Tungsten rhenium thermocouple
Special features of alloying
Tungsten-rhenium thermocouple is capable of measuring the highest temperatures. There should be noted, however, a number of factors. A plasticity and a temperature of recrystallizing rises to a positive tungsten-rhenium electrode due to 5% of rhenium impurities In order to get as great as practicable thermo-emf, rhenium content in a negative electrode must maximally differ from a quantity in a positive electrode. Nevertheless, with increasing rhenium quantity to 32% and more, it can be observed that in alloy composition at a temperature higher than 1100 °C has become to form intermetallic phases. It leads to decreasing of index accuracy. Besides, this alloy will be heterogeneous because rhenium flashes at hot temperatures. It increases inaccuracy too. An accuracy of WRe5, WRe20 thermocouples decreases linearly at a temperature 900−2100°С. In order to increase a stability, tungsten-rhenium thermocouple alloys with a help of silica masteralloys (KCl 0,1−0,5%, Аl 0,1−0,5%, 0,1−0,5% SiO2). It decreases a measurement error almost thrice.
|Number||Grade||% Re||Size (mm)||Rolles stock type||Price|
|1||WRe5||5%||Ø 0,35−0,5||a wire||contractual|
|2||WR20||20%||Ø 0,35−0,5||a wire||contractual|
|3||WR5||5%||Ø more than 0,5||a bar||contractual|
Special features of division
Emf-temperature table was invented in 1968. A complete research of inaccuracy increase that was caused by electrode discontinuity at high temperatures has been done by Gordov — a professor of D.I. Mendeleyev Institute for Metrology and Doctor of Engineering Science. Technical conditions of tungsten-rhenium (TR) thermocouples manufacturing have been got past in 1973 and in 1977 they have become listed in GOST 3044−74 and GOST 6616−74. Besides, there was listed a normalized nominal static characteristic (NCC) which has been engineered on a basis of International Practical Temperature scale.
Division of tungsten-rhenium thermocouples was made in a few scientific organizations: in Ural Institute of Weights and Measures (Yekaterinburg), KB «Termopribor» (Lviv), in Scientific Development and Production Center «Luch» (Podolsk). NCC of thermocouples was calculated in a vacuum furnace with a help of melting wires from silver, cuprum, nickel, platinum, rhodium, iridium, tantalum which were coiled on a thermojunction. Up to a platinum melting point, a mean-square error of division was evaluated in a point of rhodium. Indexes of TR-thermocouples were compared with indexes of optical pyrometr and thermocouples PR30/6 (t°до 1800°С). Specialists of scientific establishments made a conclusion that a division of 80% TR-thermocouples had a dispersion of indications to 1%.
Variation of percentage composition
Due to technical conditions, on a thermal electrode wire is considered acceptable a variation of rhenium content (not more than 0,5%) and a quantity of impurities (not more than 0,1%) in the alloy. Consequently, different wire lots have a dispersion of thermo-electromotive force. A tolerable dispersion of thermo-emf is up to 50 mkV in one wire coil to a maximum t° of 1500 °C.
In the 80's of past century, a manufacture of thermocouple wire WRe20 and WRe5 amounted to about 150 kg in a year.
Thermocouple wire was used for different works of aircraft engineering departments and research institutes in producing technology of composite materials. Metals industry was the main customer of this material.
Nowadays, tungsten-rhenium wire of WRe5 and WRe20 grades (0,35−0,5 mm of diameter) is annually produced weighing as much as 90 kg and with an overall length up to 20 km. A wire is produced with a help of metal powder industry. A mixture of tungsten powder and ammonium salts perrhenate (NH4 ReO4) is pressed in billets and after that, it should be alloyed and expanded into a wire. To realize an essential optimization of thermocouple thermoelectric characteristics could not be done because of «floating» percentage of rhenium in alloy composition. Nevertheless, in recent times in a micrometallurgy have been achieved some successes with a help of alloying nano-additives and improving an analysis of metals.
Thermo-generators on a basis of WRe5 and Wre20 grades were listed in Federal Agency of Technical Regulating and Metrology. They are used only in an inert atmosphere or in a vacuum environment. For expansion of a field of thermocouple usage, there has been invented a special pressure-tight casing that was filled with an inert-gas for protection in oxidize and carbon atmospheres (for instance in vacuum furnaces with graphite heaters).
A pressure-tight thermos-generator, placed in a molybdenum casing, is intended to control sintering mode of powdered tablets in a vacuum furnace. An endurance of such a generator is 1000 hours at a temperature 1750 °C in an aggressive environment. Besides, there have been invented other TRs — pressure-tight generators in cases, made of leucosapphire (aluminium monocrystal) which service life is 2−3 thousand hours at a temperature of 1600 ° C in oxidizing environments. They are also used in thermoprobes to control melting mode of salts, metals, glass.
Such thermal generators are utilized in very aggressive environments, and their service life directly depends on a firmness of protective case. If in the range of 1000 — 1700 ° C a permissible error shall be 0.5%, a platinum-rhodium thermocouple must be used.
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