Armored thermocouples are one of the commonly used temperature instruments. They are characterized by fast response speed, corrosion resistance, and good stability, making them one of the frequently used temperature instruments. The manufacturing of armored thermocouple cables and armored thermocouples has always been valued by countries around the world. The United States, the United Kingdom, Germany, Japan, China, and other countries have formulated their own standard systems for armored thermocouple cables and armored thermocouples. ### Differences between Armored Thermocouples and Non-Armored Thermocouples 1. Their structures are different. Compared with non-armored cables, armored cables have an additional layer of metal reinforcement on the surface. 2. Their application scopes are different. Due to the reinforcement layer on the surface, armored cables are suitable for being buried underground, while non-armored cables, without such a reinforcement layer, generally cannot be used for underground burial. However, non-armored cables are commonly used in other situations. 3. Their usages are different. Armored cables are used when buried underground and are required for direct burial and laying. In contrast, non-armored cables cannot be used for underground burial and are used in all other scenarios. ### What is the Material of Armored Thermocouples? An armored thermocouple is a combined thermocouple formed by assembling a thermoelement, insulating material, and metal sleeve, which is then drawn into shape. An armored cable consists of conductors of different materials enclosed in a metal sleeve with insulating materials inside, processed into a flexible and solid assembly. Armored cables include armored thermocouples, armored thermal resistors, armored heaters, and armored lead wires. They are mainly used for temperature measurement, signal transmission, and special heating in fields such as chemical industry, metallurgy, machinery manufacturing, power generation, and scientific experiments, with armored thermocouples being the most widely used. ### Calibration Methods for Armored Thermocouples #### JJG 351-1996 Measurement Method (No Jacked Temperature-Uniforming Block in the Tube Furnace) A first-class standard platinum-rhodium 10-platinum thermocouple is covered with a high-aluminum protective tube and bundled with the tested armored K-type base metal thermocouple into a circular bundle using fine nickel-chromium wires. The measuring ends of the tested thermocouples are evenly distributed around the measuring end of the standard thermocouple, ensuring they are on the same vertical cross-section as the standard thermocouple. Then, the bundled thermocouples are inserted into the middle of the verification furnace to a depth of approximately 300 mm. The reference end of the standard thermocouple is connected to a copper wire and placed in an ice bath, and the other end of the copper wire is connected to a switch. The compensating wire of the tested armored thermocouple is connected to a copper wire, placed in the ice bath, and the other end of the copper wire is also connected to the switch. Then, the automatic testing software is started, and measurements are taken when the temperature meets the requirements. Each thermocouple is read 4 times, and the average of the 4 readings is taken. After each independent experiment, the verification furnace is cooled to room temperature, all thermocouples are taken out, re-bundled, and the experiment is repeated. The experiment tests two temperature points: 400°C and 1000°C, with results shown in Tables 1 and 2. #### JJF 1262-2010 (Jacked Temperature-Uniforming Block in the Tube Furnace) The experiment is conducted under the same laboratory environmental conditions as Method 1, using the same set of experimental equipment. The difference is that a jacked temperature-uniforming block is placed in the thermocouple verification furnace, with the geometric center of the temperature-uniforming block positioned at the center of the uniform temperature field of the verification furnace. Then, the standard thermocouple, also covered with a high-aluminum protective tube, and the tested armored thermocouple are inserted into the temperature-uniforming block until reaching the bottom of the jacks, ensuring they do not pop out. Other measurement methods are the same as in 1.2.1. The experiment tests two temperature points: 400°C and 1000°C, with results shown in Tables 3 and 4. Which calibration method is more accurate? Through comparative experiments, the jacked temperature-uniforming block significantly improves the measurement results of armored base metal thermocouples, with better reproducibility and smaller uncertainty. It can be seen that calibrating armored base metal thermocouples in accordance with JJF 1262-2010, with a jacked temperature-uniforming block installed in the tube furnace, can provide a constant and uniform temperature field with small temperature fluctuations and stable temperature distribution, resulting in more accurate and reliable experimental results. Sources: Shanghai Metrology and Testing, Zhujiang Cable