Correct selection of thermocouples and reasons for measurement errors

Thermocouples, as one of the important sensors in temperature measurement in the field of industrial automation control, have made great contributions to temperature measurement in various industries. It is widely used in the field of automated temperature control in various industries due to its simple structure, accurate temperature measurement, wide measurement range, and easy maintenance. However, it is precisely because the thermocouple itself has a simple structure and is easy to install and use that the selection of key parameters such as indexing number and insertion depth is often overlooked during the selection process, resulting in measurement errors and even damage to the thermocouple during use. In addition, measurement errors caused by a series of issues such as temperature compensation, thermal response time, thermal impedance, and thermocouple degradation during the measurement process have also brought considerable trouble and even serious economic losses to actual users. Prior to the increasing emphasis on instrument safety, measurement accuracy, service life, and stability in modern industrial automation control, a large number of researchers and thermal instrumentation engineers have conducted extensive research on the temperature measurement accuracy, adaptability, and temperature measurement structure of thermocouples. On this basis Starting from the selection and practical application of thermocouples, this article analyzes and elaborates on how to choose the adaptability of different scale marks and temperature measurement ranges of thermocouples, the selection of temperature measurement point positions, and how to correctly select the insertion depth of thermocouples. At the same time, an analysis was conducted on the measurement errors caused by temperature compensation, thermal response time, thermal impedance, and thermocouple degradation during the use of thermocouples, and solutions were provided. I hope it can be helpful for the correct temperature measurement and long-term temperature measurement of thermocouples.
1. Selection of Thermocouples
Thermocouples are widely used due to their simple operation, but in the initial selection process, more attention is often paid to parameters related to thermocouple installation and safety, such as the installation interface and the pressure resistance level of the sleeve material. However, the adaptability, temperature measurement range, and insertion depth of thermocouples can easily be ignored, which affects the temperature measurement results.
1.1 Selection of thermocouple index number
The selection of thermocouple indexing number is usually determined based on the working temperature of the measured medium. Generally, according to the temperature measurement range of different indexing numbers of thermocouples and actual usage, we roughly divide thermocouples into three categories: medium low temperature, medium high temperature, and high temperature indexing thermocouples. Below is an analysis and discussion on how to choose thermocouples for these three temperature measurement ranges.
1.1.1 Selection of Medium and Low Temperature Thermocouples
If used for low-temperature measurement (-200 ℃~+350 ℃), a T-scale thermocouple can be selected. For medium and low-temperature measurement (-40 ℃~+700 ℃), an EJ scale thermocouple can be selected. However, due to the fact that the positive electrodes of the TJ scale thermocouple are made of pure copper and pure iron materials, they are easily oxidized during the production and use of the product, so they cannot be used for temperature measurement in oxidizing environments. It is recommended to use an E-scale thermocouple for temperature measurement in an oxidizing environment. The E-index thermocouple not only covers the temperature measurement range of the J-index thermocouple, but also has a large thermoelectric potential output, high sensitivity, good stability, and better oxidation resistance than the TJ index thermocouple. It can be used in oxidizing and inert gas environments, but cannot be directly used in sulfur and reducing gas environments at high temperatures. This will result in significant errors in the thermoelectric potential uniformity of the E-index thermocouple.
1.1.2 Selection of medium and high temperature thermocouples
K. N-degree thermocouple is a widely used low-cost metal thermocouple in the current market, mainly used for temperature measurement in medium and high temperature zones (-40 ℃~+1200 ℃). Among them, K-index thermocouples occupy an absolute market share due to their good linearity, large thermoelectric potential, high sensitivity, good stability and uniformity, strong oxidation resistance, and low price. The N-index thermocouple is a latest internationally standardized thermocouple that successfully overcomes two important drawbacks of the K-index thermocouple, namely the unstable thermoelectric potential caused by the short-range ordered lattice of nickel chromium alloy between 300 ℃ and 500 ℃; The thermal electric potential instability caused by preferential oxidation of nickel chromium alloy at around 800 ℃. Its comprehensive performance is superior to that of K-index thermocouples, making it a promising thermocouple for development. However, a major drawback of N-degree thermocouples is the large nonlinear error in the low temperature range (-200 ℃~400 ℃) and the hard and difficult to machine material.
1.1.3 Selection of High Temperature Thermocouples
S R. The B-grade thermocouple is called a precious metal thermocouple because its positive and negative electrodes are made of platinum and platinum rhodium alloy. It is a thermocouple suitable for measuring in high-temperature areas (800 ℃~1600 ℃). The recommended long-term use temperature for S and R scale thermocouples is (800 ℃~1300 ℃), and the recommended long-term use temperature for B scale thermocouples is (800 ℃~1600 ℃). Due to their high temperature measurement accuracy, good stability, wide temperature measurement range, and high temperature upper limit, these precious metal thermocouples with various graduations are widely used in high-temperature measurement applications above 1000 ℃, such as metallurgy, glass, ceramics, and other industries. The shortcomings include low thermoelectric potential, low sensitivity, decreased mechanical strength at high temperatures, high sensitivity to pollution, and expensive precious metal materials, resulting in a large one-time investment. But overall, precious metal thermocouples are still the main force in temperature measurement in high-temperature areas. With the continuous improvement of thermocouple production technology, they can effectively protect electrode wires from contamination at high temperatures and improve the service life of products.