Thermocouple measuring instruments and thermocouple thermometers

Abstract

To provide a measuring instrument and thermocouple thermometer that can more easily suppress common-mode interference that occurs during temperature measurement. [Solution] The thermocouple thermometer 1 comprises a thermocouple 2 and a measuring instrument 3. The measuring instrument 3 comprises a reference junction compensator 5 to which the thermocouple 2 is connected, a differential amplifier 6 that differentially amplifies the analog signals of the thermocouple 2 and the reference junction compensator 5, an A / D converter 7 that converts the analog signals into digital signals, and a calculation device 8 that uses the digital signals to calculate the temperature of the temperature sensing junction P1 of the thermocouple 2 (temperature of the object to be measured M). Furthermore, it comprises potentiometers 10 individually provided on each of the pair of signal lines 9 connecting the reference junction compensator 5 and the differential amplifier 6, and an adjustment unit 11 that adjusts the resistance value of the potentiometers 10.

Description

Technical Field

[0001] The present invention relates to a measuring instrument for a thermocouple and a thermocouple thermometer, and more particularly to a measuring instrument for a thermocouple and a thermocouple thermometer that can more easily suppress the influence of common mode interference that occurs during temperature measurement.

Background Art

[0002] In general measuring instruments for thermocouples, differential amplification circuits such as filtering and instrumentation amplifiers for suppressing input impedance and removing common mode are used. However, since a thermocouple is composed of a pair of metal wires of different types, the resistance values of the pair of metal wires are different even if they have the same specifications (cross-sectional area, length). Therefore, due to the difference in the resistance values of the pair of metal wires, common mode interference connected by stray capacitance or the like is converted into the normal mode, and the common mode removal of filtering and instrumentation amplifiers does not work effectively, which becomes a factor of measurement error.

[0003] A thermocouple in which the first conductor and the second conductor are mechanically asymmetric and electrically symmetric with each other has been proposed (see Patent Document 1). Electrically symmetric means that the first conductor and the second conductor have the same resistance value. Therefore, in the invention proposed in Patent Document 1, the conversion of common mode interference into the normal mode is suppressed. However, in order to make the resistance values of the first conductor and the second conductor equal, a great deal of labor is required because the wire diameter and length must be adjusted according to the other side. In addition, when the wire diameter and length are changed, the mechanical strength of each conductor decreases or the balance of heat conduction is disrupted. Therefore, there is room for improvement in more easily suppressing the influence of common mode interference that occurs during temperature measurement.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] The object of the present invention is to provide a measuring instrument for thermocouples and a thermocouple thermometer that can more easily suppress the effects of common-mode interference that occur during temperature measurement. [Means for solving the problem]

[0006] The thermocouple measuring instrument of the present invention, which achieves the above objective, comprises a reference junction compensator to which a thermocouple is connected, a differential amplifier that differentially amplifies the analog signals of the thermocouple and the reference junction compensator, an A / D converter that converts the analog signals into digital signals, and a calculation device that calculates the temperature of the thermometer's temperature junction using the digital signals, wherein the thermocouple measuring instrument comprises a potentiometer provided on at least one of a pair of signal lines connecting the reference junction compensator and the differential amplifier, and an adjustment unit that adjusts the resistance value of the potentiometer.

[0007] The thermocouple thermometer of the present invention is characterized by comprising the thermocouple measuring instrument described above and the thermocouple. [Effects of the Invention]

[0008] According to the present invention, by adjusting the resistance value of the potentiometer using the adjustment unit based on the difference in the resistance values ​​of the pair of metal wires of the thermocouple, the resistance values ​​of the pair of signal paths through which the analog signal flows from the temperature sensing junction of the thermocouple to the differential amplifier can be made equal. This simple adjustment suppresses the conversion of common-mode interference to normal mode, allowing filtering and common-mode rejection by instrumentation amplifiers to work effectively. As a result, more stable temperature measurement accuracy can be ensured in high-voltage environments, noisy environments where the effects of common-mode interference are significant, and in environments where the difference in resistance values ​​between the positive and negative legs due to the wire diameter and length of the thermocouple is large. [Brief explanation of the drawing]

[0009] [Figure 1] This is an explanatory diagram illustrating an example of a measuring instrument and thermocouple thermometer for thermocouples used in the measurement process. [Figure 2] This is an explanatory diagram illustrating an example of a measuring instrument and thermocouple thermometer for thermocouples used in the preparation process. [Figure 3] This flowchart illustrates the procedure for a measurement method using thermocouple measuring instruments and thermocouple thermometers. [Figure 4] This is a schematic explanatory diagram illustrating a portion of Figure 2 when measuring the resistance value of the positive leg of a thermocouple. [Figure 5] This is a schematic explanatory diagram illustrating a portion of Figure 2 when measuring the resistance value of the negative leg of a thermocouple. [Figure 6] This is an explanatory diagram illustrating a portion of Figure 1 after the preparation process has been completed. [Modes for carrying out the invention]

[0010] The thermocouple measuring instrument and thermocouple thermometer of the present invention will be described below based on the embodiments shown in the figures.

[0011] The embodiment of the thermocouple thermometer 1 illustrated in Figures 1 and 2 comprises a thermocouple 2 and a measuring instrument 3, and is used to measure the temperature of a target object M. This measuring instrument 3 is an embodiment of the thermocouple measuring instrument of the present invention. Figure 1 shows the thermocouple thermometer 1 in measurement step S200 (measurement step S200 in Figure 3, described later) for measuring the temperature of a target object M. Figure 2 shows the thermocouple thermometer 1 in preparation step S100 (preparation step S100 in Figure 3, described later) prior to measurement step S200.

[0012] The thermocouple 2 and measuring instrument 3 of the thermocouple thermometer 1 are separate components, with the thermocouple 2 being detachably connected to the measuring instrument 3. The thermocouple thermometer 1 is not limited to a configuration in which the thermocouple 2 and measuring instrument 3 are separate components; it may also have a configuration in which the thermocouple 2 and measuring instrument 3 are integrated.

[0013] The thermocouple 2 is composed of a pair of metal wires 4a and 4b of different types. The measuring instrument 3 includes a reference junction compensator 5, a differential amplifier 6, an A / D converter (analog-to-digital converter) 7, and a calculation unit 8. The measuring instrument 3 also includes potentiometers 10 and adjustment units 11 (calculation unit 8) individually provided on each of the pair of signal lines 9 connecting the reference junction compensator 5 and the differential amplifier 6. Furthermore, the measuring instrument 3 includes a constant current source 12, a pair of constant current signal lines 13, contact terminals 14, and a reference potential signal line 15.

[0014] To simplify the complexity of Figures 1 and 2, the reference potential signal line 15 is shown divided by the reference potential indicated by the triangle in the figure. However, the actual reference potential signal line 15 has one end connected to the contact terminal 14, an intermediate position connected to the reference potential, and the other end connected to the input ports (AIN1, AIN6) of the multiplexer 20 of the differential amplifier 6. Filters and bias current resistors are not shown in the figure, but can be used as appropriate. A filter is installed between the potentiometer 10 and the differential amplifier 6. Examples of filters include inter-line capacitors to equalize the inter-line capacitance components between signal lines 9, and line-to-GND capacitors between signal line 9 and the reference potential to reduce the impedance between the reference potential and the common-mode interference source. The bias current resistor is interposed between the signal line 9 on the thermocouple 2 side of the fixed resistor 22 and the reference potential. The bias current resistor is, for example, about 1 MΩ and is related to the resistance value of thermocouple 2 by voltage division.

[0015] Thermocouple 2 can be any known type of thermocouple, such as grounded, ungrounded, or exposed type. Examples of thermocouples 2 include Type K, which uses Chromel for the positive leg and Alumel for the negative leg; Type T, which uses copper for the positive leg and Constantan for the negative leg; and Type E, which uses Chromel for the positive leg and Constantan for the negative leg. In thermocouple 2, the metal wire 4a on the right side of Figures 1 and 2 is the positive leg (+ leg), and the metal wire 4b on the left side is the negative leg (- leg). The black dots in Figures 1 and 2 each indicate the temperature sensing junction P1 and a pair of reference junctions P2 of thermocouple 2. The temperature sensing junction P1 is the junction end of the pair of metal wires 4a and 4b, and the pair of reference junctions P2 are the open ends of the pair of metal wires 4a and 4b. In the embodiment, grounded or exposed type thermocouple 2 is preferred because the resistance values ​​of the positive and negative legs of thermocouple 2 are measured using the resistance value measurement circuit C2 in the preparation step S100 described later. Furthermore, since grounded and exposed types are more susceptible to noise than ungrounded types, the common-mode rejection effect of measuring instrument 3 has a greater impact. In the case of ungrounded types, the resistance value of the positive and negative legs is measured as a series sum, so the resistance values ​​of the positive and negative legs can be calculated using the series sum of the metal wires 4a and 4b and the known resistivity based on the metal materials that make up those wires 4a and 4b.

[0016] The measuring instrument 3 has each device housed in a housing 16. The measuring instrument 3 can utilize various types of measuring instruments, such as stationary measuring instruments, embedded measuring instruments incorporated into other devices, and handheld measuring instruments carried and used by the user.

[0017] The reference junction compensator 5 has a pair of terminals 17 and a reference junction temperature sensor 18. The pair of reference junctions P2 of the thermocouple 2 (the open ends of a pair of metal wires 4a and 4b) are connected to the pair of terminals 17 of the reference junction compensator 5, and the reference junction temperature sensor 18 acquires the temperature of the pair of terminals 17 that are evenly heated to each other, i.e., the temperature of the reference junctions P2.

[0018] The pair of terminals 17 is made of a metal material such as stainless steel, aluminum, or copper. Known thermocouple terminals such as crimp terminals to which Y terminals or round-hole terminals can be connected, and receptacles to which plugs can be connected can be used for the terminals 17. These pair of terminals 17 electrically connect the pair of reference contacts P2 of the thermocouple 2 and the pair of signal lines 9. A contact terminal 14, which will be described later, is interposed between the pair of terminals 17, and the pair of terminals 17 and the contact terminal 14 are insulated by an insulating member 19. The respective terminals of the pair of terminals 17 and the contact terminal 14 are installed so as to be able to have equal heat. The insulating member 19 can use various known insulating members such as polyimide. The thickness of this insulating member 19 only needs to be such that the pair of terminals 17 and the contact terminal 14 can have sufficient equal heat. In addition, when the resistance values of the positive leg and the negative leg of the thermocouple 2 are not measured using the resistance value measuring circuit C2 in the preparation step S100, but the resistance values of the positive leg and the negative leg are obtained by using the series combined resistance of the positive leg and the negative leg and a known resistivity, the contact terminal 14 can be omitted, and the pair of terminals 17 may be installed so as to be able to have equal heat only through the insulating member 19.

[0019] The reference contact temperature sensor 18 can use known temperature sensors such as a resistance temperature detector, a linear resistor, a thermistor, and a semiconductor temperature sensor (diode temperature sensor). The reference contact temperature sensor 18 is in contact with the contact terminal 14, and measures the temperature of the contact terminal 14, that is, the temperature of the pair of terminals 17 as the temperature of the reference contact P2. The reference contact temperature sensor 18 of the embodiment is, for example, a resistance temperature detector. This resistance temperature detector is connected to the input ports (AIN4, AIN5) of the differential amplifier 6 via a signal line. When the contact terminal 14 is omitted, the reference contact temperature sensor 18 may be in contact with both of the pair of terminals 17.

[0020] The differential amplifier 6 differentially amplifies the input analog signal (voltage signal). This differential amplifier 6 has a multiplexer (MUX) 20 and an instrumentation amplifier 21. The multiplexer 20 switches the analog signals from the thermocouple 2 and the reference junction temperature sensor 18 at a predetermined timing and outputs them to the instrumentation amplifier 21. A constant current source 12 is connected to this multiplexer 20. This constant current source 12 outputs a constant current to the reference junction temperature sensor 18 and a constant current signal line 13 described later. The instrumentation amplifier 21 amplifies each analog signal from the thermocouple 2 and the reference junction temperature sensor 18 and outputs it to the A / D converter 7. The A / D converter 7 converts each analog signal amplified by the differential amplifier 6 into each digital signal and outputs the converted digital signals to the arithmetic unit 8. The differential amplifier 6 and the A / D converter 7 may be constituted by one integrated circuit. Also, the multiplexer 20 is not essential, and a plurality of instrumentation amplifiers 21 and a plurality of A / D converters 7 may be used instead of the multiplexer 20.

[0021] The arithmetic unit 8 is constituted by a computer, various data are input and stored therein, and data processing for calculating the temperature of the measurement object M is performed using these data. The arithmetic unit 8 can use various known computers. The arithmetic unit 8 has an arithmetic processing unit (CPU), a main storage unit (memory), and an auxiliary storage unit (for example, HDD), and is communicably connected to an output unit (display) that outputs the calculated temperature.

[0022] A pair of signal lines 9 connect the reference junction compensator 5 and the differential amplifier 6. One end of each of the pair of signal lines 9 is electrically connected to the thermocouple 2 via a pair of terminals 17, and the other end of each is connected to the input ports (AIN2, AIN3) of the multiplexer 20 and is electrically connected to the instrumentation amplifier 21. Known signal lines can be used for the pair of signal lines 9. The pair of signal lines 9 do not need to have the same specifications (metal type, cross-sectional area, length) as long as their respective resistance values are, for example, 1 Ω or less and are smaller than the resolution of the potentiometer 10 described later. When the multiplexer 20 is not used, the other ends of the pair of signal lines 9 are directly connected to the instrumentation amplifier 21.

[0023] The potentiometer 10 can be any known potentiometer, such as a rotary potentiometer, linear potentiometer, or digital potentiometer. The potentiometer 10 may be installed at any point on the signal line 9. The potentiometer 10 can adjust its resistance value in multiple steps within a predetermined range. The predetermined range and the number of adjustable steps can be arbitrarily set according to the type and specifications of the thermocouple 2 (cross-sectional area and length of the metal wires 4a and 4b). For example, the potentiometer 10 may have a predetermined range of 160Ω to 2.5kΩ, an adjustable number of steps from 1 to 256, and the resistance value can be adjusted in increments of approximately 10Ω.

[0024] The potentiometer 10 only needs to be provided on at least one of the pair of signal lines 9, but it is preferable that an individual potentiometer 10 be provided on each of the pair of signal lines 9, as in the embodiment. Providing a potentiometer 10 for each signal line 9 widens the adjustable range, which is advantageous for adapting to thermocouples 2 of various types and specifications. Alternatively, the two potentiometers 10 can be configured as a single digital potentiometer with two channels.

[0025] The magnitude of the resistance values ​​of each signal path A and B should be made large enough to be negligible, depending on the input resistance of the differential amplifier 6 used. Therefore, it is preferable to provide fixed resistors 22 in each signal path A and B. Fixed resistors 22 offer superior stability compared to potentiometers 10. Accordingly, it is preferable to use fixed resistors 22 to increase the ratio of the resistance value of fixed resistors 22 to the total resistance values ​​of each signal path A and B. Furthermore, the potentiometer 10 should have its initial value set to a low resistance value that can be set excluding the lower limit (e.g., 160Ω), for example, around 200Ω. Setting the initial value of the potentiometer 10 to the lower limit increases the influence of overcurrents, etc., so it is preferable to exclude the lower limit from the initial value. The magnitude of the resistance values ​​of each signal path A and B should be determined considering input protection, filtering responsiveness, thermal noise, etc. Although it is possible to omit the fixed resistor 22 and use only the resistance values ​​of the potentiometer 10 to supply the resistance values ​​of each signal path A and B, it is preferable to use the fixed resistor 22.

[0026] The adjustment unit 11 can use a calculation unit 8 that is electrically connected to the potentiometer 10 via a control signal line 23. The adjustment unit 11 may also use a separate device from the calculation unit 8, and can use known input devices such as rotary potentiometers, slide potentiometers, DIP switches, or numeric keypads that are directly connected to the potentiometer 10 via a signal line. The adjustment of the resistance value of the potentiometer 10 by the adjustment unit 11 is performed automatically when the adjustment unit 11 is composed of a calculation unit 8, and when the adjustment unit 11 is composed of known input devices, it is performed by human operation of the input device.

[0027] The constant current source 12 outputs a constant current of approximately 0.5 mA, for example. An on / off circuit, a current inversion circuit, etc., may be added to the output side of the constant current source 12 as appropriate. The constant current source 12 outputs a constant current to the reference contact temperature sensor 18 and a pair of constant current signal lines 13 at predetermined timings via a controlled changeover switch or on / off mechanism, and this constant current is used as an excitation signal. The constant current source 12 may be increased as needed, for example, for other applications such as wire break detection, when multiple current values ​​are required, or when adding a current inversion circuit to cancel parasitic electromotive force during resistance measurement, and may also be routed through other multiplexers or switches.

[0028] A pair of constant current signal lines 13 connect the reference junction compensator 5 and the constant current source 12. One end of each pair of constant current signal lines 13 is electrically connected to the thermocouple 2 via a pair of terminals 17, and the other end of each is connected to the output ports (IOUT1, IOUT2) of the multiplexer 20 and electrically connected to the constant current source 12. In the preparation step S100 described later, it is preferable to use the four-terminal method for measuring the resistance values ​​of each metal wire 4a and 4b of the thermocouple 2, and a Kelvin connection is used in which the pair of constant current signal lines 13 are force lines, the pair of signal lines 9 and the reference potential signal line 15 are sense lines. As a result, the constant current does not pass through the fixed resistor 22 and the potentiometer 10, which is advantageous for accurately measuring the resistance values ​​of each metal wire 4a and 4b.

[0029] The contact terminal 14 is preferably made of a material with excellent conductivity, and is made of a metallic material such as stainless steel, aluminum, or copper, similar to the pair of terminals 17. When the temperature sensing junction P1 of the thermocouple 2 is in direct contact with the contact terminal 14, the temperature sensing junction P1 is electrically connected to the contact terminal 14 (current is flowing).

[0030] The contact terminal 14 is interposed between a pair of terminals 17, adjacent to each terminal 17 via a thin insulating member 19, and is heated uniformly to the temperature of the reference junction P2 by being heated uniformly with the pair of terminals 17. In the preparation step S100, the temperature measuring junction P1 comes into direct contact with the contact terminal 14, so that the temperatures of the temperature measuring junction P1 and the reference junction P2 become the same, and no electromotive force is generated in the thermocouple 2. This state in which no electromotive force is generated in the thermocouple 2 is suitable for measuring the resistance values ​​of the positive and negative legs of the thermocouple 2 in the preparation step S100, and is advantageous for measuring the resistance values ​​of the positive and negative legs with high accuracy.

[0031] The reference potential signal line 15 is connected to the reference potential of the measuring instrument 3. One end of the reference potential signal line 15 is electrically connected to the contact terminal 14, the middle end is connected to the reference potential, and the other end is connected to the input ports (AIN1, AIN6) of the multiplexer 20. In preparation step S100, the temperature measuring junction P1 of the thermocouple 2 makes direct contact with the contact terminal 14, and the reference potential signal line 15 is electrically connected (energized) to each of the pair of metal wires 4a and 4b of the thermocouple 2 via the contact terminal 14.

[0032] Next, the procedure for measuring temperature using the thermocouple thermometer 1 will be described in detail. In the temperature measurement method illustrated in Figure 3, the preparation step S100 and the measurement step S200 are performed in order. The details of the preparation step S100 (S110~S150) and the measurement step S200 (S210, S220) will be explained below.

[0033] Preparation step S100 is performed using the resistance measurement circuit C2 illustrated in Figure 2 above. Before performing preparation step S100, the resistance values ​​of each potentiometer 10 are adjusted to their initial values, and the resistance values ​​of signal paths A and B differ by the difference between the resistance values ​​of the pair of metal wires 4a and 4b of the thermocouple 2.

[0034] In step S110, the thermocouple 2 to be used in measurement step S200 is connected to the pair of terminals 17 of the reference junction compensator 5. In step S120, the temperature sensing junction P1 of the thermocouple 2 is brought into direct contact with the contact terminal 14. Direct contact between the temperature sensing junction P1 and the contact terminal 14 means that the temperature sensing junction P1 is electrically connected to the contact terminal 14 and is in a state where it can conduct electricity.

[0035] In step S130, a constant current is output from the constant current source 12 to one of the constant current signal lines 13, and the arithmetic unit 8 performs data processing to measure the resistance value of the positive leg (metal wire 4a) of the thermocouple 2. In step S140, a constant current is output from the constant current source 12 to the other constant current signal line 13, and the arithmetic unit 8 performs data processing to measure the resistance value of the negative leg (metal wire 4b) of the thermocouple 2.

[0036] The shaded arrows in Figure 4 represent the flow of constant current in step S130. In step S130, the constant current output from the constant current source 12 flows through the output port (IOUT1) to the constant current signal line 13 due to the switching of the multiplexer 20. This constant current passes through the constant current signal line 13, the positive leg (metal wire 4a) of the thermocouple 2, the contact terminal 14, and the reference potential signal line 15 in that order. A specified current flows through the positive leg (metal wire 4a) of the thermocouple 2, exciting a voltage value ΔVa corresponding to its resistance. Next, the voltage value ΔVa is amplified by the instrumentation amplifier 21 via the respective input ports (AIN1, AIN2), and the voltage value ΔVa is converted into a digital signal by the A / D converter 7 and input to the arithmetic unit 8. Then, the resistance value of the positive leg (metal wire 4a) of the thermocouple 2 is determined by data processing that calculates the resistance value using that value.

[0037] The shaded arrows in Figure 5 represent the flow of constant current in step S140. In step S140, the constant current output from the constant current source 12 flows through the output port (IOUT2) to the constant current signal line 13 due to the switching of the multiplexer 20. This constant current passes through the constant current signal line 13, the negative leg (metal wire 4b) of the thermocouple 2, the contact terminal 14, and the reference potential signal line 15 in that order. A specified current flows through the negative leg (metal wire 4b) of the thermocouple 2, and a voltage value ΔVb corresponding to its resistance value is exemplified. Next, the voltage value ΔVb is amplified by the instrumentation amplifier 21 via the respective input ports (AIN3, AIN6), and the voltage value ΔVb is converted into a digital signal by the A / D converter 7 and input to the arithmetic unit 8. Then, the resistance value of the negative leg (metal wire 4b) of the thermocouple 2 is determined by data processing that calculates the resistance value using that value.

[0038] Aside from the individual resistance values ​​of the pair of metal wires 4a and 4b of the thermocouple 2, the resistance values ​​of the potentiometer 10, fixed resistor 22, signal line 9, etc., can be ignored by using Kelvin connections. Also, since the contact terminal 14 is a block of metal material, its resistance value is 1Ω or less, and it is added equally to both voltage values ​​ΔVa and ΔVb in the derivation process, so it can be ignored. Therefore, the calculation unit 8 can calculate the resistance values ​​of the positive and negative legs of the thermocouple 2 based on the digital signals (voltage values ​​ΔVa, ΔVb) output from the A / D converter 7 and the current value of the constant current (e.g., 0.5mA). However, if a bias current resistor is provided between the constant current signal line 13 and the reference potential or between the reference potential signal line 15 and the reference potential in order to provide a return path for the input bias current of the instrumentation amplifier 21, or if a bias voltage is applied to a resistor provided on the constant current signal line 13, these resistors may become a source of error. Individual differences in the measuring instrument 3 may also become a source of error. Therefore, it is advisable to connect a 0Ω resistor and a specified resistance value near the input upper limit, for example, around 1kΩ, to terminal 17 to perform calibration and to obtain error correction data from the ideal value in advance. By storing the previously obtained error correction data in the arithmetic unit 8 and using it in the data processing to calculate the resistance value, errors in resistance measurement can be absorbed.

[0039] In step S150, the calculation unit 8 functions as the adjustment unit 11 and performs data processing to adjust the resistance value of the potentiometer 10. In this step S150, the resistance value of the potentiometer 10 is automatically adjusted by the calculation unit 8. Specifically, the calculation unit 8 performs data processing to calculate the absolute value of the difference between the resistance values ​​of the positive leg and the negative leg of the measured thermocouple 2. Next, the calculation unit 8 selects the potentiometer 10 provided on the signal line 9 connected to the metal wire 4a with the smaller resistance value (for example, the positive leg) as the target for adjustment, and performs data processing to adjust the resistance value of the selected potentiometer 10 to a value obtained by adding the reference resistance value and its absolute value. For example, if a type K thermocouple 2 is used, in this step S150, the resistance value of the potentiometer 10 provided on the signal line 9 connected to the negative leg of the type K thermocouple 2 is adjusted.

[0040] In step S150, the resistance value of the potentiometer 10 provided on the signal line 9 connected to the metal wire 4b with the higher resistance value may be adjusted. In that case, the fixed resistor 22 and the potentiometer 10 must be connected in parallel, rather than in series, in order to lower the resistance value.

[0041] In step S150, if the adjustment unit 11 is composed of a known input device, the resistance value of the potentiometer 10 is adjusted by manually operating the input device. Specifically, the resistance values ​​of the metal wires 4a and 4b measured in steps S130 and S140 described above are output to an output unit such as a monitor, and the input device is operated based on these output resistance values.

[0042] The measurement process S200 is performed using the temperature measurement circuit C1 illustrated in Figure 1 above. In this temperature measurement circuit C1, the resistance values ​​of signal paths A and B are matched by the preparation process S100.

[0043] Figure 6 schematically shows a part of the thermocouple thermometer 1 during the measurement process S200 after the preparation process S100 has been performed. In Figure 6, the upper metal wire 4a is the positive leg of the thermocouple 2, and the lower metal wire 4b is the negative leg of the thermocouple 2. The resistance value of the negative leg of the thermocouple 2, measured during the preparation process S100, is 300Ω, and the resistance value of the positive leg is 200Ω. The initial resistance value of the potentiometer 10 is 200Ω, and the resistance value of the fixed resistor 22 is 1kΩ. The adjustment unit 11 adjusts the resistance value of the potentiometer 10, which is installed on the signal line 9 connected to the positive leg, to 300Ω, which is the initial value of 200Ω plus 100Ω, the absolute value of the difference between the resistance values ​​of the positive and negative legs of the thermocouple 2.

[0044] In step S210, the temperature sensing junction P1 of the thermocouple 2 is brought into contact with the object to be measured M. This causes the temperature sensing junction P1 of the thermocouple 2 to heat up to the temperature of the object to be measured M, and an electromotive force corresponding to that temperature is generated at the temperature sensing junction P1. Next, an analog signal using this electromotive force as a power source is input to the differential amplifier 6 via signal path A and signal path B, respectively. At this time, common-mode interference is formed between the analog signal and the object to be measured M, but as illustrated in Figure 6 above, the resistance values ​​of signal path A and signal path B are equal, so the common-mode interference can be amplified by the differential amplifier 6 (instrumentation amplifier 21) while suppressing its conversion to normal mode. Next, the analog signal amplified by the differential amplifier 6 is converted to a digital signal by the A / D converter 7, and the converted digital signal is input to the arithmetic unit 8. Similarly, the signal from the reference junction temperature sensor 18 is also converted to a digital signal and input to the arithmetic unit 8.

[0045] In step S220, the arithmetic unit 8 performs data processing to calculate the temperature of the object to be measured M based on each input digital signal. For this data processing, known calculation methods using a standard thermoelectric power table for thermocouples and a standard resistance value table for resistance thermometers can be used. The temperature of the object to be measured M calculated in step S220 is output to an output unit such as a monitor, and the measurement process S200 is completed.

[0046] As described above, according to this embodiment, in the preparation step S100, the adjustment unit 11 can easily adjust the resistance value of the potentiometer 10 provided on the signal line 9 according to the difference in resistance values ​​between the positive leg and the negative leg of the thermocouple 2. This makes it possible to equalize the resistance values ​​of the signal paths A and B from the temperature sensing junction P1 to the differential amplifier 6. Therefore, in the measurement step S200, measurement can be performed while suppressing the conversion of common-mode interference occurring between the measurement target M and the device to be measured into normal-mode interference.

[0047] Common-mode interference formed between the measurement target M and the measurement target becomes more pronounced in noisy environments where the measurement target M is a high-voltage product such as a battery or electric motor. In other words, the thermocouple thermometer 1 and measuring instrument 3 of this embodiment are more suitable for temperature measurement in noisy environments.

[0048] The specifications (cross-sectional area, length) of the pair of metal wires 4a and 4b of thermocouple 2 are generally the same. Therefore, when measuring the temperature of the object M using a general thermocouple thermometer or measuring instrument different from the thermocouple thermometer 1 or measuring instrument 3 of this embodiment, the resistance values ​​of the pair of signal paths connecting the reference junction compensator and the differential amplifier will differ due to the difference in the resistance values ​​of the pair of metal wires 4a and 4b. For example, when using type K as thermocouple 2, the Chromel wire on the positive leg has a volume resistivity of 70 μΩcm at 0°C and 70.6 μΩcm at 20°C, while the Alumel wire on the negative leg has a volume resistivity of 28.1 μΩcm at 0°C and 29.4 μΩcm at 20°C. In other words, in a typical thermocouple thermometer using a Type K thermocouple 2, if the positive and negative legs are of the same specifications, the resistance of the positive leg will be greater than the resistance of the negative leg. Consequently, the total resistance of the signal path connected to the positive leg will be greater than the total resistance of the signal path connected to the negative leg. When the resistances of each part of the signal path are different in this way, the common-mode interference formed with the object being measured M is converted to normal-mode interference. This normal-mode interference is then amplified directly by the differential amplifier.

[0049] In the thermocouple thermometer 1 and measuring instrument 3 of this embodiment, after the preparation step S100 is performed, the resistance value of the potentiometer 10 is adjusted by the adjustment unit 11 based on the difference in resistance values ​​of signal paths A and B, that is, the difference in resistance values ​​between the pair of metal wires 4a and 4b of the thermocouple 2. This adjustment of the resistance value of the potentiometer 10 by the adjustment unit 11 in the preparation step S100 is performed automatically by the calculation unit 8 or manually by operating the input device. Therefore, the resistance values ​​of signal paths A and B can be matched more easily than by adjusting the wire diameter and length of the pair of metal wires 4a and 4b of the thermocouple 2 to match the other side. It is desirable that the matched resistance values ​​of signal paths A and B be equal, but they do not need to be strictly equal.

[0050] Furthermore, the adjustment of the resistance value of the potentiometer 10 by the adjustment unit 11 can be performed at any desired timing. Therefore, even if the resistance values ​​of the pair of metal wires 4a and 4b of the thermocouple 2 change due to changes in the measurement environment (for example, changes in the temperature around the thermocouple thermometer 1), the adjustment unit 11 can adjust the resistance value of the potentiometer 10 each time, making it possible to adapt to the change. In addition, by configuring the system to compare the change in resistance value with the previous measurement, a function to diagnose abnormalities or partial breaks in the thermocouple 2 can also be added.

[0051] When the temperature sensing junction P1 of the thermocouple 2 is brought into contact with the object to be measured M, heat is transferred from the object to be measured M to the thermocouple 2. This amount of heat transfer is proportional to the cross-sectional area of ​​the pair of metal wires 4a and 4b. Therefore, the smaller the cross-sectional area of ​​the pair of metal wires 4a and 4b, the smaller the amount of heat transferred, and the larger the cross-sectional area of ​​the pair of metal wires 4a and 4b, the larger the amount of heat transferred. In other words, by making the cross-sectional area of ​​the pair of metal wires 4a and 4b smaller, the amount of heat transferred from the object to be measured M to the thermocouple 2 is reduced, which is advantageous for improving the accuracy of temperature measurement. However, the smaller the cross-sectional area of ​​the pair of metal wires 4a and 4b, the more difficult it is to manufacture, so it becomes difficult to match the resistance values ​​when adjusting the wire diameter and length of each wire of the pair of metal wires 4a and 4b to match the other side. Also, since the resistance difference increases as the wire diameter decreases, this embodiment is very effective in suppressing the mode conversion of noise in the thermocouple 2.

[0052] One noise countermeasure is to cover the metal wires 4a and 4b of thermocouple 2 with shielding. However, this method increases costs. Also, the thickness of thermocouple 2 increases due to the shielding. Furthermore, when used in a high-voltage environment, care must be taken to prevent the shield from touching high-voltage parts, and if the housing 16 or connector of the measuring instrument 3 is made of resin, the shield may float or the connection may become high impedance, preventing the shield from being effective. In this embodiment, such concerns are not present, and the shield functions effectively even in situations where it would not normally be effective.

[0053] In preparation step S100, the resistance values ​​of the positive and negative legs of the thermocouple 2 can be measured using the resistance measurement circuit C2, eliminating the need to prepare a separate circuit meter for measuring resistance. Furthermore, since the resistance values ​​of the positive and negative legs can be measured with high accuracy in an environment close to the measurement environment, noise mode conversion can be suppressed more reliably. In addition, by using the calculation unit 8 as the adjustment unit 11, the resistance value of the potentiometer 10 can be automatically adjusted by the calculation unit 8. This eliminates the influence of human error on the adjustment of the resistance value of the potentiometer 10, which is advantageous for more accurate temperature measurement.

[0054] In steps S130 and S140 of the preparation process S100 described above, the resistance values ​​of the positive and negative legs of the thermocouple 2 were measured using the resistance measurement circuit C2. However, these resistance values ​​may also be measured using a known circuit meter, or estimated from the catalog specifications of the thermocouple 2 or the measurement environment. For example, the resistance values ​​of the positive and negative legs can be estimated using the type of thermocouple 2, the cross-sectional area and length of the pair of metal wires 4a and 4b, and the temperature of the measurement environment. Furthermore, if the resistance values ​​of the positive and negative legs of the thermocouple 2 are known in advance, it is not necessary to measure them. For example, if the thermocouple 2 has tags with the resistance values ​​of the positive and negative legs written on them, or IC tags with the resistance values ​​stored on them, the resistance values ​​written on or stored on those tags can be used. Therefore, if resistance measurement by the resistance measurement circuit C2 is unnecessary, the thermocouple thermometer 1 and measuring instrument 3 may omit the pair of constant current signal lines 13, contact terminals 14, and reference potential signal line 15.

[0055] If the type of thermocouple 2 to be connected is limited, the measuring instrument 3 may have a potentiometer 10 only on the signal line 9 connected to the metal wire with the lower resistance value of the pair of metal wires 4a and 4b. Specifically, if the thermocouple 2 used in the measurement process S200 is limited to type K, it is sufficient to have a potentiometer 10 only on the signal line 9 connected to the negative leg of the thermocouple 2. In this case, the signal line 9 connected to the positive leg of the thermocouple 2 may be equipped only with a fixed resistor 22, and the reference resistance value of the potentiometer 10 and the resistance value of the fixed resistor 22 should be the same.

[0056] The thermocouple thermometer 1 may have a thermocouple 2 connected to the measuring instrument 3 via a pair of compensating wires. The pair of compensating wires are made of the same type of metal wire as the pair of metal wires 4a and 4b, with one end connected to the thermocouple 2 and the other end connected to a pair of terminals 17. Therefore, in the preparation step S100 in this case, the other end of the pair of compensating wires is considered as the reference junction P2, and the resistance value of the potentiometer 10 is adjusted accordingly. Specifically, the difference between the sum of the resistance values ​​of the positive leg of the thermocouple 2 and the compensating wire connected to that positive leg, and the sum of the resistance values ​​of the negative leg of the thermocouple 2 and the compensating wire connected to that negative leg is used to adjust the resistance value of the potentiometer 10.

[0057] The reference junction compensator 5 is not limited to the configuration described above. For example, the reference junction compensator 5 may have a pair of connection terminals connected by a pair of compensating wires to a pair of terminals 17, and a thermocouple 2 may be connected to the pair of connection terminals. In this reference junction compensator 5, the pair of terminals 17 are installed inside the housing 16 closer to the differential amplifier 6, and the pair of connection terminals are arranged on the outer circumference of the housing 16 or outside the housing 16. The reference junction temperature sensor 18 considers the other end of the pair of compensating copper wires as the reference junction P2 and acquires the temperature of the pair of terminals 17, i.e., the temperature inside the measuring instrument 3, as the temperature of the reference junction P2. In addition, this reference junction compensator 5 can also use general-purpose wires other than compensating wires to connect the pair of connection terminals to the pair of terminals 17. In this case, it is preferable to have a terminal thermocouple that acquires the temperature of the pair of connection terminals and a pair of thermocouple terminals to which the terminal thermocouple is connected. The terminal thermocouple has a temperature-measuring junction that contacts the pair of connection terminals, and a reference junction that is connected to the pair of thermocouple terminals. The pair of thermocouple terminals are heated to the same temperature as the pair of terminals 17. As a result, the reference junction temperature sensor 18 acquires the internal temperature of the measuring instrument 3, and the terminal thermocouple acquires the temperature of the pair of connection terminals, i.e., the external temperature (ambient temperature) of the measuring instrument 3. The temperature of the object to be measured M is then calculated using the acquired temperatures.

[0058] The present invention is not limited to any particular embodiment, and various modifications and changes are possible within the scope of the gist of the invention. [Explanation of Symbols]

[0059] 1. Thermocouple thermometer 2 thermocouples 3 Measuring Instruments 4a, 4b metal wire 5 Reference junction compensator 6. Differential Amplifier 7 A / D Converters 8 Arithmetic unit 9 signal lines 10 Potentiometers 11 Adjustment part 12 Constant current source 13 Constant current signal line 14 Contact terminal 15. Reference potential signal line 16 cabinets 17 terminals 18. Reference junction temperature sensor 19 Insulating material 20 multiplexer 21 Instrumentation Amplifier 22 Fixed resistor 23 Control signal line C1 Temperature measurement circuit C2 Resistance Measurement Circuit P1 Temperature sensing junction P2 Reference Contact

Claims

1. A thermocouple measuring instrument comprising a reference junction compensator to which a thermocouple is connected, a differential amplifier that differentially amplifies the analog signals of the thermocouple and the reference junction compensator, an A / D converter that converts the analog signals into digital signals, and a calculation device that calculates the temperature of the thermocouple's temperature sensing junction using the digital signals, A measuring instrument for a thermocouple, comprising a potentiometer provided on at least one of a pair of signal lines connecting the reference junction compensator and the differential amplifier, and an adjustment unit for adjusting the resistance value of the potentiometer.

2. The system comprises a constant current source, a pair of constant current signal lines connecting the pair of terminals and the constant current source, contact terminals made of a conductive material that heat evenly with the pair of terminals, and a reference potential signal line connected to a reference potential that connects the contact terminals and the differential amplifier. In the measurement process for measuring the temperature of the object to be measured, the temperature measuring junction of the thermocouple connected to the reference junction compensator comes into contact with the object to be measured, forming a temperature measurement circuit for measuring the temperature of the object. The thermocouple measuring instrument according to claim 1, wherein in a preparation step prior to the measurement step, the temperature-measuring junction of the thermocouple connected to the reference junction compensator is in direct contact with the contact terminal, and a resistance value measuring circuit is formed to individually measure the resistance values ​​of each of the pair of metal wires.

3. The thermocouple measuring instrument according to claim 2, wherein the adjustment unit is the calculation device connected to the potentiometer by a control signal line, and in the preparation step, the resistance values ​​of each of the pair of metal wires measured by the resistance value measurement circuit are input to the calculation device, and data processing is performed by the calculation device to adjust the resistance value of the potentiometer based on the difference between the input resistance values ​​of each of the pair of metal wires.

4. The thermocouple measuring instrument according to claim 1, further comprising an individual potentiometer for each of the pair of signal lines.

5. A thermocouple thermometer comprising a measuring instrument for a thermocouple according to any one of claims 1 to 4 and the thermocouple.

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