Tungsten-rhenium thermocouples are available measuring the highest temperatures. They appeared in the Soviet Union in 1956 admixture of 5% rhenium to the positive-electrode BP increases its ductility and recrystallization temperature. To obtain the highest possible thermal EMF proportion of rhenium in the negative thermoelectrodes should differ from the maximum content in the positive. But at 32% or more of rhenium in the alloy, W-Re above 1100 ° C and intermetallic phases are formed falls accuracy of sensor readings. This alloy is heterogeneous — in significant heating of rhenium will evaporate, which also increases the accuracy. Therefore, using 25−26% rhenium alloy in the USSR in the US — 20% of the alloy. However, the accuracy of the thermocouple VR5 / 20 at t ° 900−2100 ° C decreases linearly. In order to improve the stability of VR-doped thermocouple SILICA additives (0,1−0,5% KCl, 0,1−0,5% Al ;, O3, 0,1−0,5% SiO2). This reduces the error in the measurement of high temperatures is almost 3 times.
The calibration table of the thermocouple VR5 / 20 was developed in 1968, most fully investigated the errors increase due to the heterogeneity of BP-electrodes and bypass the insulation at high temperatures, D.Sc. Professor Gordov (VNIIM). TU production of BP-thermocouple were taken in 1973 and listed in GOST 6616−74. In GOST 3044−74 in 1977, was introduced and standardized NSH (nominal static characteristic), developed based on the temperature scale IPTS-68.
Calibration of the thermocouple VR5 / 20 conducted in several institutions: in Yekaterinburg in the Ural Institute of Metrology, in Lviv, CB «Termopribor» in Podolsk in the NPO «Luch». NSH was determined in a vacuum furnace thermocouple wires melting of silver, copper, nickel, platinum, rhodium, iridium, tantalum, wound on the hot junction of the thermocouple. Before the melting point of platinum mean square error evaluated at calibration rhodium. BP-thermocouple readings were compared to readings of thermocouple AP30 / 6 (1800 ° C) and an optical pyrometer. It turned out that the grading of 80% BP-thermocouple had spread in values of up to 1%, the minimum error for thermocouple VR5 / 20 Class 2 was set at the level of error thermocouple type B (AP30 / 6) 0.5%. At BP-thermocouple third error class more than 0.5%.
Fluctuations in the percentage composition
According to specifications for thermocouple wire, it is permissible fluctuation proportion of rhenium in the alloy to 0.5% and the proportion of impurities down to 0.1%. As a consequence, different batches of wire have spread the thermo-emf. In one bay wire admissible spread thermoelectric voltage to 50 mV at 1500 ° C.
GOST 3044−77 standardizes the three closest to the thermocouple calibration VR5 / 20: A-1, A-2 and A-3. A calibration-2 and A-3 are disposed respectively above and below the primary grading of A-1 with an operating range of 1000 to 2500 ° C and only go up to 1800 ° C.
In the 80 years of the twentieth century, thermocouple wire VR5 (VAR5) and VR20 produced about 150 kg per year. The wire was on the needs of research institutes and aircraft KB, used in the manufacture of composite materials technology. The main consumer was metallurgy. When measuring the temperature of molten metal thermocouples VR has successfully replaced the expensive Platinum Rhodium, both in the Soviet Union and CMEA countries. The collapse of the socialist camp in the early 90s led to a crisis in industrial production and decay thermocouple wire consumption by 60%. Now the recovery of production requires more robust high-temperature sensors, such as BP-thermocouple.
Today in Russia annually produce about 90 kg of wire thermocouple VR5 and VR20 0.35−0.5 mm in diameter and a total length of 20 km. The alloy is produced by powder metallurgy. A mixture of tungsten powder and ammonium salts perrhenate (NH4ReO4) was compressed into rods and then melted and rolled into wire. Because of the «floating» the rhenium content in alloys VR5 and VR20 is unable to radically optimize the thermoelectric characteristics of thermocouples. However, recent successes micrometallurgy, the emergence of nano-alloying additions and improvements are already beginning the analysis of metals to improve the situation.
Thermal-based thermocouple VR5 / 20 made in the Register of measuring instruments R. F. These thermocouple is used only in a vacuum or inert atmosphere. To extend their scope special hermetic cover was designed filled with inert gas, carbon thermocouple protecting environment (e.g., in a vacuum furnace with graphite heaters) and in oxidizing environments. So sealed pouch in a molybdenum thermal converter for temperature control in a vacuum sintering furnace fuel pellets, its working life up to 1000 hours at 1750 ° C in an aggressive environment. Developed and other sealed BP-thermocouples in their cases from a single crystal alumina (sapphire) with a working life of 2000−3000 hours at 1600 ° C in oxidizing environments. They also apply to the thermal probe for controlling the temperature of molten metals, salts, or glass.
These converters are used in particularly aggressive environments, and the service life depends on the strength of a protective cover. At temperatures of 1000 — 1700 ° C they replace Platinum Rhodium Thermocouple if margin of error — not less than 0.5%.
Certification thermocouple wire
Held in a conventional high-temperature furnace. Thermocouples of the segments BP-wire taken from the beginning and the end bays, placed in a shroud made of sapphire and sealed there. Calibration is carried out in the range of 600−1700 ° C by the reference thermocouple AP30 / 6 placed in the same oven. This method is more simple and cheaper than the calibration in a vacuum oven to 90% is realized requests certification thermocouple wire. Since the BP-thermocouple are quite reliable, and often the only possible, means of measuring the temperature in the range of 1500−2500 ° C are included in the CIS standards and demand is now everywhere.
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