High-temperature thermocouple calibration method based on platinum melting point

Abstract

The invention provides a high-temperature thermocouple calibration method based on a platinum melting point. The high-temperature thermocouple calibration method comprises the following steps: enabling two thermocouples to respectively penetrate into double holes of a double-hole insulating tube; welding the end parts of the measuring ends of the two thermocouples to be calibrated together; putting the end part of the bundled measuring end into a working area of a high-temperature furnace; the reference ends of the two thermocouples are respectively connected with an input end outgoing line and an output end outgoing line of the data acquisition module, and a connection point of the outgoing lines is placed in a constant-temperature test tube in the freezing point thermostat; raising the temperature of the high-temperature furnace to a first preset temperature, and keeping the temperature for a preset time; determining a preset heating rate; acquiring a thermoelectric force-time curve acquired by the data acquisition module; and taking the average value of all thermoelectric forces on the melting section curve or the temperature deviation of the thermocouple as a calibration result. The method can solve the technical problems that the uncertainty introduced by an existing calibration method is large, and the calibration requirement of accurate measurement on the high-temperature thermocouple cannot be met.

Description

Technical Field

[0001] The invention relates to the technical field of high temperature measurement, and in particular to a high temperature thermocouple calibration method based on the melting point of platinum. Background Art

[0002] In the fields of metallurgy, casting, chemical industry, manufacturing, aerospace and other military industries, high-temperature thermocouples are widely used to measure high temperatures on solid surfaces or inside media, with an upper limit of 2300°C. In the current metrology system, there are no standard thermocouples above 1500°C, and only photoelectric pyrometers based on non-contact measurement principles can be used as standards. The measurement uncertainty of the photoelectric pyrometer itself is large, and there are large differences in the temperature measurement targets and measurement principles between the calibrated thermocouple and the photoelectric pyrometer. Therefore, the uncertainty introduced in the calibration process is large, generally around 10°C, which does not meet the calibration requirements for high-temperature thermocouples for accurate measurement. Summary of the invention

[0003] The present invention provides a high-temperature thermocouple calibration method based on the melting point of platinum, which can solve the technical problem that the existing calibration method introduces large uncertainty and fails to meet the calibration requirements of the high-temperature thermocouple for accurate measurement.

[0004] The present invention provides a high-temperature thermocouple calibration method based on the melting point of platinum, the method comprising:

[0005] Take two thermocouples, insert them into the double holes of the double-hole insulating tube respectively, and make the measuring ends of the two thermocouples extend from one end of the double-hole insulating tube, and the reference ends of the two thermocouples extend from the other end of the double-hole insulating tube;

[0006] Weld together the ends of the measuring ends of the two thermocouples to be calibrated;

[0007] Use platinum fuse to tie the ends of the welded measuring ends;

[0008] Place the bundled end of the measuring end into the working area of ​​the high-temperature furnace;

[0009] The reference ends of the two thermocouples are connected to the input lead wire and the output lead wire of the data acquisition module respectively, and the connection points of the lead wires are placed in a constant temperature test tube in an ice point thermostat. The data acquisition module is used to collect the thermoelectric potential of the thermocouples;

[0010] Raising the temperature of the high-temperature furnace to a first preset temperature and maintaining it for a preset time, wherein the first preset temperature is between 0.9 and 0.95 times the melting point of platinum;

[0011] Determine a preset heating rate based on the rate of change of heat absorption of the platinum fuse, the specific heat capacity of the platinum fuse and the mass of the platinum fuse;

[0012] When the temperature fluctuation of the high-temperature furnace is less than the preset fluctuation value, the high-temperature furnace is heated for a second time at a preset heating rate until the temperature reaches a second preset temperature, wherein the second preset temperature is between 1.05 and 1.1 times the melting point of platinum;

[0013] Acquire the thermoelectric potential-time curve collected by the data acquisition module;

[0014] The time when the platinum melting point is reached during the secondary heating process is taken as the melting starting point of the platinum fuse;

[0015] Draw a straight line with a preset heating rate as the slope and tangent to the thermoelectric potential-time curve after the melting start point, and take the time corresponding to the tangent point as the melting end point of the platinum fuse;

[0016] The thermoelectric potential-time curve between the melting start point and the melting end point is taken as the melting section curve, and the average value of all thermoelectric potentials on the melting section curve or the temperature deviation of the thermocouple is taken as the calibration result.

[0017] Preferably, the preset heating rate is determined by the following formula:

[0018]

[0019] In the formula, represents the rate of change of heat absorption of platinum fuse, C represents the specific heat capacity of platinum fuse, m represents the mass of platinum fuse, Indicates the preset heating rate.

[0020] Preferably, the average value of all thermoelectric potentials is obtained by the following formula:

[0021]

[0022] The temperature deviation is obtained by the following formula:

[0023] Δe=e m -e r

[0024]

[0025] In the formula, e m represents the average value of all thermoelectric potentials on the melting curve, e1, e2, ..., e n It represents the thermoelectric potential of the thermocouple obtained by the first, second, ..., nth measurement on the melting section curve, n represents the number of measurements on the melting section curve, Δe represents the deviation potential of the thermocouple to the graduation table, e r It represents the standard potential value of the thermocouple, Δt represents the temperature deviation of the thermocouple on the melting section curve, and S represents the differential electromotive force of the thermocouple.

[0026] Preferably, after welding the measuring ends of the two thermocouples together, the method further comprises: selecting a diameter of the platinum fuse based on the electrode diameter of the thermocouple.

[0027] Preferably, bundling the end of the welded measuring end with the platinum fuse comprises: tightly winding the end of the welded measuring end with the platinum fuse for several turns along the axis direction of the thermocouple.

[0028] Preferably, the number of turns of the platinum fuse is 3 to 5.

[0029] Preferably, when the thermocouple is a tungsten-rhenium thermocouple, the double-hole insulating tube is placed in a protective sleeve for packaging.

[0030] Preferably, the platinum fuse is made of metal platinum wire with a purity of 99.99% or more.

[0031] The application of the technical solution of the present invention has the following beneficial effects:

[0032] 1. Use fixed point absolute method for calibration, no need to use standard thermocouple;

[0033] 2. Compared with the existing photoelectric pyrometer, the measurement uncertainty is at a better level (the measurement uncertainty of the existing photoelectric pyrometer is about U=10°C (k=2), and the measurement uncertainty of the present invention is U=1.2°C (k=2)); 3. It is suitable for high-precision calibration of various high-temperature thermocouples. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The included drawings are used to provide a further understanding of the embodiments of the present invention, which constitute a part of the specification, are used to illustrate the embodiments of the present invention, and together with the text description, explain the principles of the present invention. Obviously, the drawings in the following description are only some embodiments of the present invention, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0035] Figure 1 A flow chart of a high temperature thermocouple calibration method based on platinum melting point provided according to an embodiment of the present invention is shown;

[0036] Figure 2 A schematic diagram of a high-temperature thermocouple calibration method based on platinum melting point provided according to an embodiment of the present invention is shown;

[0037] Figure 3 A thermoelectric potential-time curve diagram of a thermocouple melting zone provided in accordance with an embodiment of the present invention is shown;

[0038] Figure 4 A schematic diagram of platinum fuse winding according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0039] It should be noted that, in the absence of conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its application or use. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0040] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0041] Unless otherwise specifically stated, the relative arrangement of the parts and steps described in these embodiments, numerical expressions and numerical values ​​do not limit the scope of the present invention. At the same time, it should be understood that, for ease of description, the sizes of the various parts shown in the accompanying drawings are not drawn according to the actual proportional relationship. The technology, method and equipment known to ordinary technicians in the relevant field may not be discussed in detail, but in appropriate cases, the technology, method and equipment should be regarded as a part of the authorization specification. In all examples shown and discussed here, any specific value should be interpreted as being merely exemplary, rather than as a limitation. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters represent similar items in the following drawings, so once a certain item is defined in an accompanying drawing, it does not need to be further discussed in subsequent drawings.

[0042] like Figure 1 and Figure 2 As shown, the present invention provides a high temperature thermocouple calibration method based on the melting point of platinum, the method comprising:

[0043] S10. Take two thermocouples, insert the two thermocouples into the double holes of the double-hole insulating tube respectively, and make the measuring ends of the two thermocouples extend from one end of the double-hole insulating tube, and the reference ends of the two thermocouples extend from the other end of the double-hole insulating tube; specifically, take out the thermocouples from their protection tubes and insulating tubes, and insert the two electrodes of the thermocouples into the corresponding high-purity alumina double-hole insulating tubes respectively; when disassembling and assembling the thermocouples, the insulating tubes should be marked with positive and negative poles, and it is strictly forbidden to interchange the positive and negative holes;

[0044] S20, welding the ends of the measuring ends of the two thermocouples to be calibrated together; after welding, the thermoelectric potential can be stabilized during the melting process to reduce jumps;

[0045] S30, using a platinum fuse to bundle the ends of the welded measuring ends; wherein, in order to make the melting temperature basically stable, a metal platinum wire with a purity of more than 99.99% is used as the platinum fuse;

[0046] S40. Place the bundled end of the measuring end into the working area of ​​the high-temperature furnace and fix its position so that it does not touch the wall;

[0047] S50, connecting the reference ends of the two thermocouples to the input lead wire and the output lead wire of the data acquisition module respectively, and placing the connection point of the lead wire into a constant temperature test tube in the ice point thermostat, the data acquisition module is used to collect the thermoelectric potential of the thermocouple, wherein the constant temperature test tube adopts a glass tube with a diameter of (6-7) mm, and the insertion depth should not be less than 150 mm; the data acquisition module can use a digital voltmeter, which is preheated after being turned on;

[0048] S60, raising the temperature of the high temperature furnace to a first preset temperature and maintaining it for a preset time, wherein the first preset temperature is between 0.9 and 0.95 times the melting point of platinum; for example, starting the high temperature furnace and raising the temperature to 1758° C. and maintaining it for 10 minutes, so that the thermocouple and the air in the furnace can fully exchange heat;

[0049] S70, determining a preset heating rate based on the change rate of heat absorption of the platinum fuse, the specific heat capacity of the platinum fuse and the mass of the platinum fuse, wherein the preset heating rate is set to 3.0° C. / min;

[0050] S80, when the temperature fluctuation of the high-temperature furnace is less than the preset fluctuation value, the high-temperature furnace is heated for a second time at a preset heating rate until the temperature rises to a second preset temperature, wherein the second preset temperature is between 1.05 and 1.1 times the melting point of platinum; for example, when the temperature fluctuation is less than 0.1-0.3°C / min, the next step of heating and melting process is performed, the temperature is set to 1778°C, the heating rate is 3.0°C / min, the high-temperature furnace is heated, and when the high-temperature furnace rises to 1778°C, the heating and melting process ends;

[0051] S90, obtaining the thermoelectric potential-time curve collected by the data acquisition module, such as Figure 3 As shown, the sampling rate is set to 5 times / s, and the data acquisition time is set to the heating and melting process;

[0052] S100, taking the time when the platinum melting point (1768° C.) is reached during the secondary heating process as the melting starting point of the platinum fuse;

[0053] S110, drawing a straight line with a preset heating rate as a slope and tangent to the thermoelectric potential-time curve after the melting start point, and taking the time corresponding to the tangent point as the melting end point of the platinum fuse;

[0054] S120, taking the thermoelectric potential-time curve between the melting start point and the melting end point as the melting section curve, and taking the average value of all thermoelectric potentials on the melting section curve or the temperature deviation of the thermocouple as the calibration result.

[0055] In addition, after the calibration is completed, the thermocouple is cut off (10-15) mm from the measuring end, and the measuring end is re-welded after cleaning.

[0056] The application of the technical solution of the present invention has the following beneficial effects:

[0057] 1. Use fixed point absolute method for calibration, no need to use standard thermocouple;

[0058] 2. Compared with the existing photoelectric pyrometer, the measurement uncertainty is at a better level (the measurement uncertainty of the existing photoelectric pyrometer is about U = 10 ° C (k = 2), and the measurement uncertainty of the present invention is U = 1.2 ° C (k = 2));

[0059] 3. Suitable for high-accuracy calibration of various high-temperature thermocouples.

[0060] According to an embodiment of the present invention, after welding the measuring ends of the two thermocouples together, the method further includes: selecting a platinum fuse diameter based on the electrode diameter of the thermocouple, as shown in the following table:

[0061] Table 1 Selection of platinum wire diameter

[0062] Electrode diameter / mm Platinum wire diameter / mm 0.5 0.3 0.8 0.5 1.0 0.5

[0063] According to an embodiment of the present invention, in S30 of the present invention, bundling the end of the welded measuring end with a platinum fuse includes: using the platinum fuse to tightly wrap the end of the welded measuring end for several turns along the axis direction of the thermocouple, such as Figure 4 shown.

[0064] Specifically, the measuring end of the thermocouple is exposed from the double-hole insulating tube (15-20) mm. For ease of operation, the measuring end of the thermocouple is wound from top to bottom, there should be no obvious gap between the fuse wires, and it should be in close contact with the welding point; the number of winding turns of the platinum fuse wire is 3-5 turns, so that the thermoelectric potential is stable during the melting process with less jumps, avoiding the failure to find the melting temperature plateau due to too few turns, and avoiding too many turns resulting in material waste and long melting time.

[0065] In this embodiment, by controlling the diameter of the platinum fuse and the number of windings, it is ensured that the melting temperature plateau occurs without causing waste of fuse material, thereby reducing the calibration cost.

[0066] According to an embodiment of the present invention, when the thermocouple is a tungsten-rhenium thermocouple, the double-hole insulating tube is placed in a protective sleeve for packaging to prevent oxidation of the thermocouple electrodes during the calibration process.

[0067] According to an embodiment of the present invention, in S70 of the present invention, the preset heating rate is determined by the following formula:

[0068]

[0069] In the formula, It represents the rate of change of heat absorption of platinum fuse, the unit is J / s, C represents the specific heat capacity of platinum fuse, the unit is J / (kg﹒K), m represents the mass of platinum fuse, the unit is kg, Indicates the preset heating rate in K / s.

[0070] It is obtained through the above formula that when the heating rate is controlled to 3.0℃ / min, the melting temperature plateau appears and has the least impact on the calibrated thermocouple.

[0071] According to an embodiment of the present invention, in S120 of the present invention, the average value of all thermoelectric potentials is obtained by the following formula:

[0072]

[0073] The temperature deviation is obtained by the following formula:

[0074] Δe=e m -e r

[0075]

[0076] In the formula, e m It represents the average value of all thermoelectric potentials on the melting curve, in mV, e1, e2, ..., e nIt represents the thermoelectric potential of the thermocouple obtained by the first, second, ..., nth measurement on the melting section curve, in mV, n represents the number of measurements on the melting section curve, Δe represents the deviation potential of the thermocouple to the scale, in mV, e r It represents the standard potential value of the thermocouple in mV, Δt represents the temperature deviation of the thermocouple on the melting curve in °C, and S represents the differential electromotive force of the thermocouple in μV / °C.

[0077] After the above steps, the electromotive force e of the high temperature thermocouple can be obtained. m The main sources of measurement uncertainty and the uncertainty of each component are shown in the following table. The measurement uncertainty is calculated to be U = 1.2 °C (k = 2).

[0078] Table 2 Measurement uncertainty list

[0079] Sources of uncertainty category symbol Uncertainty component / μV 1 Measurement repeatability A <![CDATA[u1]]> 0.12 2 Reference junction temperature B <![CDATA[u2]]> 0.03 3 Fuse purity B <![CDATA[u3]]> 0.17 4 Uneven thermocouples B <![CDATA[u4]]> 0.51 5 Furnace temperature uniformity B <![CDATA[u5]]> 0.29 6 Data processing B <![CDATA[u6]]> 0.13

[0080] Calculate the combined standard uncertainty u c :

[0081]

[0082] Taking the coverage factor k = 2, the expanded uncertainty U = 1.2°C.

[0083] In summary, the present invention provides a high-temperature thermocouple calibration method based on the melting point of platinum, which uses the physical phenomenon that the temperature of metal platinum remains unchanged during melting to calibrate the high-temperature thermocouple. The outstanding advantage is that it can still be calibrated without a high-temperature standard thermocouple and the measurement uncertainty can reach U = 1.2 ° C (k = 2).

[0084] For ease of description, spatially relative terms such as "above", "above", "on the upper surface of", "above", etc. may be used here to describe the spatial positional relationship between a device or feature and other devices or features as shown in the figure. It should be understood that spatially relative terms are intended to include different orientations of the device in use or operation in addition to the orientation described in the figure. For example, if the device in the accompanying drawings is inverted, the device described as "above other devices or structures" or "above other devices or structures" will be positioned as "below other devices or structures" or "below other devices or structures". Thus, the exemplary term "above" can include both "above" and "below". The device can also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatially relative descriptions used here are interpreted accordingly.

[0085] In addition, it should be noted that the use of terms such as "first" and "second" to limit components is only for the convenience of distinguishing the corresponding components. If not otherwise stated, the above terms have no special meaning and therefore cannot be understood as limiting the scope of protection of the present invention.

[0086] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A high temperature thermocouple calibration method based on the melting point of platinum, characterized in that: The method comprises: Take two thermocouples, insert them into the double holes of the double-hole insulating tube respectively, and make the measuring ends of the two thermocouples extend from one end of the double-hole insulating tube, and the reference ends of the two thermocouples extend from the other end of the double-hole insulating tube; Weld together the ends of the measuring ends of the two thermocouples to be calibrated; Use platinum fuse to tie the ends of the welded measuring ends; Place the bundled end of the measuring end into the working area of ​​the high-temperature furnace; The reference ends of the two thermocouples are connected to the input lead wire and the output lead wire of the data acquisition module respectively, and the connection points of the lead wires are placed in a constant temperature test tube in an ice point thermostat. The data acquisition module is used to collect the thermoelectric potential of the thermocouples; Raising the temperature of the high-temperature furnace to a first preset temperature and maintaining it for a preset time, wherein the first preset temperature is between 0.9 and 0.95 times the melting point of platinum; Determine a preset heating rate based on the rate of change of heat absorption of the platinum fuse, the specific heat capacity of the platinum fuse and the mass of the platinum fuse; When the temperature fluctuation of the high-temperature furnace is less than the preset fluctuation value, the high-temperature furnace is heated for a second time at a preset heating rate until the temperature reaches a second preset temperature, wherein the second preset temperature is between 1.05 and 1.1 times the melting point of platinum; Acquire the thermoelectric potential-time curve collected by the data acquisition module; The time when the platinum melting point is reached during the secondary heating process is taken as the melting starting point of the platinum fuse; Draw a straight line with a preset heating rate as the slope and tangent to the thermoelectric potential-time curve after the melting start point, and take the time corresponding to the tangent point as the melting end point of the platinum fuse; The thermoelectric potential-time curve between the melting start point and the melting end point is taken as the melting section curve, and the average value of all thermoelectric potentials on the melting section curve or the temperature deviation of the thermocouple is taken as the calibration result.

2. The method according to claim 1, characterized in that The preset heating rate is determined by the following formula: In the formula, represents the rate of change of heat absorption of platinum fuse, C represents the specific heat capacity of platinum fuse, m represents the mass of platinum fuse, Indicates the preset heating rate.

3. The method according to claim 1 or 2, characterized in that: The average value of all thermoelectric potentials is obtained by the following formula: The temperature deviation is obtained by the following formula: Δe=e m -e r In the formula, e m represents the average value of all thermoelectric potentials on the melting curve, e1, e2, ..., e n It represents the thermoelectric potential of the thermocouple obtained by the first, second, ..., nth measurement on the melting section curve, n represents the number of measurements on the melting section curve, Δe represents the deviation potential of the thermocouple to the graduation table, e r It represents the standard potential value of the thermocouple, Δt represents the temperature deviation of the thermocouple on the melting section curve, and S represents the differential electromotive force of the thermocouple.

4. The method according to any one of claims 1 to 3, characterized in that: After welding the measuring ends of the two thermocouples together, the method further includes: selecting a diameter of the platinum fuse based on the electrode diameter of the thermocouple.

5. The method according to claim 1, characterized in that: The method of bundling the end of the welded measuring end with the platinum fuse includes: tightly winding the end of the welded measuring end with the platinum fuse for several turns along the axis direction of the thermocouple.

6. The method according to claim 5, characterized in that The number of windings of the platinum fuse is 3 to 5.

7. The method according to claim 1, characterized in that When the thermocouple is a tungsten-rhenium thermocouple, a double-hole insulating tube is placed in a protective sleeve for encapsulation.

8. The method according to claim 1, characterized in that The platinum fuse uses a metal platinum wire with a purity of more than 99.99%.

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