Temperature probe and method for manufacturing a temperature probe

Abstract

The invention relates to a temperature probe for determining a temperature according to the three-point probe method, the temperature probe (1) having a sensor element (2) which provides a temperature value, wherein a several-meter-long three-wire line (3) consisting of a first connecting line (4), a second connecting line (5) and a third connecting line (6) is connected to the sensor element (2), wherein the connecting lines (4, 5, 6) are made of a first material and serve for transmitting energy and the measured temperature value, wherein an electrically conductive element (7, 8) made of a second material is inserted into each of the second connecting line (5) and the third connecting line (6), the electrical resistivity of the second material being higher than the electrical resistivity of the first material, and wherein the two inserted electrically conductive elements (7, 8) are designed such that the second connecting line (5) and the third connecting line (6) have substantially the same electrical resistance as the first connecting line (4).

Description

Technical Field

[0001] This invention relates to a temperature probe for high-precision measurement. Background Technology

[0002] Thermometers with resistive elements typically include so-called thin-film sensor elements, or resistance temperature detectors (RTDs). Typically, such sensor elements have a substrate on which leads are disposed and a metal coating is applied to the front surface. A metal coating may also be used on the rear surface. Platinum is commonly used as the sensor element, and is commercially available under names such as PT10, PT100, and PT1000. In many cases, the sensor element is preferably encapsulated or embedded in pure ceramic powder, and the connecting leads are guided in a guide tube to electronic devices, such as temperature transmitters. Details of industrial platinum resistance thermometers can be found, for example, in the European standard EN60751.

[0003] Various measurement methods for determining temperature using resistive elements have become known from the prior art. Typically, a transmitted signal in the form of a current is applied to the sensor element, and a received signal, typically in the form of a voltage drop across the sensor element, is detected and evaluated relative to temperature.

[0004] In the simplest case, temperature is determined using a temperature-dependent resistor, where the sensor element is simply in contact via two wires. The drawback of this solution is that the resistance of the wires is included as an error in the temperature determination. According to another method, the sensor element is in contact via three wires. By tapping a drop voltage between pairs of two of the three wires in each case, the resistance of one or more wires can be greatly compensated if it can be assumed that the three wires have the same resistance. It is also known to measure temperature using four wires.

[0005] RTD Pt100 temperature sensors are widely used as sensor elements in process monitoring. They are among the most commonly used standard temperature sensors on the market. In some applications of process automation, very long sensors are required to reach the areas where temperature should be detected and / or monitored. These temperature sensors can have a total length of up to one hundred meters.

[0006] In such applications, the sensor cable must ensure the necessary robustness. Typically, it is made of MgO cable with an outer metal protective sheath. The protective sheath is usually made of stainless steel or a nickel alloy. A number of conductors, in most applications copper wire, form the connection wires used to connect the temperature sensor to the electronics.

[0007] The most well-known solution for measuring temperature with very long temperature probes involves measuring the resistance of a Pt100 probe using a 4-wire terminal sensing method. This so-called 4-point probe method involves injecting current using two wires and measuring voltage using the remaining two wires, such as... Figure 1 The diagram is schematically illustrated. The advantage of this measurement method is that the measurement is unaffected by the resistance of the connecting wire between the probe and the temperature probe's electronics, thus the measurement is independent of the length of the connecting cable.

[0008] Another very common measurement method required by the market is the 3-wire terminal sensing, 3-wire connection, or 3-point probe method. Typically, this method is required to reduce the cost of temperature measurement equipment, or for design reasons: in fact, in probes with multiple sensing elements (two or more), reducing the number of necessary wires in the cable has the benefit of making the cable more compact (smaller diameter and therefore less invasive) or allowing for an increase in the number of possible measurement points in the same cable.

[0009] Compared to the four-wire method, this approach has a fundamental limitation: the measurement can only compensate for the resistance of the connecting wires without any additional error if the resistance of the cables is the same in all three cables used in the measuring device. This limitation can be easily demonstrated by analyzing how the measurement is performed – see [link to relevant documentation]. Figure 2 :

[0010] In this case, the resistance of the Pt100 probe is the result of the resistance of the two loops:

[0011] In a circuit, the resistance Rc1 is measured between the common connection point C and point 1:

[0012] Rc1=R common+R1+R Pt100.

[0013] Then take measurements between point 1 and point 2:

[0014] Rc2=R1+R2

[0015] Assuming that the three connection resistors are identical, the result is calculated as follows:

[0016] Measurement = Rc1 – Rc2 = R common + R1 + R Pt100 – (R1 + R2) = RPt100 + R common – R2

[0017] Other calculation methods are possible and applied by different measuring devices, but the result is always the same: the measuring device can measure Pt100 without any error only when the three cables have the same resistance. When considering... Figure 3 This becomes clear when the circuit shown is presented.

[0018] Measurement 1 = Va / I = R common + R Pt100 (the current in R2 is zero)

[0019] Measurement 2 = Vb / I = R1

[0020] Final measured value = (Va – Vb) / I = R common + R Pt100 – R1 = Pt100 (if R common = R1)

[0021] This method is typically a good compromise, but it assumes that the resistance of the connecting cables is uniform. This is usually not true. Unfortunately, the wires of an MgO cable do not have uniform resistance. This is due to the wire-stretching process during manufacturing. In these components, there is typically a resistance difference of about 1-3% depending on the total value of the cable length. For very long sensors, the total resistance can reach 20 ohms or more, which also depends on the cable diameter, i.e., the diameter of the inner wires.

[0022] The resistance difference between the wires of a cable can negatively affect the measurement accuracy that can exceed the required limits.

[0023] If calibration is performed in an ice + water reference bath at 0°C, the measurement error of the Pt100 probe can be as follows:

[0024] Temperature error = (R Pt100–100) / 0.39 = Resistance error / 0.39 0°

[0025] Class AA: Maximum permissible temperature (T) error < 0.10℃; maximum permissible temperature (R) error < 0.039Ω

[0026] Class A: Maximum permissible T error < 0.15℃; Maximum permissible R error < 0.0585Ω

[0027] Class B: Maximum permissible T error < 0.10℃; Maximum permissible R error < 0.117Ω

[0028] If the known three-wire method is used, these measurement errors of different classes are generally not achievable with the construction of very long MgO cables. Summary of the Invention

[0029] The purpose of this invention is to provide a temperature probe operating according to a three-wire method, which enables highly accurate temperature measurement. Additionally, the purpose of this invention is to provide a processing method.

[0030] To achieve this objective, the present invention includes a temperature probe for determining temperature according to a three-point probe method, the temperature probe having a sensor element that provides a temperature value, wherein a three-wire line of several meters, consisting of a first connecting wire, a second connecting wire, and a third connecting wire, is associated with the sensor element, wherein the connecting wires are made of a first material and are used to transmit energy and the measured temperature value, wherein a conductive element made of a second material is inserted into each of the second and third connecting wires, the second material having a resistivity greater than that of the first material, and wherein the inserted conductive element is designed such that the second and third connecting wires have substantially the same resistance as the first connecting wire.

[0031] The solution according to the invention is particularly suitable for 3-wire cables of relatively long temperature probes to ensure a preferred high level of accuracy: resistance compensation of the connecting wires is preferably achieved by inserting a relatively short conductive material with a higher resistivity than the material of the connecting wires into two of the three wires of the 3-wire cable. Equal resistance in each of the three connecting wires is achieved by inserting conductive elements of a certain length and / or diameter into two of the three wires.

[0032] According to an embodiment of the temperature probe, the resistivity of the second material is proposed to be at least five times higher than that of the first material. Preferably, the connecting wire is made of copper, and the inserted conductive element is made of constantan. Resistance compensation is achieved by selecting the correct material for the conductive element. For efficient construction, good compensation, and short length, the material must have a much higher resistivity than the original wire. Most wires in MgO cables are made of copper. Analysis by comparing different materials and the resistance values ​​that must be compensated leads to the conclusion that constantan is the preferred material for the conductive element. Constantan has a high resistivity and good and robust mechanical properties compared to, for example, copper.

[0033] According to an embodiment of the temperature probe, an inserted conductive element made of at least one second material is arranged within a transition bushing of the probe. Two segments of a three-wire cable are connected together within this transition bushing. To provide an extended temperature probe, a transition bushing is typically used to connect the MgO cable to the flexible extension cable. Resistance-compensating conductive elements are inserted between the end segments of the corresponding wires of the MgO cable and the flexible extension cable. They can be connected by any known method, such as soldering, brazing, tinning, or crimping. Each connection can be protected by an additional Kapton or heat-shrinkable insulated cable to isolate it from other connections. Finally, the entire bushing can be sealed by resin potting.

[0034] According to an alternative design for the temperature sensor, a conductive element made of at least a second material is inserted into the connection area through which a three-wire circuit can be connected to external electronics: the conductive element, which compensates for the resistance difference of the connecting wires, is attached to the terminal to which the main cable's connecting wire is connected. Depending on the length of the main cable, this can be an MgO cable or a flexible extension cable. The two wires with the conductive element inserted can be stripped and broken. The conductive element is inserted between the connecting wire and the terminal. Similarly, the connection can be soldered, brazed, tinned, or crimped. The wires are insulated from each other, for example, using heat-shrink tubing. Additional heat-shrink tubing insulation can be applied to protect the connection.

[0035] According to an alternative design of the temperature sensor, a conductive element made of at least a second material is inserted into the connection area. Preferably, this connection area is arranged within a flexible extension cable. The two wires into which the conductive element is inserted can be stripped and broken. The conductive element is inserted between the connecting wire and the terminal. Similarly, the connection can be soldered, brazed, tinned, or crimped. The conductive element and wire connection is directly connected to the terminal as in the foregoing embodiments or by using a rigid support as a reinforcement.

[0036] According to an embodiment of the temperature probe, the resistance of each of the two conductive elements inserted in the second and third connecting lines is designed such that the temperature probe provides a measurement value with a predetermined measurement accuracy. For example, the accuracy class could be A or B.

[0037] Further specifying, the sensing element is a resistance temperature detector (RTD) element, preferably a platinum measuring resistor PT100. Any other suitable sensor element can be used in conjunction with the solution of this invention.

[0038] Regarding a method for producing a temperature probe used to determine temperature according to a three-point probe method, the temperature probe has a sensor element, preferably designed as a platinum measuring resistor, the sensor element providing a temperature measurement value, wherein a three-wire circuit several meters long, consisting of a first connecting wire, a second connecting wire, and a third connecting wire, is associated with the sensor element, wherein the connecting wire is made of a first material having a predetermined specific resistance and is used to transmit energy and to transmit the measured temperature value, wherein the following method steps are proposed:

[0039] - Measure the resistance of each of the three connecting lines;

[0040] - Identify the connection line with the highest resistance, hereinafter: the first connection line;

[0041] - Insert a first conductive element into the second connecting wire, wherein the first conductive element is made of a second material with a resistivity greater than that of the first material, and wherein the dimensions of the inserted first conductive element are designed such that the second connecting wire has the same resistance as the first connecting wire;

[0042] - Insert a second conductive element into the third connecting line, wherein the second conductive element is made of the second material, and wherein the dimensions of the inserted second conductive element are designed such that the third connecting line has the same resistance as the first connecting line.

[0043] In the development of this method, the conductive element is welded, brazed, soldered, or crimped for insertion into the corresponding connecting wire. Attached Figure Description

[0044] The invention will be explained in more detail with reference to the following figures.

[0045] Figure 1 A schematic diagram of a circuit for measuring temperature using the 4-point probe method and the corresponding temperature probes is shown.

[0046] Figure 2 A schematic diagram of a circuit for measuring temperature using the three-point probe method and the corresponding temperature probes is shown.

[0047] Figure 3 Showing more details Figure 2 The circuit,

[0048] Figure 4 A table showing the measurement error based on the length of an exemplary cable with three connecting wires is provided.

[0049] Figure 5 The temperature probe of the present invention is illustrated schematically.

[0050] Figure 6 A table showing the resistivity of different conductive materials is provided.

[0051] Figure 7 A first embodiment of the temperature probe of the present invention is shown, and

[0052] Figure 8 A second embodiment of the temperature probe of the present invention is shown.

[0053] Figure 9 A third embodiment of the temperature probe of the present invention is shown.

[0054] exist Figures 1 to 3 Different existing technology solutions for temperature probe 1 and corresponding methods for measuring temperature have been described in the document. Detailed Implementation

[0055] For temperature sensor 1 with a resistance thermometer element 2, such as Pt100, MgO cable 14 is typically used. A cable length greater than 50 m is usually required to measure temperature at a remote location. Additional requirements include a predetermined high measurement accuracy (e.g., Class A) and the use of a three-wire cable. Due to the technical characteristics of MgO cables, achieving the required accuracy level is difficult, or in some cases, impossible. The problem is that the internal connecting wires 4, 5, and 6 of the MgO cable 14 typically do not have the same resistance. Manufacturers typically declare the accuracy between wires 4, 5, and 6 of a three-wire cable 3, which has a line resistance of approximately 0.002 ohms / m, on a typical 6 mm MgO cable 14.

[0056] Corresponding experimental studies have confirmed that, statistically, the resistance differences of lines 4, 5, and 6 can be expected to have a standard deviation of approximately 1% of the total measured values.

[0057] As an example, Figure 4 A table is shown that visualizes the measurement error based on the length of the three-wire cable 3 with three connecting lines 4, 5, and 6 for the temperature probe 1. Specifically, the table shows the maximum cable length beyond which the required accuracy class A or B for temperature measurement can no longer be maintained. Therefore, by considering very long temperature probes 1, we have... Figure 4 As shown, once the connecting lines 4, 5, and 6 exceed a certain length, the measured value deviates from the given accuracy level.

[0058] According to the temperature probe 1 of the present invention, the resistance difference of the three connecting lines 4, 5, and 6 is compensated by adding an additional resistance. Preferably, the resistance of two of the three lines 4, 5, and 6 is equal to the resistance of the connecting line with the highest resistance (e.g., 4). The temperature probe 1 of the present invention is simple and inexpensive to manufacture because the compensation method is less invasive, yet provides high-precision temperature measurement. A conductive element 7, 8 with a high resistivity and a defined size is required to modify the resistance of the remaining two connecting lines 5, 6 such that each of the connecting lines 4, 5, and 6 has the same resistance.

[0059] Figure 5 A schematic diagram of the temperature probe 1 of the present invention, which determines temperature according to the three-point probe method, is shown. Figure 6 A table showing the resistivity of different conductive materials is provided.

[0060] The calculation of the linear resistance of connecting lines 4, 5, and 6 is very simple:

[0061] Linear resistance = Material resistivity / Linear cross-section

[0062] By using standard constantan wire with a diameter between 0.2 mm and 0.5 mm, it is possible to compensate for the resistance difference between the three connecting wires 4, 5, and 6 of the MgO cable 14 by adding 10 mm conductive elements 7 and 8 to the 50 mm constantan wire.

[0063] The lengths of conductive elements 7 and 8 are calculated using the following formula:

[0064] Compensation length = resistance difference / linear line resistance.

[0065] The steps for compensating for the resistance difference on the three lines are described below:

[0066] The process begins by measuring the resistance of each of the three lines 4, 5, and 6.

[0067] Line 4, which has the highest resistance, is identified, and the difference between its maximum value and the resistance values ​​of the two remaining lines 5 and 6 is calculated.

[0068] For each of the remaining two lines 5 and 6, the compensation length of the conductive elements 7 and 8, preferably made of constantan, is calculated such that the resistance of each of them is equal to the resistance of the first line 4, which has the highest resistance value.

[0069] Figure 7 A first embodiment of the temperature probe 1 according to the invention is shown. The focus is on attaching conductive elements 7, 8 to two or at least one of the connecting wires 4, 5, 6. The conductive elements 7, 8, made of at least one second material, such as constantan, are arranged in a transition bushing 9 of the temperature probe 1. This transition bushing 9 is used to connect two different sections 10, 11 of the three-wire cable 3.

[0070] For the extended temperature probe 1, this transition bushing 9 is typically used to connect the MgO cable 14 to the flexible extension cable 15. Compensating conductive elements 7 and 8 are inserted between the end sections of the corresponding wires of the MgO cable 14 and the flexible extension cable 15. They can be connected by any known method, such as welding, brazing, soldering, or crimping. For electrical insulation, each joint can be protected by an additional Kapton or heat-shrinkable insulating sleeve or cap 16. Finally, the entire bushing 9 can be sealed by resin potting 17.

[0071] Figure 8A view of a second embodiment of the temperature probe of the present invention is shown. According to this alternative design of the temperature probe, conductive elements 7, 8 made of at least a second material are inserted into a connection region 12 through which a three-wire cable 3 can be connected to an external electronic device 13. The conductive elements 7, 8, which compensate for the resistance difference of the connecting wires 4, 5, 6, are attached to the terminals 18 to which the connecting wires 4, 5, 6 of the main cable 3 are connected. Depending on the length of the main three-wire cable 3, this can be an MgO cable 14 or a flexible extension cable 15. The two wires 4, 5, 6 into which the designed conductive elements 7, 8 are inserted can be stripped and broken. The conductive elements 7, 8 are inserted between the connecting wires 4, 5, 6 and the terminal 18. Similarly, the connection can be soldered, brazed, tinned, or crimped. For electrical insulation, each joint can be protected by an additional Kapton or heat-shrinkable insulating sleeve or cap 16. Alternatively, a heat-shrinkable tube insulator 19 can be used to protect the joint.

[0072] Figure 9 A third embodiment of the temperature probe 1 of the present invention is shown. Here, the conductive elements 8 made of a second material and the corresponding wire connections 20 connecting the conductive elements 7, 8 to the wires 5, 6 are protected by a casing 21. The conductive elements 7, 8 and the wire connections 20 are protected by the casing 21. Preferably, the casing is inserted into the flexible extension cable 15. They can also be directly inserted into the terminal portion 19 through which the temperature probe 1 is connected to the external electronics 13. The conductive elements 7, 8 and the wire connections 20 are inserted into a suitable heat-shrink tube 16 or insulating tape. Then, the connection area 12 is protected by another casing 21. The connection area 12 can be at the end or in the middle of the flexible extension cable 15.

[0073] List of reference numerals

[0074] 1 Temperature probe

[0075] 2 sensor components

[0076] 3 Three-wire cable

[0077] 4 First connecting line / First line

[0078] 5 Second connecting line / second line

[0079] 6. Third connecting line / third line

[0080] 7 First conductive element

[0081] 8 Second conductive element

[0082] 9 bushings

[0083] 10 sections of three-wire cable

[0084] Section 11 of the three-wire cable

[0085] 12 connection areas

[0086] 13 Electronic Components

[0087] 14MgO cable

[0088] 15 Flexible Extension Cable

[0089] 16 insulating tube

[0090] 17 Resin potting

[0091] 18-tube insulator

[0092] 19 terminals

[0093] 20-wire connection

[0094] 21-packaging box.

Claims

1. A temperature probe (1) for determining temperature according to the three-point probe method, the temperature probe having a sensor element (2) that provides a temperature value, wherein, A three-wire line (3) several meters long, consisting of a first connecting line (4), a second connecting line (5), and a third connecting line (6), is connected to the sensor element (2). The connecting lines (4, 5, 6) are made of a first material and are used to transmit energy and measure temperature values. Conductive elements (7, 8) of a certain length and diameter, made of a second material, are inserted into each of the second connecting line (5) and the third connecting line (6). The resistivity of the second material is at least 5 times higher than that of the first material. The dimensions of the two inserted conductive elements (7, 8) of the length and diameter are designed such that the second connecting line (5) and the third connecting line (6) have substantially the same resistance as the first connecting line (4).

2. The temperature probe according to claim 1, in, The connecting wires (4, 5, 6) are made of copper, and the inserted conductive elements (7, 8) are made of constantan.

3. The temperature probe according to claim 1, in, The inserted conductive elements (7, 8) made of the second material are arranged in the bushing (9), and the two sections (10, 11) of the three-wire line (3) are interconnected at the bushing (9).

4. The temperature probe according to any one of claims 1-3, in, The conductive elements (7, 8) made of the second material are arranged in the connection area (12), and the three-wire line (3) can be connected to an external electronic device (13) via the connection area (12).

5. The temperature probe according to any one of claims 1-3, in, The resistances of the two conductive elements (7, 8) inserted in the second connecting line (5) and the third connecting line (6) are designed such that the temperature probe (1) provides a measurement value with a predetermined measurement accuracy.

6. The temperature probe according to any one of claims 1-3, in, The sensor element (2) is a resistance temperature detector element.

7. The temperature probe according to claim 6, wherein, The resistance temperature detector element is a platinum measuring resistor PT100.

8. A method for manufacturing a temperature probe for determining temperature according to a three-point probe method, the temperature probe having a sensor element (2) that provides a temperature measurement, wherein, A three-wire line (3) several meters long, consisting of a first connecting wire (4), a second connecting wire (5), and a third connecting wire (6), is associated with the sensor element (2), wherein the connecting wires (4, 5, 6) are made of a first material having a predetermined specific resistance and are used to transmit energy and to transmit measured temperature values, wherein the following method steps are provided: Measure the resistance of each of the three connecting lines (4, 5, 6); Identify the connection line (4, 5, 6) with the highest resistance, hereinafter: the first connection line (4); A first conductive element (7) is inserted into the second connecting line (5), wherein the first conductive element (7) is made of a second material with a resistivity greater than that of the first material, and wherein the dimensions of the inserted first conductive element (7) are designed such that the second connecting line (5) has the same resistance as the first connecting line (4); A second conductive element (8) is inserted into the third connecting line (6), wherein the size of the inserted second conductive element (8) is designed such that the third connecting line (6) has the same resistance as the first connecting line (4).

9. The method according to claim 8, in, The sensor element is designed as a platinum measuring resistor.

10. The method according to claim 8, in, The second conductive element (8) is made of the second material.

11. The method according to claim 8, in, The conductive elements (7, 8) are soldered, brazed, tinned or crimped to be inserted into the corresponding connecting wires (5, 6).

12. The method according to any one of claims 8-11, in, The inserted conductive elements (7, 8) made of the second material are arranged in the bushing (9), and the two sections (10, 11) of the three-wire line (3) are interconnected at the bushing (9).

13. The method according to any one of claims 8-11, in, The inserted conductive elements (7, 8) made of the second material are arranged in the connection area (12), and the three-wire line (3) can be connected to an external electronic device (13) via the connection area (12).

14. The method according to any one of claims 8-11, in, The conductive elements (7, 8) made of the second material and the corresponding wire connections (20) connecting the conductive elements (7, 8) to the wires (5, 6) are protected by a package (21), wherein the conductive elements (7, 8) protected by the package (21) are inserted into a flexible extension cable (15).

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