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Based on the principle of temperature measurement using thermocouples, cold junction compensation is an essential element in thermocouple-based temperature measurement; however, it is often overlooked in practical applications. Ensuring the accuracy of cold junction compensation is a crucial factor in maintaining the precision of thermocouple temperature measurements. Furthermore, in applications requiring rapid temperature measurement, we often pay close attention to the sampling period of secondary instruments such as data collectors, but overlook the response time of the thermocouples used alongside them; as a result, inappropriate selection of thermocouples can affect the actual temperature measurement accuracy. Next, we will provide a brief explanation regarding cold-end compensation and thermocouple response time. Cold-end compensation Cold-end compensation is mainly divided into internal compensation and external compensation. The internal compensation schematic is shown in Figure 1. Figure 1 Principle of internal compensation Internal compensation refers to the provision of cold-end compensation by secondary instruments such as recorders. To ensure the accuracy of this internal cold-end compensation, in addition to using high-precision temperature sensors, it is also very important to achieve equal temperature at the terminals. The Yokogawa recorder ensures terminal isothermality through the optimal configuration of temperature sensors (transistors) and the manufacturing process using metal-core printed circuits (see Figure 2). Nevertheless, when there are significant changes in the ambient temperature of the environment in which the terminal (measuring instrument) is placed, it has a substantial impact on the temperature readings obtained by the thermocouple; therefore, it is crucial to maintain stability in the environment surrounding the measuring instrument. Figure 2: Metal core printed circuit board manufacturing process. The principle of external cold-end compensation is shown in Figure 3. An external electronic ice point compensator (ZERO-CON) can be used to further improve the temperature measurement accuracy. Note that the wiring between the external compensator and the measuring instrument is ordinary wire rather than thermocouple wire. Figure 3 Principle of external compensation Thermocouple response time The thermocouple response time is primarily determined by the diameter of the thermocouple wire and the type of processing at the front end of the thermocouple. The front-end types of thermocouples are shown in Figure 4, mainly including exposed type, grounded type, and insulated type. Figure 4 Comparison of thermocouple front-end types. It can be seen that the more commonly used insulated thermocouples have a slower response time compared to the other two types, but they offer advantages in terms of noise resistance. The quantitative relationship between the front-end processing type and wire diameter of thermocouples and their response time is described in specific terms by different manufacturers of temperature sensors. For example, Figure 5 shows the relationship between wire diameter and response time for American OMEGA thermocouples (grounded/ exposed type) under specific test conditions. If an insulated thermocouple is selected, the response time constant should be multiplied by 1.5. Response time is defined as the time required to reach 63.2% of the instantaneous temperature change. The test conditions were room temperature and atmospheric pressure with an air flow velocity of 20 m per second. In high-speed temperature measurement applications, the sampling time of the measuring instrument and the response time of the thermocouple must be matched. Figure 5 Relationship between OMEGA thermocouple wire diameter and response time