Environmental Impact on the Use of Thermocouples

1) Selective Oxidation

With regard to Ni-Cr alloys containing Fe, if the partial pressure of oxygen is lower than a certain value, Cr, which has a high affinity with O2, will undergo selective oxidation, which is grain boundary oxidation unique to Ni-Cr alloys. If you look at the surface oxide layer with a microscope, you can see green precipitates. This phenomenon is often called "green etch." Especially when the temperature is in the range of 800°C to 1050°C and the system contains CO, H2 and other regenerative gases, the positive electrode of the K type thermocouple is more susceptible to selective oxidation. This low thermoelectric potential due to the reduction in Cr content has become a limiting factor for the long-term use of K-type thermocouples in the heat treatment industry. If the gas used is pure and the system contains no oxygen, it can extend the life of the thermocouple. However, if the thermocouple wire has an oxide layer on its surface, it can still provide enough oxygen for the selective oxidation of Cr. Therefore, when using in a non-oxidizing atmosphere, clean, polished wire should be used. At the same time, the use of inert gases with trace amounts of oxygen or low partial pressure of oxygen should be avoided as much as possible. When the length and diameter of the maintenance pipe is relatively large (ie, the maintenance pipe is very thin), due to poor air circulation, an oxygen-deficient state is formed, and the remaining small amount of oxygen can still provide conditions for the selective oxidation of Cr.

2) Countermeasures for selective oxidation

In order to avoid or slow down the degradation of K type thermocouples due to selective oxidation, in addition to improvement in materials, corresponding measures should be taken in the construction of thermocouples:

1 Select a metal with stronger oxygen affinity than Cr as a getter, enclosed in the maintenance pipe to avoid selective oxidation of Cr, and increase the oxygen content by increasing the diameter of the maintenance pipe or by blowing air.

2 Assembled thermocouple solidification. The patented product developed by the author—a thermocouple used in a solid carburizing furnace—developed a fabricated thermocouple with a sealed structure to avoid selective oxidation of Cr attack, through the Wazhou Group, FAW, DCM, and Ipsen industrial furnaces. The use of more than a dozen companies such as Shen, Shen, Shen Qian and Qianjiang Motorcycles has proven that this plan is effective. With an operating life of 12 months or more, users are very satisfied [4]. 3) The influence of the atmosphere on the thermocouple's stability varies depending on the operating temperature and the atmosphere. The maximum operating temperature of the same type of sensor, such as the K-type thermocouple, also varies with the diameter, and K-type thermocouples with the same diameter are also used. Due to the different structures, their stability is also very different. In the choice of thermocouple, it is necessary to consider the following conditions for the use of conditions: 1 commonly used temperature and maximum operating temperature

2 Oxidation recovery and other operating atmosphere

3 Anti-vibration performance As for the assembled thermocouple, the influence of the atmosphere depends on the material of the maintenance pipe and the thermocouple structure. Therefore, it is necessary to familiarize with and control the physical and chemical properties of various maintenance pipe materials. For example: In the powder metallurgy industry, molybdenum tubes are commonly used as thermocouple maintenance tubes. In the 1600 °C H2 atmosphere, the use of better results. However, the molybdenum tube is corroded by oxidation in a short time in an oxidizing atmosphere. Secondly, based on the operating atmosphere, choose a suitable thermocouple, select a platinum-rhenium thermocouple in an oxidizing atmosphere above 1300°C, and use a tungsten-rhenium thermocouple under recovery and vacuum conditions. The K type thermocouple is suitable for working in an atmosphere such as air or O2, but when it is used in H2, its appearance is restored by H2 and has no influence in a short time. If it is exposed to H2 for a long time, it will accelerate recovery and it will The wire grows and breaks when the wire grows. In a regenerative atmosphere such as CO or coal gas, the deterioration will be significantly accelerated and it will be worse. With regard to armored thermocouples, the atomic radius of hydrogen is so small that it easily enters the interior through the jacket. It also accelerates deterioration, resulting in a significant decrease in the thermoelectric value.

4) Effect of insulation resistance

Thermocouple insulation, at high temperatures, the insulation resistance decreases rapidly with increasing temperature, therefore, there will be a leakage current, the current through the insulation resistance once fell into the insulation, so that the instrument indicates instability or measurement Inaccuracies may also occur as a result of the chaos of the recorder.

Stainless Steel Expanded Metal Mesh

Material: Aluminum plate, Thin low carbon steel plate, Stainless steel boards, AL-Mg alloy plate, Copper plate, Nickel plate.

Weaving Type: It is punched to mesh, according to the hole shape, it is divided into diamond, square, round, triangle, scale hole.

Features: skid resistance, wear resistance, high intensity, heavy load, high temperature resistance, strong and wear-resisting, artistic and tasteful etc.

Applications: Widely used in the construction of highway, railways, civil architecture, hydraulic engineering, and various machinery, electrical appliances, window protection and aquaculture, etc.

Middle expanded metal

Heavy expanded metal

Mild Steel Expanded Metal Mesh,Expanded Mesh Sheet,Stainless Steel Expanded Metal Mesh,Diamond Expansion Metal Mesh

Balanat Import&Export Co.,Ltd. , https://www.balanatwiremesh.com