10 is made of, for example, a synthetic resin such as plastic, and includes a first tube member 101 and a second tube member 102 that branches from the first tube member 101 and merges so as to form a loop. An annular flow path is formed.

  A toroidal core is attached to the first tube member 101 so that the first tube member 101 penetrates, and the primary side magnetic flux ring 111 is formed inside the toroidal core. A coil 11 is wound. Similarly, a toroidal core is attached to the second tube member 102 so that the second tube member 102 penetrates, and the secondary flux ring 112 is formed inside the toroidal core. A secondary coil 12 is wound around the toroidal core.

  These toroidal cores are made of, for example, cylindrical ferrite, and both the primary coil 11 and the secondary coil 12 are made of windings such as copper wires.

  A power supply unit 13 is connected to the primary coil 11, and when a predetermined AC voltage is applied from the power supply unit 13 to the primary coil 11, the secondary coil is passed through the measurement object introduced into the annular tube member 10. An induced current is generated at 12 depending on the conductivity of the measurement object.

  The induced current generated in the secondary coil 12 is converted into direct current by the rectifier 15, amplified by the amplifier 16 with a predetermined amplification factor, and input to the information processing control device 14.

  As shown in FIG. 3, the information processing control device 14 includes a memory 52, an input / output channel 53, an input unit 54 such as a keyboard, and an output unit 55 such as a display, in addition to the CPU 51. 53 is connected to an analog-digital conversion circuit such as an A / D converter 56 and a D / A converter 57.

  Then, the CPU 51 and its peripheral devices cooperate in accordance with a program stored in a predetermined area of the memory 52, so that the information processing control device 14 includes a measuring unit 141 and a range switching unit 142 as shown in FIG. , Function as a sensitivity calibration circuit switching unit 143, a measurement sensitivity calculation unit 31, a reference sensitivity storage unit 32, and a calibration unit 33.

  For example, the measurement unit 141 stores the secondary coil 12 based on the reference data that is stored in the reference data storage unit 144 set in a predetermined area of the memory 52 and indicates the relationship between the induced current and the conductivity as a proportional relationship. The conductivity is measured from the induced current generated in the circuit.

  The range switching unit 142 outputs a range signal for determining the measurement range of the measurement unit 141 to the amplifier 16 and the measurement unit 141, and changes the amplification factor of the amplifier 16, thereby changing the measurement range of the measurement unit 141 from among a plurality of measurement ranges. To switch to either.

The sensitivity calibration circuit switching unit 143 uses, for example, the usage circuit storage unit 145 that stores the correspondence relationship between the measurement range and the sensitivity calibration circuit 2 used during calibration, which is set in a predetermined area of the memory 52. Based on the circuit data and the range signal acquired from the above-described range switching unit 142 , the impedance of the sensitivity calibration circuit 2 used for calibration is switched from a plurality to the impedance corresponding to each measurement range.

More specifically, the sensitivity calibration circuit switching unit 143 outputs an open / close signal to the open / close switch 22 described later based on the use circuit data and the acquired range signal, and sets each open / close switch 22 to a closed state or an open state. By switching, the impedance of the sensitivity calibration circuit 2 is switched corresponding to each measurement range.

  Next, the sensitivity calibration circuit 2 provided to calibrate the measurement sensitivity of the measurement unit 141 for each measurement range will be described with reference to FIGS.

  The sensitivity calibration circuit 2 is arranged in parallel with the measurement object introduced into the annular tube member 10, and the resistor 212 and the electric cable 211 that connects both terminals of the resistor 212 to form a closed circuit. And an open / close switch 22 that is provided on the electric cable 211 and switches the impedance circuit 21 to either a closed circuit state or an open circuit state.

  In the present embodiment, as illustrated, the electric cable 211 is common to each impedance circuit 21 and is provided so as to penetrate the primary side magnetic flux ring 111 and the secondary side magnetic flux ring 112.

  A plurality of resistors 212 for determining the resistance value of the sensitivity calibration circuit 2 corresponding to each measurement range (that is, the conductivity of the sensitivity calibration circuit 2) are connected to the electrical cable 211 in parallel.

  In the present embodiment, the resistance value of each resistor 212 is a value at which the conductivity of the sensitivity calibration circuit 2 corresponding to each measurement range becomes the measurement upper limit value of the measurement range.

  As shown in the figure, the open / close switch 22 is a two-terminal analog switch configured to move independently based on an open / close signal from the sensitivity calibration circuit switching unit 143.

The open / close switch 22 configured as described above switches each impedance circuit 21 to either a closed circuit state or an open circuit state as described above, and at the same time, the impedance of the sensitivity calibration circuit 2 corresponds to each measurement range. It is possible to switch to the impedance of the sensitivity calibration circuit 2 used at the time of calibration.

Since these open / close switches 22 move independently, the plurality of impedance circuits 21 can be simultaneously closed , corresponding to the plurality of open / close switches 22 having the sensitivity calibration circuit 2 closed. It can be regarded as a circuit having a combined impedance obtained by combining the impedances of the resistor 212 .

When the sensitivity calibration circuit 2 having an impedance corresponding to each measurement range is switched from the open circuit state to the closed circuit state in a state where a predetermined AC voltage is applied to the primary coil 11 by the open / close switch 22, An induced current depending on the impedance of the sensitivity calibration circuit 2 (that is, the conductivity of the sensitivity calibration circuit 2) is generated in the coil 12, and the conductivity or conductivity obtained from the measurement unit 141 is calculated. The conductivity related value changes.

  Subsequently, referring to FIGS. 2, 4, and 5, in order to calculate and calibrate the measurement sensitivity of the measurement unit 141 based on the change in the conductivity or the conductivity-related value obtained from the measurement unit 141 described above. The measurement sensitivity calculation unit 31, the reference sensitivity storage unit 32, and the calibration unit 33 will be described.

  The measurement sensitivity calculation unit 31 calculates the measurement sensitivity of the measurement unit 141 based on the change in the conductivity or conductivity-related value obtained from the measurement unit 141. In this embodiment, the sensitivity calibration circuit 2 is used. Is changed from the open circuit state to the closed circuit state, and the change in the current value obtained from the measurement unit 141 is calculated as the measurement sensitivity.

  The reference sensitivity storage unit 32 is set in a predetermined area of the memory 52, and stores a reference sensitivity that is a reference measurement sensitivity for each measurement range.

More specifically, the reference sensitivity storage unit 32 switches the sensitivity calibration circuit 2 having the impedance corresponding to each measurement range from the open circuit state to the closed circuit state at the initial time such as factory shipment. The change in the current value obtained from the measuring unit 141 is stored as the reference sensitivity.

  The calibration unit 33 acquires the measurement sensitivity calculated by the measurement sensitivity calculation unit 31 and the corresponding reference sensitivity stored in the reference sensitivity storage unit 32 in each measurement range, and the sensitivity is set so that the measurement sensitivity becomes the reference sensitivity. It is to calibrate.

  In the present embodiment, the calibration unit 33 outputs a calibration signal for setting the measurement sensitivity as the reference sensitivity to the measurement unit 141, and measures when the sensitivity calibration circuit 2 is changed from an open circuit state to a closed circuit state. The change of the current value obtained from the unit 141 is calibrated so as to be the change of the current value stored in the reference sensitivity storage unit 32 as the reference sensitivity.

  The information processing control device 14 outputs a warning signal when the conductivity obtained from the measurement unit 141 when the sensitivity calibration circuit 2 is in the closed circuit state exceeds the measurement upper limit value of the measurement range (illustrated). No) A function as a warning unit 34 is further provided.

  According to the conductivity meter 100 according to the present embodiment configured as described above, the sensitivity calibration circuit 2 for calibrating the measurement sensitivity by the measurement unit 141 is provided so as to be switched corresponding to each measurement range. Therefore, the measurement sensitivity can be accurately calibrated for each measurement range, and the measurement accuracy can be ensured while increasing the measurement accuracy or widening the measurement range.

  In addition, the measurement sensitivity calculation unit 31 calculates the measurement sensitivity of each measurement range, and the calibration unit 33 calibrates the measurement sensitivity so as to be the corresponding reference sensitivity stored in the reference sensitivity storage unit 32. Workability can be improved by automating calibration.

  Furthermore, the resistance value of each resistor 212 is a value at which the conductivity of the sensitivity calibration circuit 2 corresponding to each measurement range becomes the measurement upper limit value of the measurement range, so that accurate calibration can be performed in each measurement range. .

  By simultaneously setting the plurality of impedance circuits 21 in the closed circuit state, the sensitivity calibration circuit 2 in the closed circuit state can be regarded as a circuit having a combined impedance obtained by combining the respective impedances, which is smaller than the number of measurement ranges. The impedance circuit 21 enables sensitivity calibration in each measurement range.

Further, by using the sensitivity calibration circuit 2 including a plurality of resistors 212 and obtaining a plurality of measurement values from the measurement unit 141, the linearity of the measurement sensitivity change and the zero point can be confirmed.

  In addition, since the information processing control device 14 has a function as the warning unit 34, the user can determine that the conductivity measured by the measurement unit 141 when the sensitivity calibration circuit 2 is in the closed circuit state is the measurement upper limit value of the measurement range. It can be easily recognized that sensitivity calibration cannot be performed.

  In this case, by setting the impedance circuit 21 having an impedance larger than the impedance circuit 21 of the sensitivity calibration circuit 2 corresponding to the measurement range to a closed circuit state, the conductivity by the measurement unit 141 is within the measurement range. Sensitivity calibration is possible without changing the measurement range.

  Next, the second embodiment will be described. In addition, the same code | symbol shall be attached | subjected to the element which shows the structure similar to 1st Embodiment.

In the first embodiment described above, the open / close switch 22 is a two-terminal analog switch provided in the sensitivity calibration circuit 2 and moves independently so that the sensitivity calibration circuit 2 is in a closed circuit state or an open circuit. In the second embodiment, as shown in FIG. 6, the open / close switch 22 is one of a terminal provided on one side and a plurality of terminals provided on the other side. It is configured to close between two terminals, and the sensitivity calibration circuit 2 having a specific impedance among a plurality of impedances can be switched to a closed circuit state.

Even in the conductivity meter 100 of the second embodiment configured as described above, the sensitivity calibration circuit 2 can be switched corresponding to each measurement range, and the measurement sensitivity is accurately calibrated for each measurement range. In addition, the switching of the open circuit state or the closed circuit state of the sensitivity calibration circuit 2 by the open / close switch 22 and the impedance switching of the sensitivity calibration circuit 2 corresponding to each measurement range are facilitated.

  The present invention is not limited to the above embodiment.

  For example, when the sensitivity calibration is performed in a state where the liquid sample is filled in the annular tube member 10 during the measurement, the resistance value of each resistor 212 is the sensitivity calibration circuit 2 corresponding to each measurement range as in this embodiment. Is the measurement upper limit value of the measurement range, the conductivity obtained from the measurement unit 141 exceeds the measurement upper limit value of the measurement range, and sensitivity calibration cannot be performed. Therefore, by setting the resistance value of each resistor 212 to a value smaller than the measurement upper limit value, the sensitivity may be calibrated even during the measurement of the conductivity of the measurement target.

  The resistance values of the resistors 212 do not have to be different from each other, and some of the resistors 212 may have the same resistance value.

Furthermore, it is not necessary that the impedance of the sensitivity calibration circuit 2 used for calibration differs for each measurement range, and some impedances of the sensitivity calibration circuit 2 corresponding to each measurement range are the same. It doesn't matter.

  Further, when the warning unit 34 outputs a warning signal, the sensitivity calibration circuit switching unit 143 automatically switches to the sensitivity calibration circuit 2 having a larger impedance than the sensitivity calibration circuit 2 corresponding to each measurement range. Also good.

  By configuring as described above, the sensitivity calibration of the measurement unit 141 can be reliably performed even during the measurement of the conductivity of the measurement target.

  In the first embodiment, the measurement sensitivity calculation unit 31 is configured to calculate the measurement sensitivity based on the change amount of the current value obtained from the measurement unit 141. However, the conductivity obtained from the measurement unit 141 is calculated. The measurement sensitivity may be calculated based on the rate change.

  The calibration unit 33 outputs the calibration signal to the measurement unit 141 and calibrates the sensitivity so that the measurement sensitivity becomes the reference sensitivity. However, by changing the magnitude of the voltage applied by the power supply unit 13, the measurement sensitivity becomes the reference sensitivity. Sensitivity calibration may be performed so that

  In addition, the range switching unit 142 is configured to output the range signal to the amplifier 16 and change the amplification factor, but by outputting the range signal to the power supply unit 13 and changing the magnitude of the applied voltage. You may be comprised so that the measurement range of the measurement part 141 may be switched.

  Furthermore, as shown in FIG. 7, the primary coil 11 and the secondary coil 12 may be arranged in series and both may be attached to the second pipe member 102.

Moreover, in the said 1st Embodiment, although it was used when measuring the electrical conductivity of a liquid sample, the density | concentration of the measuring object contained in the liquid sample can also be measured from the measured electrical conductivity. it can.
Furthermore, it can also be used as an ohmmeter for measuring the resistance value of a liquid sample.

  Further, although the measurement object is a liquid, it may be a metal that forms a closed loop.

  In addition, the present invention is not limited to the above embodiment, and various modifications can be made without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 100 ... Conductivity meter 14 ... Information processing control device 141 ... Measurement part 142 ... Range switching part 143 ... Sensitivity calibration circuit switching part 2 ... Sensitivity calibration circuit 21 ... Impedance circuit 22: Open / close switch 31 ... Measurement sensitivity calculation unit 32 ... Reference sensitivity storage unit 33 ... Calibration unit

Claims (6)

A primary coil that forms a primary flux ring, a secondary coil that forms a secondary flux ring, a power supply unit that applies a predetermined AC voltage to the primary coil, and the primary coil and secondary coil that are connected to the flux ring A measurement unit that measures the conductivity of the measurement target from the induced current generated in the secondary coil when the measurement target that forms a closed loop passes through, and a range switching unit that switches a measurement range by the measurement unit In a conductivity meter comprising:
An electric cable penetrating the primary side magnetic flux ring and the secondary side magnetic flux ring, a plurality of impedance elements connected to one end of the electric cable and provided in parallel with each other, and provided in series with the impedance elements; And a circuit for sensitivity calibration having a plurality of open / close switches connected to the other end of the electric cable,
A used circuit storage unit that stores the used circuit data indicating the relationship between the measurement range by the measurement unit and the impedance of the sensitivity calibration circuit used during calibration in the measurement range;
Based on the range signal acquired from the range switching unit and the use circuit data, an open / close signal is output to each open / close switch, and each open / close switch is switched to a closed state or an open state, thereby the sensitivity calibration circuit. A sensitivity calibration circuit switching unit that switches impedance to impedance corresponding to the measurement range indicated by the range signal;
In a state where a predetermined alternating voltage is applied to the primary coil, the sensitivity calibration circuit switching unit switches the impedance of the sensitivity calibration circuit to an impedance corresponding to the measurement range indicated by the range signal, thereby the measurement unit. When the electrical conductivity by exceeds the measurement upper limit value of the measurement range indicated by the range signal, the sensitivity calibration circuit switching unit outputs an open / close signal to the open / close switch to switch the open / closed switch to a closed state or an open state. Thus, the impedance of the sensitivity calibration circuit is increased. A measurement sensitivity calculation unit that calculates measurement sensitivity based on a change in conductivity or conductivity-related value by the measurement unit when the sensitivity calibration circuit is switched between a closed circuit state and an open circuit state;
A reference sensitivity storage unit that stores a reference sensitivity, which is a reference measurement sensitivity, for each measurement range;
A calibration unit that calibrates the measurement sensitivity of each measurement range obtained by the measurement sensitivity calculation unit so as to be the corresponding reference sensitivity stored in the reference sensitivity storage unit. Item 2. The conductivity meter according to Item 1.   In a state in which a predetermined AC voltage is applied to the primary coil, the range switching unit switches the impedance of the sensitivity calibration circuit to an impedance corresponding to the measurement range indicated by the range signal, whereby the conductivity by the measurement unit is set. 3. The conductivity meter according to claim 1, further comprising a warning unit that outputs a warning signal when the value exceeds a measurement upper limit value of the measurement range indicated by the range signal.   4. The conductive circuit according to claim 1, wherein an impedance of the sensitivity calibration circuit used in calibration in each measurement range stored in the used circuit storage unit is different for each measurement range. Rate meter.   5. The conductivity meter according to claim 1, wherein the sensitivity calibration circuit includes an impedance element having an impedance predetermined for each measurement range.   6. The conductivity meter according to claim 5, wherein the impedance element is a resistor having a known resistance value.