Tungsten rhenium thermocouple
|Grade||analogue||W. Nr.||Aisi Uns||En||Order|
|VR5||WRe20||Buy from stock, view availability|
|BP20||WRe20||Buy from stock, view availability|
The tungsten rhenium thermocouple is capable of measuring the highest temperatures. In doing so, a number of factors should be taken into account. The ductility and recrystallization temperature increase towards the positive VR electrode due to the admixture of five percent rhenium. To obtain the highest possible thermo-EMF, the content of rhenium in the negative electrode should be as different as possible from the proportion in the positive. However, with an increase in the proportion of rhenium to 32% or more in the composition of the alloy at t ° above 1100 ° C, intermetallic phases begin to form, which leads to a drop in the accuracy of the readings. In addition, this alloy will be inhomogeneous; with high heating, rhenium will begin to evaporate, which also increases the error. The accuracy of thermocouples VR5 and VR20 at a temperature of 900−2100°C decreases linearly. To increase stability, tungsten-rhenium thermocouple is alloyed with silicon-alkali additives (KCl 0.1−0.5%, Al 0.1−0.5%, 0.1−0.5% SiO 2), which is almost three times reduces the error in the measurement process.
|Number||Brand||%Re||Section (mm)||Rental type||price|
|3||VR5||5%||Ø more than 0.5||bar||negotiable|
The thermocouple calibration table was developed back in 1968. The most complete study of the increase in error due to the inhomogeneity of electrodes — VR and at high temperatures of insulation shunting was carried out by Professor VNIIM, Doctor of Technical Sciences Gordov. Technical conditions for the production of thermocouples — BP were adopted in 1973, and in 1977 they were included in GOST 3044−74 and GOST 6616−74. A standardized nominal static characteristic (NSH) was also introduced, which was developed on the basis of the MPTSh-68 temperature scale.
The calibration of tungsten rhenium thermocouples was carried out in several scientific organizations: at the Ural Institute of Metrology (Yekaterinburg), at the Termopribor Design Bureau (Lvov) and at the NPO LUCH (Podolsk). NSH thermocouples were determined in a vacuum furnace by melting silver, copper, nickel, platinum, rhodium, iridium, tantalum wires wound on the working junction of thermocouples. The root-mean-square error of calibration to the platinum melting point was estimated at the rhodium point. The readings of BP-thermocouples were compared with the readings of an optical pyrometer and thermocouples PR30/6 (t° up to 1800°C) Specialists of scientific institutions came to the conclusion that the calibration of 80% of BP thermocouples had a spread of readings up to one percent.
According to the technical specifications for the thermal electrode wire, fluctuations in the content of rhenium in the alloy are not more than 0.5%, the proportion of impurities is not more than 0.1%. Therefore, different batches of wire have a spread of thermal electromotive force. Up to t ° 1500 ° C in one coil of wire, a spread of thermo-EMF up to 50 μV is allowed.
In the 80s of the last century, the production of thermoelectrode wire VR20 and VR5 (VAR5) was approximately 150 kilograms per year. Thermoelectrode wire has been used for various jobs by aircraft design bureaus and research institutes in the technology of composite materials production. The main consumer of this material was the metallurgical industry.
Wire is produced by powder metallurgy. A mixture of tungsten powder and ammonium salts (NH 4 ReO 4) perrhenate is pressed into rods, after which they are fused and rolled into a wire. It was not possible to carry out cardinal optimization of the thermoelectric characteristics of the thermocouple, due to the «floating» percentage of rhenium in the composition of the alloy. Despite this, there have been some recent advances in micrometallurgy, thanks to nano-alloying additives and improvements in metal analysis.
They are used only in an inert atmosphere or in a vacuum. To expand the field of application of thermocouples, a sealed special case was developed, which is filled with an inert gas for protection in oxidizing and carbon environments (for example, in vacuum furnaces with graphite heaters).
Hermetic thermal converter, located in a molybdenum case, is designed to control the sintering mode of powder pellets in a vacuum furnace. The operating life of such a converter is at t°1750°C, up to 1000 hours in an aggressive environment. Other VRs have also been developed — sealed transducers in cases made of leucosapphire (a single crystal of aluminum), whose service life at a temperature of 1600 °C in oxidizing environments is 2−3 thousand hours. They are also used in thermal probes to control the melting of salts, glass and metals.
Such thermal converters are used in very aggressive environments, and the working life directly depends on the durability of its protective cover. If in the range of 1000 — 1700 ° C the permissible error should be 0.5%, a platinum-rhodium thermocouple is used.
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