RU90898U1 - THERMOELECTRIC CONVERTER (OPTIONS), THERMOCOUPLING CABLE FOR MANUFACTURING THERMOELECTRICAL CONVERTER FOR THE FIRST OPTION - Google Patents

Abstract

1. The thermoelectric cable Converter, which includes two thermocouples with the same nominal static characteristic, and the thermoelectrodes of various thermocouples are made of wire with different concentration of alloying elements, and / or from wire with different nominal cross-section, and / or from wire that has passed preliminary heat treatment of various kinds. ! 2. A thermocouple cable for manufacturing a thermoelectric converter according to claim 1, consisting of four wires made in pairs of two different thermoelectrode alloys, the wires are placed in a metal sheath made of heat-resistant steel or alloy and insulated from each other and from the sheath by mineral insulation from magnesium oxide or alumina, while the wires made of the same thermoelectrode alloy differ from each other in the concentration of alloying elements, and / or the nominal cross section, and / or the type of preliminary heat treatment. ! 3. A thermoelectric converter comprising a switching unit, a protective case, a thermocouple attachment unit made with a through hole designed to house a control or reference measuring instrument inside the protective case, two sensing elements in the form of thermocouples with the same nominal static characteristic, and thermocouples differ in the type of materials that insulate the thermoelectrodes, and / or in the cross section of the thermoelectrodes, and / or in the concentration of alloying elements in the thermoelectrodes, and / or in the type of their dvaritelnoy heat treatment, and / or the number of elements of mechanical protection thermoelectrodes, Geometry

Description

The claimed group of utility models relates to thermometry and can be used to measure temperature on equipment used in long-term technological processes.

Known thermoelectric converter / RU 41864, G01K 15/00, 2004 /, which contains protective fittings consisting of outer and inner shells, a ceramic insulator, inside of which are placed thermoelectrodes connected in pairs by hot junctions. In this case, the main thermoelectric converter has the ability to periodically install a test transducer into it, which is placed in an insulator common to all thermoelectrodes. The thermoelectrodes of the main transducer and the test transducer are placed in the through channels of the insulator located along the length of the main transducer, the junctions of the thermoelectrodes of the main transducer are located in the grooves made in the end part of the insulator. The inner shell is made in the form of a ceramic cap, worn on the end part of the insulator, or in the form of a winding of refractory ceramic thread that covers the hot junctions of thermoelectrodes.

Also known is a device for measuring temperature in the form of a thermoelectric converter / RU 2299408, G01K 7/02, 2007 /, comprising a protective cover, a thermometric insert made of a thermocouple cable in a metal sheath with mineral insulation, and the thermometric insert is equipped with a unit for mounting it in the converter , terminal block for connecting the thermometric insert to the patch wires. A distinctive feature of the known device is that the thermometric insert is not aligned with it in the protective cover, and the mounting unit is made with a through hole designed to accommodate the working part of the control or reference measuring means inside the protective cover.

The use of known devices does not allow to determine the exit time of the dependence of thermo-emf. thermocouples from temperature outside the specified tolerance. More precisely, the exit time can be determined by comparing the thermocouple readings and the temperature control means at fairly short intervals, but it is impossible to predict how many such comparisons need to be made and how accurately the set period of time will be reproduced during further operation. Moreover, each comparison requires a time investment of about 20 minutes.

Thermocouple cables are known (see, for example, RU 66857, НВВ 9/00, 2007; RU 54250, НВВ 13/00, 2006; RU 30459, Н01В 7/00, 2003). Of the known thermocouple cables it is impossible to make the claimed device for measuring temperature.

The objective of the claimed group of utility models is the creation of thermoelectric converters, allowing to determine the need for verification (calibration), i.e. determination of the point in time when the readings of the transducers may not reflect the actual temperature, and, therefore, they must be replaced or periodically calibrated (calibrated). Timely replacement or verification (calibration), which allows you to introduce corrections to the readings, will provide a technical result consisting in increasing the reliability of measurement results, the reliability of technological processes and the quality of products.

To solve the problem, as well as to achieve the claimed technical result, it is proposed according to the first embodiment, a thermoelectric cable converter containing two thermocouples with the same nominal static characteristic. Moreover, the thermoelectrodes of various thermocouples are made of wire with a different concentration of alloying elements and / or of a wire with a different nominal cross section, and / or of a wire that has undergone preliminary heat treatment of various types.

To manufacture a thermoelectric converter according to the first embodiment, a thermocouple cable is proposed, consisting of four wires made in pairs of two different thermoelectrode alloys. The wires are placed in a metal sheath made of heat-resistant steel or alloy and are isolated from each other and from the sheath by mineral insulation from magnesium oxide or aluminum oxide. In this case, wires made of the same thermoelectrode alloy differ from each other in the concentration of alloying elements and / or nominal cross-section, and / or type of preliminary heat treatment.

To solve the problem, as well as to achieve the claimed technical result, it is proposed according to the second embodiment, a thermoelectric converter containing a switching unit, a protective cover, a thermocouple mounting unit made with a through hole designed to accommodate a control or reference measuring instrument inside the protective cover, two sensitive an element in the form of thermocouples with the same nominal static characteristic. Moreover, thermocouples differ in the type of materials that insulate the thermoelectrodes, and / or in the cross section of the thermoelectrodes, and / or in the concentration of alloying elements in the thermoelectrodes, and / or in the form of their preliminary heat treatment, and / or in the number of elements for mechanical protection of the thermoelectrodes, and its geometric parameters .

In addition, according to the second embodiment, it is proposed to use a standard-type cable thermocouple in a sheath made of heat-resistant steel or an alloy with mineral insulation, for example, magnesium oxide or aluminum oxide, as the first sensitive element. Standard thermocouple cable means a thermoelectric cable converter (cable thermocouple) made in accordance with the requirements of the International Electrotechnical Commission No. 1515 “Thermocouples and thermocouple cables with mineral insulation” and / or in accordance with the requirements of GOST 23847-79 “Thermoelectric cable converters. Technical conditions. " And as a second sensitive element, use a thermocouple with thermoelectrodes isolated from each other, from the main sensitive element and the protective cover with a single or multi-channel ceramic straw, or glass fiber, or silica thread.

In addition, according to the second embodiment, it is proposed that the first sensitive element be made in the form of a cable thermocouple of a standard layout with an outer diameter of 4 to 8 mm, and the second sensor element should be made in the form of a cable thermocouple of a standard layout with an outer diameter of 1 to 3 mm.

In addition, according to the second variant, it is proposed to use a cable thermocouple with an increased sheath thickness or with a sheath consisting of two coaxially arranged layers of different metal, or with a steel or alloy sheath with a chemical composition close to the composition of thermoelectrodes and having a linear expansion coefficient, as the first sensitive element comparable to the coefficient of linear expansion of thermoelectrodes, or a cable thermocouple with mineral insulation, which is larger than magnesium oxide and about vivo Recording aluminum, insulation resistance across the temperature range, and as a second sensing element to apply cabling standard thermocouple arrangement with electrodes with a diameter not exceeding the diameter of the main thermoelectrodes sensor element.

From numerous publications it is known that cable thermoelectric converters have higher stability compared to wire ones. Thus, a change in the readings of cable thermocouples of type XK with a diameter of 4 mm (electrode diameter 0.85 mm) at 425 ± 10 ° C for 10,000 hours does not exceed 0.5 ° C, and for 25,000 hours it is 1.15 ° C / Gerashchenko O. A., Gordov A.N., Eremina A.K. and other temperature measurements. Directory. Kiev: Naukova Dumka, 1989., p. 387 /, while for wire reaches 1 ° C in 10,000 hours.

Comparative tests of type XA thermocouples showed that the change in thermo-emf. a cable thermocouple with an outer diameter of 3 mm (diameter of thermoelectrodes 0.65 mm) at a temperature of 800 ° С for 10,000 hours is approximately 2.5 ° С, whereas for a conventional ТХА thermocouple with thermoelectrodes of 3.2 mm in diameter it reaches 3 ° С with an electrode diameter of 0.7 mm exceeds 200-250 μV (5-6 ° C) under the same conditions / Rogelberg I.L., Beilin V.M. Alloys for thermocouples. Directory. M .: Metallurgy, 1983, p. 84 /.

The increased stability of cable thermocouples is explained by the difficulty in oxidizing thermoelectrodes due to the limited amount of oxygen inside the cable, as well as additional protection of thermoelectrodes from exposure to the working medium with a metal sheath and magnesium oxide.

The metrological stability of thermocouples can also be increased due to a small change in the chemical composition of thermoelectrodes. Alloying is done with elements that increase the resistance to gas corrosion. In this case, the alloying elements or slightly change the thermo-emf. alloys, or their influence is compensated by other alloying elements. Thus, the positive influence of tenths of a percent of silicon on the heat resistance of chromel also leads to an increase in its stability. Alloying the same alloy with different elements can cause a change in the direction of drift of the thermoelectric power. For example, alloying chromel with aluminum increases its thermo-emf, while silicon decreases it. The introduction of both elements into the alloy reduces chromel instability to a negligible value.

In [Bentley R.E. Optimizing the thermoelectric stability of ID-MIMS type K thermocouples by adjusting the levels of Mn and Al. Proceedings of international symposium "Temperature-92. It's measurement and control in science and industry", v.6, part 1, American institute of physics, New York, 1992, pp.591-594.], Devoted to optimization of the composition of alloying elements of thermoelectrodes cable thermocouple XA, indicate the optimal initial concentration in the alumel electrode of manganese at 0.8% and aluminum at 0.7%. Their presence in the alloy affects the change in thermopower. in the opposite direction than reversible transformations in chromel. As a result, the thermocouple will have maximum stability.

Another way to increase the thermoelectric stability of alloys is their heat treatment. For example, chromel thermoelectrodes annealed at a temperature not only much higher than the temperature of the onset of recrystallization, or annealed at high temperature, but then subjected to annealing for “ordering” at 400-450 ° С, have stability exceeding the stability of thermoelectrodes annealed by standard conditions, 30-40%.

Thus, the different metrological stability of two thermocouples absolutely identical in design can be predetermined by preliminary heat treatment or the presence of alloying additives in thermoelectrodes.

It should also be noted that the instability of a cable thermocouple slightly depends on the diameter of the cable if it exceeds 3 millimeters. For small diameters (0.5-2.0 mm), it increases significantly with a decrease in the diameter of the shell and, accordingly, thermoelectrodes. This is due primarily to the nature of the mechanisms that cause a change in the static characteristics of cable thermocouples. These reasons limit the upper limit for the use of cable thermocouples depending on their diameter. So, in the standard ASTM E 608 / E 608M-00 (2004) "Typical specifications for thermocouples made of base metals with mineral insulation, in a metal sheath" the following recommended upper temperature limits for cable thermocouples are given:

Table 1 The upper temperature limit for various shell diameters, ° C [° F] Nominal shell diameter Thermocouple type mm inch T J E K, N 0.5 0.020 260 (500) 260 (500) 300 (570) 700 (1290) ... 0.032 260 (500) 260 (500) 300 (570) 700 (1290) 1.0 0.040 260 (500) 260 (500) 300 (570) 700 (1290) 1.5 0.062 260 (500) 440 (825) 510 (950) 920 (1690) 2.0 ... 260 (500) 440 (825) 510 (950) 920 (1690) ... 0.093 260 (500) 480 (900) 580 (1075) 1000 (1830) 3.0 0.125 315 (600) 520 (970) 650 (1200) 1070 (1960) 4.5 0.188 370 (700) 620 (1,150) 730 (1350) 1150 (2100) 6.0 0.250 370 (700) 720 (1330) 820 (1510) 1150 (2100) 8.0 0.375 370 (700) 720 (1330) 820 (1510) 1150 (2100)

The different susceptibility of cable thermocouples with different diameters to thermal shock, as a factor affecting their metrological characteristics, is shown in Figure 1. Thermal shock in studies conducted by TESEY PK means the immersion of a thermocouple from room temperature in a furnace with a temperature of 1000 ° C. for one minute, holding to stabilize the readings, then removing the thermocouple from the furnace at the same time, followed by cooling to room temperature due to natural convection. Heating in the first minute is about 700 ° C. The thermocouples of the KTNN type (nichrosil-nisil) with a diameter of 4.5 were investigated; 3 and 2 mm. For each modification, the change in dependence was monitored after 500 thermal shocks. As can be seen from the presented results, the difference in changes in characteristics for thermocouples with a diameter of 3 and 4.5 mm is in the range of 0.2-0.3 ° C, which fits into the uncertainty of the measurements. The readings of thermocouples with a diameter of 2 mm after 500 shocks differ from the readings of thermocouples with a diameter of 3 and 4.5 mm by an amount equal to or greater than 0.15% of the measured temperature.

Cables of special designs are also produced, which differ from standard designs by the presence of a second protective sheath from another material or by the manufacture of a sheath with a thickness 1.5–2 times increased (FIG. 2). The above designs also make it possible to increase metrological stability, ceteris paribus.

Another reason for the change in the characteristics of the cable thermoelectric converter may be the difference in the coefficients of thermal expansion of its components, which leads to mechanical deformation of thermoelectrodes TP, operating in the heat exchange mode. The resulting residual microstresses cause a change in thermo-emf. To reduce the effect of deformation, all the materials making up the thermocouple are selected in such a way that they behave as a whole during operation and have a minimal effect on each other. This is achieved by maximally equalizing the coefficients of thermal expansion of materials and by strictly dosing the content of alloying elements and impurities in thermoelectrode alloys of the thermocouple and cable sheath. Such cable thermocouples are called integrated thermocouples. In the work [Daneman H.L. The Choice of sheathing for mineral insulated thermocouples. J. Measurements & Control, 25 (1), 1991, pp.93-95] provides data on the high stability of the KTHN cable thermocouple in a sheath of modified alloy nichrosil with an outer cable diameter of 3 mm for 2200 hours at a temperature of 1100 ° C. Change in thermo-emf. did not exceed 4 ° C. For cable thermocouples KTHN with a diameter of 2 mm in a shell of an Ig grade alloy at a temperature of 1200 ° C, it was -6 ° C and -9 ° C for 2000 and 4000 hours exposure, respectively. [Bailleui G., Fourrez S. High stability type K & type N thermocouples for operation up to 1200 ° C. Proceedings of international symposium "Temperature-2002. It's measurement and control in science and industry", v.7, part 1, American institute of physics, New York, 2002].

The development and manufacture of cable thermocouples with an integrated layout allows achieving high stability not only of nichrosil-nisil thermocouples, but also of chromel-alumel [Bentley R.E. Thermoelectric behavior of Ni-based ID-MIMS thermocouples using the Nicrosil-plus sheathing alloy. Proceedings of international symposium "Temperature-92. It's measurement and control in science and industry", v.6, part 1, American institute of physics, New York, 1992, pp.585-590].

The use of insulating materials with greater than magnesium oxide insulation resistance over the entire temperature range and preventing the transition of alloying elements from a thermoelectrode to a thermoelectrode and from a shell to thermoelectrodes leads to increased stability of cable thermocouples. In [Barberree D.A., The next generation of thermocouples for the turbine engine industry. Proceedings of international symposium "Temperature-2002. It's measurement and control in science and industry", v.7, parts 1 and 2, American institute of physics, New York, 2002] provides data on increasing the stability of XA cable thermocouples with an inconel sheath and mineral insulation Ml-Dry ™ on average 4 times, and a minimum of three. The tests were carried out at 1200 ° C in vacuum. The maximum retention time of the static characteristic within ± 10 ° C of the nominal for thermocouples with magnesium oxide insulation was 500 hours. Deviations of the characteristic value from the nominal value for thermocouples with Ml-Dry ™ insulation did not exceed ± 5 ° С in 1500 hours.

Thus, the metrological stability of thermoelectric converters depends on the type of materials insulating thermoelectrodes, on the cross section of thermoelectrodes, on the concentration of alloying elements in thermoelectrodes, on the type of their preliminary heat treatment, on the number of elements of mechanical protection of thermoelectrodes and its geometric parameters.

Figure 3 shows the cross section of the thermoelectric converter according to the first embodiment (it is also the cross section of the proposed thermocouple cable), where 1, 2, 3, 4 are wires (thermoelectrodes), 1 and 2 are made of one thermoelectrode alloy, and 3 and 4 - from another, 5 - a metal sheath made of heat-resistant steel or alloy, 6 - mineral insulation from magnesium oxide or aluminum oxide, while thermoelectrodes 1 and 3 are combined into one thermocouple, and 2 and 4 - into another. Wires (thermoelectrodes) made of the same thermoelectrode alloy differ from each other in the concentration of alloying elements and / or nominal cross-section and / or type of preliminary heat treatment.

Figure 4 shows a thermoelectric converter according to the second embodiment, where 7 is a switching unit, 8 is a protective cover, 9 is a thermocouple mounting unit made with a through hole 10, designed to accommodate a control or reference measuring instrument inside the protective cover, 11 and 12 are sensitive elements in the form of thermocouples with the same nominal static characteristic, and thermocouples differ in the type of materials that insulate the thermoelectrodes, and / or in the cross section of the thermoelectrodes, and / or in the concentration of the alloying elements in in thermoelectrodes, and / or by the type of their preliminary heat treatment, and / or by the number of elements of mechanical protection of thermoelectrodes, its geometric parameters.

The device operates as follows. For a thermoelectric converter containing a protective cover 8, a switching unit 7 and two sensing elements 11 and 12 in the form of thermocouples with the same nominal static characteristic, if possible, choose thermocouples with the closest metrological characteristics. Moreover, the design of thermocouples provides a different degree of susceptibility to factors affecting their metrological stability. For example, these are cable thermocouples of different diameters or the same, but with different shell thicknesses, or cable thermocouples and thermocouples with thermoelectrodes placed in ceramic beads.

In order to verify the varying degrees of susceptibility of the selected thermocouples to factors influencing their metrological stability, an intertesting (intercalibration) interval is preliminarily set for them according to the method described in RMG 74-2004 "METHODS FOR DETERMINING INTERTEST AND INTERCALIBER INTERVALES OF INTERVALS. Moreover, the drift of readings over a specified period of time for thermocouples should differ by an amount greater than the measurement uncertainty. Many factors influence the uncertainty of temperature measurements by thermocouples, the main ones are:

- uncertainty of measurement of thermo-emf. a recording device;

- uncertainty of thermocouple calibration, i.e. determining its individual statistical characteristics;

- thermoelectric characteristic of the extension line connecting the thermocouple to the recording device;

- random effects when taking measurements

In the laboratory of PC "Theseus", equipped with modern measuring instruments, an expanded uncertainty of the calibration of thermocouples of types K and N is equal to ± 0.1% of the measured temperature in the range from 300 to 1100 ° C.

From the rules for calculating the expanded uncertainty, it follows that the determination of the difference in thermocouple readings can be made with an uncertainty of ± 0.15% of the temperature level in the range from 300 to 1100 ° C.

Thus, to ensure the operability of the device, it is necessary to choose a pair of thermocouples, the readings of which for the calibration interval established in laboratory conditions diverge, at least by an amount equal to ± 0.15% of the measured temperature level.

After placing the thermoelectric converter at the facility, the readings of both thermocouples are continuously recorded. Since the degree of susceptibility of thermocouples to factors influencing their metrological stability is different, over time the readings of thermocouples will begin to diverge. Moreover, with a high degree of probability it can be argued that the speed of the discrepancy of the readings will be greater than when establishing the calibration interval according to RMG-74. Upon reaching a certain difference, the value of which will depend on the selected thermocouple designs and operating conditions, but no less than the expanded uncertainty of determining the difference in thermocouple readings, the transducer is checked or calibrated.

To increase the reliability of verification, it is carried out without dismantling the thermoelectric converter from the object according to the method of MI 3091-2007 “GSI. Thermoelectric converters with an additional channel for a standard cable thermoelectric converter. Verification Technique. ” Then they decide to replace the converter or amend the readings of both thermocouples and continue the measurements until a new discrepancy is reached by the set value.

To prove the achievement of the technical result, 3 thermoelectric converters with two sensitive elements were manufactured, one in the form of a cable thermocouple for graduation XA with a diameter of 4.5 mm, the other in the form of a cable thermocouple for graduation XA with a diameter of 2 mm. The converters were operated in a brick annealing furnace in a cyclic mode for 3.5 months. During operation, the readings of both thermocouples were recorded. The temperature was determined by the nominal static characteristic taking into account the corrections obtained during the initial calibration of thermocouples. The results obtained for thermocouples of one of the transducers are presented in figure 5. As can be seen, after 105 days the difference in readings at a temperature of 1000 ° C was 3.4 ° C. During measurements, their expanded uncertainty was estimated at 2.5 ° C. The control calibration of the thermocouples according to the MI 3091-2007 method showed that the deviation from the initial calibration for the thermocouple with a diameter of 4.5 mm was 5.6 ° C at 1000 ° C and the thermocouple did not go beyond the tolerance established for class 2 thermocouples. The deviation of the thermocouple with a diameter of 2 mm was 10.2 ° С, which indicates that its readings went beyond the established tolerance for thermocouples of the 2nd accuracy class. The results obtained, taking into account the uncertainties of the measurement, confirm the operability of the proposed device.

Thus, the task is solved - the creation of thermoelectric converters, allowing to determine the need for periodic verification or calibration, and also achieved the technical result, which consists in increasing the reliability of measurement results, the reliability of technological processes and the quality of products.

Claims (6)

1. The thermoelectric cable Converter, which includes two thermocouples with the same nominal static characteristic, and the thermoelectrodes of various thermocouples are made of wire with different concentration of alloying elements, and / or from wire with different nominal cross-section, and / or from wire that has passed preliminary heat treatment of various kinds. 2. A thermocouple cable for manufacturing a thermoelectric converter according to claim 1, consisting of four wires made in pairs of two different thermoelectrode alloys, the wires are placed in a metal sheath made of heat-resistant steel or alloy and insulated from each other and from the sheath by mineral insulation from magnesium oxide or alumina, while the wires made of the same thermoelectrode alloy differ from each other in the concentration of alloying elements, and / or the nominal cross section, and / or the type of preliminary heat treatment. 3. A thermoelectric converter comprising a switching unit, a protective case, a thermocouple attachment unit made with a through hole designed to house a control or reference measuring instrument inside the protective case, two sensing elements in the form of thermocouples with the same nominal static characteristic, and thermocouples differ in the type of materials that insulate the thermoelectrodes, and / or in the cross section of the thermoelectrodes, and / or in the concentration of alloying elements in the thermoelectrodes, and / or in the type of their dvaritelnoy heat treatment, and / or the number of elements of mechanical protection thermoelectrodes by geometrical parameters. 4. Thermoelectric converter according to claim 3, characterized in that the first thermocouple is a cable thermocouple of a standard layout in a shell made of heat-resistant steel or an alloy with mineral insulation, for example, magnesium oxide or aluminum oxide, and a thermocouple is used as the second sensitive element with thermoelectrodes isolated from each other, from the main sensitive element and the protective cover with a single or multi-channel ceramic straw, or glass fiber, or silica thread Yu. 5. The thermoelectric converter according to claim 3, characterized in that the first sensitive element is made in the form of a cable thermocouple standard layout with an outer diameter of from 4 to 8 mm, and the second sensitive element is made in the form of a cable thermocouple standard configuration with an outer diameter of 1 to 3 mm 6. The thermoelectric converter according to claim 3, characterized in that a cable thermocouple with an increased sheath thickness or with a sheath consisting of two coaxially arranged layers of different metal, or with a sheath of steel or alloy with a chemical composition close to one another is used as the first sensing element to the composition of thermoelectrodes, and having a coefficient of linear expansion comparable to the coefficient of linear expansion of thermoelectrodes, or a cable thermocouple with mineral insulation, having a large, h I eat magnesium oxide and aluminum oxide, insulation resistance over the entire temperature range, and as a second sensitive element we used a standard-type cable thermocouple with an electrode diameter not exceeding the diameter of the thermoelectrodes of the main sensitive element.