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How to choose vacuum furnace thermocouple

The common characteristic of vacuum furnaces is the temperature measurement at a certain vacuum degree. If the thermocouple is expected to be accurate and reliable, and its service life is long, the factors affecting the accuracy of temperature measurement under vacuum environment must be deeply discussed, such as vacuum, emissivity, thermal conductivity and the insertion depth of the thermocouple. Therefore, the choice of special thermocouple for vacuum furnace is particularly important.

How to choose vacuum furnace thermocouple

Vacuum Furnace for Special Thermocouple 

2.1. Classification of thermocouples

The International Organization for Standardization (ASTME344-08) classifies thermocouples into three categories.

Standardized cheap metal thermocouples; Standardized noble metal thermocouples; 3. Refractory metal thermocouples. Refractory metal thermocouples are defined as: the unipolar material melting point of the thermocouple is higher than 1935 C, and the two poles of the thermocouple must be refractory metal or alloy.

2.2. Standardized Low-cost Metal Thermocouples

Standardized low-cost metal thermocouples are T, J, E, K and N. When the working temperature of vacuum furnace is lower than 1260 C, low-cost metal armoured thermocouple can be selected. Because K-type thermocouple has poor high temperature stability, N-type armoured thermocouple is recommended to replace S-type thermocouple under 1300 C.

1) Characteristic of Nickel Chromium Silicon-Nickel Silicon Magnesium Thermocouple (Type N)

High temperature oxidation resistance and good long-term stability;

(2) The short-term thermal cycle stability is good in the range of 250-500 C, and the magnetic transition is restrained, while the thermoelectric EMF has no obvious change.

(3) The linearity of thermoelectric characteristics of N-type thermocouples is better than that of K-type thermocouples in the range of 400-1300 C.

2) N-type thermocouple instead of S-type thermocouple

N-type thermocouple is used instead of S-type thermocouple to measure the temperature of multi-purpose furnace heater (1180 C). Five years since 2008,

3) Selection of armored thermocouples

Durable thermocouples can be divided into two categories: assembled and armored. Considering the saving of metal materials, the convenience of use and the reduction of heat loss, armoured thermocouples should be adopted as far as possible. Thermocouple armor mainly includes the following kinds:

(1) Medium-high temperature zone armor

In China, the armor in the middle temperature zone is mainly 304, 321 or 1Cr18Ni9Ti, while in the high temperature zone there are 310S, Inconel 600 and NiCr alloys. Experiments show that the drift magnitude varies with the casing material in the temperature range of 1000-1200 C. The rule is that the ratio of NiCr < Inconel 600 < 310S drift is 0.75 1 5.

(2) Nicrobell, a new high temperature armored thermocouple material

This alloy is based on the chemical characteristics of Nicrosil, adding magnesium and niobium elements, improving the oxidation resistance and high temperature strength of the alloy, using Nicrobell alloy to make a new type of armoured thermocouple. It has been used in the temperature zone above 1200 C in Fengdong Co., Ltd. and achieved good results.

2.3. Standardized noble metal thermocouples

Standardized noble metal thermocouples are S, R and B. Precious metal thermocouples can be selected in the range of 1260-1750 C.

1) It is not recommended to select precious metal thermocouples at < 1260 C. Some vacuum furnace enterprises choose S-type thermocouples and GH30 or 39 as protective tubes within this temperature range. Although the protective tubes have high strength and are not easy to be damaged, nickel-based alloys will pollute S-type thermocouples at high temperatures and cause errors. At present, special high temperature armored thermocouple has been gradually used, which is economical, economical and popular.

2) It is not recommended to use S-type bare couple under vacuum condition of T > 1280 C.

(1) For military or aviation certification enterprises, the accuracy of furnace temperature uniformity test is very high. When T > 1280 C, S-type bare couple is mostly used, but the strength of precious metal bare wire is very low. For this reason, the author developed a dispersion-enhanced S-type thermocouple. The temperature uniformity test of vacuum heat treatment furnace used in Beijing Institute of Aeronautical Materials at t > 1300 C shows that the service life of the vacuum heat treatment furnace is more than 5 stoves, while the service life of the ordinary S-type bare couple is only one stove, and the service life is significantly improved.

(2) Disadvantage of using S-type naked couple in vacuum

Under vacuum conditions, the use of S-type bare dipoles will be contaminated. Secondly, in vacuum environment, rhodium in S-type bare dipoles will volatilize to the negative. The original single platinum rhodium is transformed into double platinum rhodium thermocouple, resulting in large negative deviation.

(3) Suggestion to change S-type into B-type bare couple

In vacuum, the rhodium of positive pole must evaporate. When rhodium deposits on S-type pure platinum, it becomes platinum-rhodium alloy thermocouple, which will produce the negative deviation mentioned above. If B-type bare couple is used instead, although rhodium will still volatilize at high temperature, the effect of rhodium deposited on the negative pole of B-type thermocouple containing Rh 6% PT-Rh alloy on the composition of the negative pole can be neglected. Its service life is much longer than that of S-type thermocouple.

(4) Ceramic tubes should be selected for precious metal thermocouples when t > 1260 C.

In order to prevent metal or alloy from contaminating precious metal thermocouples. It is suggested that corundum protective tubes, especially type B thermocouples, should be selected with high purity and clean corundum tubes. In order to improve the strength of thermocouple and prevent the breakage of ceramic protective tube, metal-ceramic composite tube can be used, that is, the length of thermocouple clamping tube can be prolonged intentionally. The thermocouple has the advantages of both ceramic and metal protective tubes. Its structure looks simple, if mechanical imitation is sure to fail. The reason is that the seal is not tight and leaks air, so it can not be used in vacuum furnace.

3) in the temperature range of 1260-1750

(1) The precious metal thermocouple should be selected in this temperature range. If the temperature measurement accuracy is high (0.5% t or above) or the requirement is stable and reliable, the precious metal thermocouple should be selected. If the working temperature is 1800 C, the non-standardized platinum-rhodium 40-platinum-rhodium 20 thermocouple should be selected. In this temperature range, if the thermocouple is installed vertically, it is feasible. However, if the thermocouple is installed horizontally at t (> 1600 C), it will bend due to the weight of the cantilever at high temperature. If the refractory metal and corundum composite pipe structure is adopted, the bending deformation of the protective pipe can be effectively prevented at high temperature.

(2) Tungsten-rhenium thermocouple is selected when the temperature measurement accuracy is (+1%) t or T > 1800 (?) C. Its characteristics are as follows:

1. The thermoelectric pole has high melting point (3300 C) and high strength, and its service temperature is as high as 2500 C.

2. Thermoelectric potential is large, sensitive and three times as high as B-type thermocouple.

3. It is very easy to oxidize.

4. Cheap price, only 1/5 of S-type thermocouple.

With the development of advanced science and technology fields such as aviation, aerospace and nuclear energy, and the rigid requirement of replacing precious metal thermocouples to reduce costs, the application prospect of W-Re thermocouples is extremely attractive.

Under the condition of market economy, vacuum furnace manufacturers and users constantly pursue high performance and quality of products. Therefore, the requirement for accurate measurement of temperature in vacuum furnace is higher and higher, and the life expectancy of thermocouple is longer and longer. Therefore, besides improving product quality in instrument factory, users can reasonably select special thermocouple for vacuum furnace according to the use environment, atmosphere, temperature and installation mode, which is also an important aspect that can not be ignored.

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