Device for calibrating high-temperature thermocouples

Abstract

The present invention relates to temperature measurement. A device for calibrating high-temperature thermocouples includes: a protective housing made of a high-melting-point material and having a mounting flange; a thermocouple with a ceramic insulator; and a radiation unit. The device is placed vertically, the protective housing is hermetically sealed with the working end through a plug, and has a carrier tube coaxially mounted therein, to which the hot end of the thermocouple is attached, and the carrier tube is sealed at the working end through a reflector plug. The protective housing and the carrier tube have holes for allowing the filling of an inert gas. The protective housing containing the carrier tube and the thermal element of the thermocouple attached thereto is disposed in the working chamber of the radiation unit, and the latter is attached to a composite electric heater. A hole for observing a pyrometer is provided on the opposite side of the radiation unit relative to its working chamber. The present invention enables the calibration process to be performed on one or more high-temperature thermocouples up to 2500 °C.

Description

[0001] The present invention relates to the field of temperature measurement and can be used for the calibration of high-temperature thermoelectric transducers including tungsten-rhenium thermoelectric transducers, which are designed to measure the temperature of a working medium in the range of 1700 °C to 2500 °C.

[0002] There are known A-type tungsten-rhenium thermocouples (WRe thermocouples) (WRe5 - WRe20), where the rhenium content in the thermoelectric elements is 5% and 20% respectively, and C-type tungsten-rhenium thermocouples (WRe5 - WRe26), where the rhenium content in the thermoelectric elements is 5% and 26% respectively. Such thermocouples have the widest measurement temperature range among contact sensors. According to the IEC60584-1:2013 standard, for C-type thermocouples, the upper limit of the measured temperature reaches 2300 °C, and for A-type thermocouples, the upper limit of the measured temperature reaches 2500 °C. However, due to the lack of reliable metrological control and reliable calibration during the manufacturing stage and during operation, the widespread use of A-type and C-type thermocouples at temperatures above 1700 °C is limited.

[0003] According to the article "Ulanovsky A., Edler F., Fischer Y., Oleynikov P., Zaitsev P., Pokhodun A, Features of high-temperature calibration of tungsten-rhenium thermocouples / / International Journal of Thermophysics, 2015, Vol. 36(2 - 3), pp. 433 - 443", a calibration method for Russian WRe thermocouples is known, where the calibration is performed in a horizontal furnace with a graphite heater, which is inside a graphite radiation cavity installed in the heater. The temperature of the radiation cavity is measured by a reference pyrometer, and then the WRe thermocouple is immersed in the cavity, and the WRe thermocouple is calibrated according to the measured temperature of the cavity. During these operations, the stability of the temperature value is maintained by the control pyrometer (feedback pyrometer) of the furnace. The calibration method in the graphite radiation cavity of the horizontal furnace has some disadvantages:

[0004] 1. The thermocouple to be calibrated cannot be protected from the influence of graphite vapor, which can lead to distortion of the calibration curve due to the shunting of the thermocouple signal.

[0005] 2. Poor protection of the thermocouple forces calibration to be carried out in the shortest possible time, which can lead to rapid heating and cooling of the thermocouple. This can also lead to distortion of the calibration curve.

[0006] 3. Due to the low productivity of expensive processing, the calibration of individual thermocouples is not suitable for large-scale sensor production.

[0007] 4. The horizontal thermocouple position in the furnace causes bending of the thermoelectric element at high temperatures and increases the risk of accidental short circuits of the heater or furnace element.

[0008] According to the patent CN 106370322 A with a priority date of November 24, 2016, a verification system for tungsten-rhenium thermocouples is known. The system includes a high-temperature furnace with a black body inside and a temperature control unit. The temperature control unit is part of an automated measurement system. The thermocouple to be calibrated is connected to a data acquisition module and a cold junction compensation device, and all data is transmitted to a common data exchange fieldbus, which is also connected to the control unit. The measured temperature range is from 1500 °C to 2300 °C. The patent describes the components of a WRe thermocouple calibration unit, including a vacuum system, a cooling system, a gas path, and a measurement method. Block diagrams of the measurement and the basic parts of the facility are given.

[0009] Closest in technical essence to the claimed device is a protective tube for calibrating high-temperature tungsten-rhenium thermocouples according to the patent CN 102944332 A with a priority date of December 3, 2012. The protective tube provides protection for the tungsten-rhenium thermocouple during high-temperature calibration in the temperature range of 1500 °C to 2300 °C. The calibration process is carried out inside the protective tube, which has an independent cavity at the working end and is structurally close to a black body. The tungsten-rhenium thermocouple is inserted from the upper end of the protective tube with a mounting flange and passes through an upper grating and a lower grating composed of tungsten sheets with holes for thermocouple leads. Inserting the thermocouple positions the working junction of the thermocouple in the isothermal zone of the protective tube. The upper flange is sealed to protect the furnace atmosphere, including the internal protective tube, from air infiltration. Before starting to heat the furnace, the protective tube is evacuated and filled with an inert gas. After the furnace temperature reaches the set value and stabilizes, the temperature is measured by a reference pyrometer installed outside the furnace. The pyrometer observes through a quartz glass window in the side wall of the furnace at a hole in the lower part of the protective tube. Since the isothermal zone of the protective tube is near the radiation hole, the measured temperature values of these two zones are considered the same. The thermoelectromotive force value of the thermocouple is calibrated by the temperature measured by the reference pyrometer. Thus, the deviation between the reading of the calibrated thermocouple and the nominal value at the current temperature is determined. Tungsten is used as the material of the protective tube. The length of the isothermal zone along the axis of the tube is about 20 mm, and the size of the radiation cavity is 8 mm. It is assumed that the emissivity value of the cavity is at least 0.98. The heating element of the furnace can be graphite or a tungsten tube. During the calibration process, the protective tube protects the thermocouple from contamination caused by evaporation and transfer of the heating element material.

[0010] The disadvantages of the thermocouple calibration described in the above patents are as follows:

[0011] 1. Each load can calibrate up to three thermocouples, which greatly complicates large-scale production.

[0012] 2. When the cavity emissivity is equal to 0.98, temperature measurement at a single wavelength using a monochromatic reference pyrometer will have additional errors due to the correlation between tungsten spectral emissivity and temperature.

[0013] 3. The additional fused silica element at the optical measurement axis will add a new component to the total measurement uncertainty of the true temperature inside the cavity.

[0014] 4. Due to the thermal expansion of the furnace element, the radiation cavity will deviate from the optical axis of the reference pyrometer, which will require additional adjustment of the pyrometer at each temperature level.

[0015] The present invention solves the technical problem of eliminating these drawbacks, that is, the present invention provides a wide range and high performance for the calibration process of high-temperature thermocouples including tungsten-rhenium thermocouples in the temperature range from 1000 °C to 2500 °C.

[0016] The technical result of the present invention provides a calibration process for more than three high-temperature thermocouples including tungsten-rhenium thermocouples in the temperature range from 1000 °C to 2500 °C due to the design features of the device.

[0017] To achieve the claimed technical result, a device for calibrating high-temperature thermocouples is proposed. The device includes: a protective tube made of refractory material with a closed end having a mounting flange, a thermocouple with a ceramic insulator, and a radiation block. The distinctive feature of the device is that the protective tube is hermetically sealed at the working end using a plug, and a carrier tube to which the thermocouple is attached is coaxially installed inside the protective tube. The carrier tube is hermetically sealed at the working end using a reflector plug. The protective tube and the carrier tube have holes for filling the internal space with an inert gas. At the outlet of the protective tube, there is an insulator plug with a number of holes through which the thermoelectric element is led out. The assembly including the protective tube and the carrier tube with the attached thermocouple is placed in the working cavity of the radiation block, which is fixed inside the central region with a uniform temperature of an electric heater. On the back side of the radiation block opposite to its working cavity, there are holes for observing the control pyrometer of the furnace. The device has a vertical position.

[0018] The essence of the technical solution is illustrated by the following description and drawings.

[0019] Figure 1 A device for calibrating high-temperature thermocouples is shown, where:

[0020] 1. The working junction of the thermocouple;

[0021] 2. The thermocouple wire;

[0022] 3. Thermocouple ceramic insulator;

[0023] 4. Carrier tube;

[0024] 5. Reflector plug;

[0025] 6. Protection tube;

[0026] 7. Protection tube plug;

[0027] 8. Mounting flange;

[0028] 9. Insulator plug;

[0029] 10. Hole for inert gas;

[0030] 11. Radiation block;

[0031] 12. Working cavity of the radiation block;

[0032] 13. Tubular electric heater composed of a graphite ring stack;

[0033] 14. Pyrometer observation hole.

[0034] Figure 1 The figure shows the device vertically located inside the electric heater. The working junction 1 of the thermocouple is fixed to the carrier tube 4. The working end of the carrier tube (view B) is sealed by the reflector plug 5, which has a shaped surface that scatters radiation inside the tube. The thermocouple wire 2 is placed in the ceramic insulator 3.

[0035] The length of the carrier tube 4 is not less than 50 times the value of the inner diameter of the carrier tube. This makes the radiation cavity parameters as close as possible to those of a black body radiation cavity. The carrier tube 4 with the thermoelectric element 2 attached is inserted into the protective tube 6. The protective tube 6 is hermetically sealed with a plug 7 at the working end (View B). The protective tube 6 and the carrier tube 4 are centered together and are within the working volume of the heating furnace on the mounting flange 8. The protective tube 6 and the carrier tube 4 are electrically isolated from each other. The outlet opening (View A) between the protective tube 6 and the carrier tube 4 is closed by an insulator plug 9 through which the thermocouple wire 2 is led out. There are holes 10 in the upper parts of the protective tube 6 and the carrier tube 4 for filling the internal volume with an inert gas. There is no inert gas flow near the working junction 1 of the thermocouple that can cool the working junction 1. The protective tube 6 with the thermoelectric element 2 is inserted into the working cavity 12 of the radiation block 11, which is fixed in the furnace zone with a uniform temperature between the annular elements of the composite electric heater 13. It is not allowed for the protective tube 6 to contact the wall or bottom of the cavity 12 of the radiation block 11. The depth of the cavity is not less than twice the diameter of the protective tube 6. The radiation block 11 serves as an additional element for equalizing the temperature field near the working junction 1 of the calibrated thermocouple. On the back side of the radiation block 11 opposite to the working cavity 12, there is a hole 14 for observing the replication pyrometer thereon.

[0036] The design of the proposed device for calibrating high-temperature thermocouples enables the following results to be achieved:

[0037] 1. For high-temperature thermocouples including tungsten-rhenium thermocouples, the full range and high performance of the expensive calibration process are achieved;

[0038] 2. The protective tube made of refractory material and hermetically sealed at the working end protects the thermocouple wire from contamination by vapors of other materials that appear at high temperatures;

[0039] 3. The presence of the carrier tube in the device enables more than three thermocouples to be placed on the carrier tube, and the working junctions of the thermocouples are fixed in the isothermal zone of the furnace and close to the reflector plug, on which the reference pyrometer is observed;

[0040] 4. The carrier tube with the reflector plug enables the measurement of the axial and azimuthal non-uniformities of the temperature field in the measurement area;

[0041] 5. The reflector plug with a shaped surface scatters the radiation inside the carrier tube, making it closer to the parameters of a black body;

[0042] 6. The sudden heating and cooling of the calibrated thermocouple are excluded;

[0043] 7. The vertical arrangement of the thermocouple and the protective tube eliminates the risk of accidental short circuits of the heater or furnace elements;

[0044] 8. The radiation cavity of the carrier tube has no displacement relative to the optical axis of the reference pyrometer;

[0045] 9. Along the optical measurement axis, there is no intermediate element between the lens of the reference pyrometer and the radiation cavity of the carrier tube;

[0046] 10. By means of the radiation block, approaching blackbody parameters, the extension of the isothermal zone is achieved, which is necessary for thermocouple calibration;

[0047] 11. The hole at the back side of the radiation block opposite to the working cavity enables the use of a replicated radiation pyrometer for controlling the temperature value of the measurement area;

[0048] 12. The device enables the installation of a unit for achieving the reference melting point of the eutectic of metal carbide "M-C" in the cavity of the radiation block. A set of such melting (fixed) points within the calibration range, for example: Pd-C (1492 °C); Rh-C (1657 °C); Pt-C (1738 °C); Cr3C2-C (1827 °C); Ru-C (1953 °C); Ir-C or Y-C (2290 °C); Re-C (2474 °C) - enables the calibration of a single thermocouple, such as a tungsten-rhenium thermocouple, with a high precision of no more than 1 Kelvin.

[0049] Figure 2 A measurement scheme for calibrating a high-temperature thermocouple based on the readings of a radiation pyrometer is shown, and all auxiliary devices are indicated in the scheme.

[0050] The measurements during the calibration process are performed as follows. After installing the thermocouple calibration device in the furnace, heating is started to the first calibration point. The temperature of the radiation block is controlled by the replicated pyrometer, the readings of which are transmitted to the temperature measurement unit, and the replicated pyrometer can also be used for controlling the heating. The radiation surface temperature of the reflector plug of the carrier tube of the device is measured by the reference pyrometer. This temperature is the true value of the temperature of the working junction of the thermocouple located near the radiation surface of the reflector plug. Checks are also made for the temperature of the radiation block measured by the replicated pyrometer. Thus, the temperature of the working cavity near the working junction of the thermocouple can be controlled from both sides. All current data are displayed in real time on the screen of a personal computer and can be saved to a separate calibration file. The free end of the thermocouple is connected to a copper wire, and the contact point is immersed in a Dewar vessel with melting ice to achieve a temperature of 0 °C for the cold junction of the thermocouple. The thermal electromotive force of each thermocouple is measured by a precision millivoltmeter and stored in the calibration file. The measured value of the thermal electromotive force of the thermocouple is adjusted to the temperature value measured by the reference pyrometer. All measurements are performed at a stable furnace temperature with a drift of no more than 0.5 degrees per minute. Then the furnace is heated to the next set calibration point. Thus, the thermocouple is calibrated within its working temperature range.

[0051] The maximum calibration temperature achievable is determined only by the properties of the ceramic insulator of the thermocouple.

[0052] Therefore, the proposed invention eliminates the drawbacks of the above-described device, namely:

[0053] 1. Protect the calibrated thermocouple from the vapors of the heater material.

[0054] 2. Enable the calibration of more than three thermocouples simultaneously to ensure mass production of temperature sensors.

[0055] 3. Eliminate the sudden heating and cooling of the calibrated thermocouple.

[0056] 4. The vertical arrangement of the thermocouple and the protective tube eliminates the risk of accidental short circuits of the heater or furnace element.

[0057] 5. There is no displacement of the radiation cavity relative to the optical axis of the reference pyrometer.

[0058] 6. The optical measurement scheme has no intermediate element between the pyrometer lens and the radiation cavity, which eliminates a component of the measurement uncertainty.

[0059] Therefore, the whole set of these design features achieves the specified technical result, which includes the possibility of simultaneously calibrating more than three thermocouples at specified points in the temperature range from 1000 °C to 2500 °C.

[0060] Example of High-Temperature Thermocouple Calibration

[0061] As a device (type A tungsten-rhenium thermocouple) for reproducing the full range of operating temperatures from 1000 °C to 2500 °C, the high-temperature furnace BB3500YY was selected. Overall dimensions of the working space of the furnace: diameter 47 mm, height 500 mm. Vertical furnace with a graphite heater, maximum operating temperature 3500 °C, protective gas - argon, water cooling of the furnace unit.

[0062] The working junctions of ten thermocouples are fixed to a carrier tube near the radiation surface of the reflector plug, and the carrier tube is coaxially placed inside a protective tungsten tube hermetically sealed from the working end. Immerse the calibration device (all components) into the working cavity of a graphite radiation block located in the isothermal zone of the furnace. Determine the dimensions of the isothermal zone in advance. The temperature of the radiation block is controlled by a replicated pyrometer on the opposite side (from the bottom) of the furnace.

[0063] The length of the carrier tube is not less than 50 times its inner diameter value to make the radiation cavity as close to a black body as possible. Holes have been made in the upper cooling part of the protective tube and the carrier tube wall to fill the internal volume with an inert gas (argon). Determine the calibration temperature by the readings of the reference pyrometer and the replicated pyrometer. Adjust the temperature value to the thermoelectromotive force value of the calibrated thermocouple. Based on the measurement results, determine a separate calibration curve for each thermocouple. An example of a calibration table for one of the thermocouples is shown in Table 1.

[0064] Therefore, ten tungsten-rhenium thermocouples were calibrated simultaneously in the temperature range from 1000 °C to 2500 °C. The results confirmed the claimed technical result.

[0065] Table 1

[0066] Calibration results for thermocouple type A, serial number 01

[0067]

[0068] Conclusion: The calibrated thermocouple type A, serial number 01, corresponds to the second accuracy class of GOSTR 8.585-2001.

Claims

1. A device for calibrating high-temperature thermocouples, comprising: A protective tube made of refractory material with a mounting flange, a thermocouple with a ceramic insulator, and a radiation block, characterized in that: the device is placed vertically, the protective tube is hermetically sealed at the working end by a plug, a carrier tube is coaxially installed inside the protective tube, wherein the thermocouple and the working junction are fixed to the carrier tube, the carrier tube is hermetically sealed with the working end by a reflector plug, the protective tube and the carrier tube have holes for filling inert gas, the protective tube and the carrier tube attached with the thermoelectric element are placed in the working cavity of the radiation block, the radiation block is fixed to a composite electric heater, and there is a hole for pyrometer observation at the back side of the radiation block opposite to the working cavity of the radiation block.

2. The device according to claim 1, characterized in that An insulator plug is placed at the outlet of the protective tube, and the thermoelectric element is led out through the insulator plug.

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