ВР5 tungsten rhenium thermocouple wire

Well-known aphorism «everything is known in comparison» clearly illustrates qualities of alloys such as VR5 and VR20.

Heat resistance VR5 and VR20

Advantages of alloys determine tungsten and rhenium. Tungsten is one of most refractory, heavy and hard metals, melts at t° 3410 °C, and boils at same temperature as on surface of Sun: 6690 °C. Tungsten alloys in terms of hardness and refractoriness lead among others. Rhenium is not much inferior to tungsten, with melting point of 3170 °C and boiling point of 5870 °C. Rhenium in terms of heat resistance leaves other metals far behind, demonstrating high strength and resistance to corrosion.

Features of alloying

Alloying tungsten with 5% rhenium improves ductility, increases temperature of recrystallization of alloy — this is especially important when using it for positive thermocouple electrode. Using an alloy as negative thermocouple electrode to achievement maximum thermoelectromotive force requires rhenium content of up to 35%, but not higher. Otherwise, at t° 1100 °C uniformity of alloy is lost, and thermo-emf is noticeably distorted. In domestic metallurgy, rhenium additives do not exceed 20%, which allows use of tungsten-rhenium thermocouples at t° up to 2500 °C.

Durability

Tungsten-rhenium alloy exhibits resistance to helium, argon, dry nitrogen and hydrogen. Doping with silicon-alkali additives allows to increase stability of alloy in temperature regime up to 2000 °C by 2−3 times. BP5/20 sensors are able to work in vacuum. For more efficient operation in vacuum at temperature of 1800 °C and above, electrodes with large diameter should be used. BP5/20 are widely used as differential thermocouple — they can consist of two equivalent thermocouples connected, as rule, opposite each other. Such thermocouples are able to measure level of temperature difference between two different junctions: working and conditional.

Thermo-EMF

Nominal value of thermo-EMF also decreases to 24.59 mV when temperature drops to 1600 °C. At t = 1500 °C this indicator is not more than 23.31 mV, and at t = 1400 °C nominal level of thermo-emf is 21.97 mV.

Disadvantages of VR5 and VR20

Disadvantages of these alloys include poor reproducibility of thermo-EMF, which is why it is necessary to group thermoelectric pairs in categories: A-1, A-2, A-3. Category A-1 allows the use of alloy at maximum temperature of 2200 °C, temperature regime of use of A-2 and A-3 is limited to 1800 °C. It is strictly forbidden to use wire from different parties, as each batch of wire has its own indicator of thermo-EMF. Tungsten and rhenium are rare metals, so their cost is very high. Price of VR5/20 is much higher than cost of thermocouples made of other materials. But still less than cost of optical pyrometers for measuring and monitoring temperature conditions in similar industrial conditions.

When nitrogen and tungsten-rhenium alloy interact under vacuum, it is necessary to observe cleanliness and especially carefully monitor mandatory removal of organic substances and dust.

Application

VR5 and VR20 are used for manufacture of measuring and converting devices, contact thermocouple sensors (TVR) for measuring highest temperatures in metallurgy — liquid metals. Up to 2500 °C, measurement error is extremely small (± 0.01 °C), which makes TVR one of most accurate instruments for measuring in such aggressive environment.

Calibration table — thermocouples VAR-5 (VR-5)/VR-20 at t° of free ends 0 °C. Table «Nominal static characteristic of conversion of VR (A) -1»:

1. Basic technical data

2. General information

Tungsten-rhenium wire for thermocouples annealed graduated thermocouples with nominal static properties of transformation VR (A) -1. Wire meets technical conditions of YaeO.021.142 TU for production of thermocouples VAR-5 (VR-5) / VR-20.

3. Recommendations for use

Use of thermocouples made of thermoelectrode wire from different series is inadmissible, even with same nominal static characteristic of thermoelectrons.

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Elektrovek-steel LLC has wide range of thermocouples in stock. Our representative offices are located in Russia and Ukraine. You can make purchase without leaving your office through Company’s website. To purchase, you only need to place order and click «buy» button. If necessary, our managers can advise you on any current issues. They will provide all necessary information regarding product, price range, conditions of sale. Lead times are short. For bulk purchases discounts.

Experience of using tungsten framing thermocouples VR5/20 in high temperature thermometry