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The usage of tungsten thermocouples
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The tungsten-rhenium thermocouples are used at the highest temperature measuring. These thermocouples appeared in 1956 in USSR. The 5% of rhenium additive reduced the alloy plasticity and recrystallization temperature of tungsten. To get the maximum index of electromotive force in negative thermoelectrode it should be significantly changed in positive content. But if the rhenium content is 32% and temperature is 1100°C the intermetallic phases appears and thermocouple errors increase. This alloy will have heterogeneous characteristics and during the substantial heating it will evaporate. In the USA the 25-26% rhenium alloy is used and in the USSR – 20% alloy.
With the aim of thermocouple stability increase at t of1000-2100°С the couples are alloyed by silicon-alkali additives (0.1– 0,5% KCl; 0,1 – 0,5% Al2O3 , 0,1 – 0,5% SiO2). In thermocouple 5/20 the alloying additives increased the temperature of collective recrystallization and stabilized the thermocouple operation at temperature of 1500-2000°C almost in 3 times.
Calibration characteristic of thermocouple 5/20 is accepted in 1968. In works of professor Gordov the thermoelectrical stability and calibrated error of the thermocouple 5/20 are perfect researched. The thermoelectrode wire specifications were developed in 1973 and included to State Standard 6616-74.
Thermocouple 5/20 calibrating was provided in several organizations: Yekaterinburg in Ural institute of metrology, Lviv in “Thermo device” and Podolsk “Luch”. The thermocouples were identified in vacuum furnace by wire melting of silver, copper, nickel, platinum, rhodium, iridium, tantalum, etc. Up to platinum melting point the mean-square error of calibration was identified by rhodium point. The thermocouple 5/20 registrations were compared with thermocouple 30/6 (up to 1800°C) registrations and optical pyrometer. It was shown that thermocouple 5/20 calibration had spread of values of 1%, minimal electromotive force error of thermocouple 5/20 of class 2 was 0.5%. Thermocouple 5/20 of third class has the error of ±0,7%.
The acceptable rhenium alloy fluctuation up to 0.5% with additive concentration of 0.1% is permissible. As a result in different wire batches there is dispersion of electromotive force. In one wire coil the permissible dispersion of electromotive force is 50mkV at temperature of 1500°C.
The State Standard 3044-77 standardizes three close thermocouple 5/20 calibrations: А-1, А-2 and А-3. Calibration А-2 and А-3 is replaced accordingly to A-1 with operating range from 1000 to 2500°C.
In 80-s of XX century the thermoelectrode wire 5 and 20 was produced in volume of 150 kg/year. These thermocouples were used in research institutions, in the production of composite materials and aircraft building. The main consumer of thermoelectrodes was metallurgy.
The disintegration of Soviet Union and Council for Mutual Economic Assistance in 90-s lead to crisis of industrial production and recession of thermoelectrode wire 5/20. Nowadays the production restoration requires the reliable high temperature sensors for the temperature 1500-2000°C measurement.
Nowadays the production of thermoelectrode wire in Russia is 90 kg/year.The alloy is produced with the help of powder metallurgy. The mixture of tungsten powder and ammonium salt (NH4ReO4) is pressed in fillets, than melted and rolled in wire. Due to non-stable rhenium content in alloy it is impossible to optimize thermoelectric characteristics of thermocouple. However the last successes in micro metallurgy, the alloyed nano-additives appearance and metal analysis perfection improved the present situation.
The assessment sequence of thermoelectrode samples was accepted in 1987. Compensating conductors made from copper alloy for this thermocouple are produced at temperature up to 1000°C. This thermocouple can be connected to secondary sensor through current converter (with output signal 4… 20 mA) produced by Russian companies.
Thermal converters with thermocouple 5/20 basis are recorded in Russian measurement register. These thermocouples are used only in vacuum or inert atmosphere and for the expansion of the application range was developed special waterproof cover filled by inert gas for thermocouple protection of in carbonic medium (for example: in vacuum furnace with graphitic heater) and in oxidizing media. Waterproof thermal converter with molybdenum cover is used for the temperature control in vacuum furnace of fuel pellet sintering; its endurance is up to 1000 hours at 1750°C in aggressive medium. Also there are other waterproof tungsten thermal converters in covers of aluminum single crystal (leucosapphire) with endurance of 2000-3000 hours at temperature of 1600°C in oxidizing media. Also there are used in thermoprobes for the melted metal, salts and glass temperature control.
Such thermal convertors are used in highly aggressive media and its endurance depends on working cover stability. At temperatures 1000-1700°C they replace platinum-rhodium thermocouples on condition that admissible error is not less than 0.5%. There is a method of thermocouple check and thermoelectrode coil wire 5/20 assessment in regular high-temperature furnace. Thermocouple pieces are placed in leucosapphire casing and are hermetically sealed. The calibration passes at temperature of 600-1700°C with standard thermocouple 30/6 placed in furnace.
International Bureau of Weights and Measures at thermocouples calibration in standard melting points of international temperature scale from 1000 to 2000°C allows the error of calibration 0.5-5.0°C. General error of electromotive force is 2.7-7.0.
As the thermal convertors with thermocouple 5/20 basis are quite reliable measurement means at temperature 1500-2500°C, they are recorded in CIS countries and quite required.