TEMPERATURE PROBE AND METHOD FOR MANUFACTURING A TEMPERATURE PROBE

Abstract

A temperature probe for determining the temperature according to the three-point probe method includes a three-wire line several meters long consisting of a first connecting line, a second connecting line, and a third connecting line connected to sensor element. The connecting lines are made of a first material and serve to transmit energy and the measured temperature values. A conductive element made of a second material is inserted in the second connecting line and in the third connecting line. The resistivity of said second material is higher than the resistivity of the first material. The two inserted conductive elements are designed in such that the second connecting line and the third connecting have the same resistance as the first connecting line. Additionally, the present disclosure refers to a method describing the manufacture of a temperature probe.

Claims

1-12. (canceled)

13. A temperature probe for determining a temperature according to a three-point probe method, the temperature probe comprising: a sensor element embodied to provide temperature values; a three-wire line several meters long, the three-wire line including: a first connecting line; a second connecting line; and a third connecting line, wherein the three-wire line is connected to the sensor element, and wherein each of the three connecting lines is made of a first material and serves to transmit energy and measured temperature values; a first conductive element made of a second material and inserted in the second connecting line; and a second conductive element made of the second material and inserted in the third connecting line, wherein a resistivity of the second material is higher than a resistivity of the first material, and wherein the two inserted conductive elements are designed such that the second connecting line and the third connecting line have a same resistance as the first connecting line.

14. The temperature probe according to claim 13, wherein the resistivity of the second material is at least 5 times higher than the resistivity of the first material.

15. The temperature probe according to claim 14, wherein the three connecting lines are made of copper, and wherein the two inserted conductive elements are made of constantan.

16. The temperature probe according to claim 13, wherein the two inserted conductive elements made of the second material are arranged within a bushing where two sections of the three-wire line are interconnected.

17. The temperature probe according to claim 13, wherein the two conductive elements made of the second material are arranged in a connection area via which the three-wire line is connectable to external electronics.

18. The temperature probe according to claim 13, wherein the resistance of the two conductive elements inserted in the second connecting line and in the third connecting line is designed such that the temperature probe provides measured values with a predetermined measurement accuracy.

19. The temperature probe according to claim 13, wherein the sensing element is a Resistance Temperature Detector (RTD) element.

20. A method of manufacturing a temperature probe for determining a temperature according to a three-point probe method with a sensor element designed as a platinum measuring resistor that provides temperature measured values, wherein a three-wire line several meters long having a first connecting line, a second connecting line, and a third connecting line is associated with the sensor element, wherein the three connecting lines are made of a first material with a predetermined specific resistance and serve for transmitting energy and for transmitting the measured temperature values, the method comprising: measuring a resistance of each of the three connecting lines; determining a connecting line having the highest resistance, hereinafter: the first connecting line; inserting a first conductive element into the second connecting line, wherein the first conductive element is made of a second material having a resistivity greater than the resistivity of the first material, and wherein the inserted first conductive element is dimensioned such that the second connecting line has the same resistance as the first connecting line; inserting a second conductive element into the third connecting line, wherein the second conductive element is made of the second material, and wherein the inserted second conductive element is dimensioned such that the third connecting line has the same resistance as the first connecting line.

21. The method according to claim 20, wherein the two conductive elements are welded, brazed, soldered, or crimped for insertion into the corresponding connecting lines.

22. The method according to claim 20, wherein the inserted conductive elements made of the second material are arranged within a bushing where two sections of the three-wire line are interconnected.

23. The method according to claim 20, wherein the inserted conductive elements made of the second material are arranged in a connection area via which the three-wire line is connectable to external electronics.

24. The method according to claim 20, wherein the conductive elements made of the second material and corresponding wire connections connecting the conductive elements to the wires are protected by an encapsulation box, whereby the conductive elements protected by the encapsulation box are inserted in a flexible extension cable.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The present disclosure is explained in more detail with reference to the following figures.

[0040] FIG. 1 shows a circuit for measuring temperature according to the 4-points-probe method, and an schematic view of a corresponding temperature probe,

[0041] FIG. 2 shows a circuit for measuring temperature according to the 3-points-probe method, and a schematic view of a corresponding temperature probe,

[0042] FIG. 3 shows in ore details the circuit of FIG. 2,

[0043] FIG. 4 shows a table of the measurement error as function of the length of an exemplary cable with three connecting wires,

[0044] FIG. 5 shows schematically an inventive temperature probe,

[0045] FIG. 6 shows a table of the resistivity of different conductive materials,

[0046] FIG. 7 shows a first embodiment of the inventive temperature probe,

[0047] FIG. 8 shows a second embodiment of the inventive temperature probe, and

[0048] FIG. 9 shows a third embodiment of the inventive temperature probe.

DETAILED DESCRIPTION

[0049] The different prior art solutions of temperature probes 1 and the corresponding methods for measuring the temperature are already described in FIGS. 1-3.

[0050] For temperature sensors 1 with resistance thermometer elements 2, for example a Pt100, MgO cables 14 are usually used. A cable length of more than 50 m is often required to measure the temperature in a remote location. Further requirements are a predetermined high measuring accuracy (e.g., class A) and the use of a 3-wire line. Due to the technical properties of the MgO cable, it is difficult, or in some cases impossible, to reach the requested accuracy class. The problem is that the inner connecting wires 4, 5, 6 of an MgO cable 14 usually do not have the same resistance. The manufacturers generally declare an accuracy among the wires 4, 5, 6 of a three-wire cable 3 of about 0.002 Ohm/m on a typical 6 mm MgO cable 14 with a wire resistance of about 0.04-0.06 Ohm/m.

[0051] Corresponding experimental investigations have confirmed that statistically a difference in resistance of wires 4, 5, 6 with a standard deviation of about 1% of the total measured value can be expected.

[0052] FIG. 4 shows, as an example, a table visualizing the measurement error as a function of the length of a three-wire cable 3 of a temperature probe 1 with three connecting wires 4, 5, 6. In particular, the table shows the maximum cable length above which a required accuracy class A or B for the temperature measurement can no longer be maintained. So, by considering very long temperature probes 1 we have a situation like it is shown in FIG. 4: The measurement values leave a given accuracy class as soon as the connecting lines 4, 5, 6 exceed a certain length.

[0053] According to the inventive temperature probe 1 the differences of the resistances of the three connecting lines 4, 5, 6 is compensated by adding an additional resistance. Preferably, the resistances of two of the three wires 4, 5, 6 are equalized to the resistance of the connecting line (for example 4) with the highest resistance. The inventive temperature probe 1 is simple and inexpensive to manufacture, as the compensation method is less invasive, but provides a high accuracy of the temperature measurement. A piece of a conductive element 7, 8 with a higher resistivity and the determined dimensions is needed to modify the resistance of the remaining two connecting lines 5, 6 in such a way that each of the connecting lines 4, 5, 6 has the same resistance.

[0054] FIG. 5 shows a schematic view of the inventive temperature probe 1 determining the temperature according to the three-point probe method. FIG. 6 shows a table of the resistivity of different conductive materials.

[0055] The calculation of the linear resistance of a connecting line 4, 5, 6 is quite simple:

[0056] Linear wire resistance=material resistivity/wire section.

[0057] By doing the calculation using a standard Constantan wire with a diameter between 0.2 and 0.5 mm we can compensate the resistance differences between the three connecting lines 4, 5, 6 of a MgO cable 14 by adding a conductive element 7, 8 of 10 mm to 50 mm of a Constantan wire.

[0058] Whereby the length of the conductive element 7, 8 is calculated by:

Compensation length=Resistance difference/Linear wire resistance.

[0059] In the following the steps for compensating resistance differences on the three wires is described:

[0060] The process starts by measuring the resistance of each of the three wires 4, 5, 6.

[0061] The wire 4 with the highest resistance is identified and the difference between the maximum value and the resistance values of the two remaining wires 5, 6 is calculated. [0062] The compensation length of the conductive element 7, 8, preferably made of Constantan, is calculated for each of the two remaining wires 5, 6 to equalize the resistance of each of them with the resistance of the first wire 4 with the highest resistance value.

[0063] FIG. 7 shows a first embodiment of the temperature probe 1 according to the present disclosure. The focus is on the attachment of the conductive elements 7, 8 to two or at least one of the connecting wires 4, 5, 6. The conductive elements 7, 8 made of the at least one second material, for example constantan, are arranged in a transition bushing 9 of the temperature probe 1. Such a transition bushing 9 serves to connect two different sections 10, 11 of the three-wire cable 3.

[0064] For extended temperature probes 1 such a transition bushing 9 is generally used to connect the MgO 14 cable to a flexible extension cable 15. The compensating conductive elements 7, 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 of the known methods, for example: welding, brazing, soldering, or crimping. For electrical insulation, each joint may be protected by an additional Kapton or heat shrink insulating sleeve or cover 16. Finally, the complete bushing 9 may be sealed by a resin potting 17.

[0065] FIG. 8 shows a view on a second embodiment of the inventive temperature probe. According to this alternative design of the temperature probe, the conductive elements 7, 8 made of at least a second material are inserted in a connection area 12 through which the three-wire cable 3 can be connected to external electronics 13: the conductive elements 7, 8 that compensate for the differences in resistance of the connection lines 4, 5, 6 are attached to the terminals 18 to which the connection lines 4, 5, 6 of the main cable 3 are connected. Depending on the length of the main three-wire cable 3, this may be the MgO cable 14 or the flexible extension cable 15. The two wires 4, 5, 6 into which the conductive elements 7, 8 of a determined design are inserted may be stripped and interrupted. The conductive elements 7, 8 are inserted between the connecting wires 4, 5, 6 and the terminals 18. Again, the connections can be welded, brazed, soldered, or crimped. For electrical insulation, each joint may be protected by an additional Kapton or heat shrink insulating sleeve or cover 16. Additionally, heat shrink tubing insulation 19 can be applied to protect the connections.

[0066] FIG. 9 shows a third embodiment of the inventive temperature probe 1. Here the conductive elements , 8 made of the second material and corresponding wire connections 20 connecting the conductive elements 7, 8 to the wires 5, 6 are protected by an encapsulation box 21. The conductive elements 7, 8 and the wire connections 20 are protected by an encapsulation box 21. Preferably tis encapsulation box is inserted in the flexible extension cable 15. They can also be directly inserted into the terminal part 19 by which the temperature probe 1 is connected to an external electronics (13). The conductive elements 7, 8 and the wire connections 20 are inserted into proper heat shrink tubing 16 or an insulating tape . Then the connection area 12 is protected by an additional encapsulation box 21. The connection area 12 may be at the end or in the middle of the flexible extension cable 15.