An expert from Yokogawa University gave a lecture on July 7: Practical principles and techniques for temperature measurement using Yokogawa recorders!

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　　An expert from Yokogawa University gave a lecture on July 7: Practical principles and techniques for temperature measurement using Yokogawa recorders! Recorders are widely used in industrial settings and scientific research, and temperature testing is one of the important requirements, with a variety of application scenarios. On July 7, 2020, Yokogawa University officially launched its broadcasts; Dr. Feng Yuelu, an expert in recorder testing, shared his years of experience in field testing online, and the response from viewers was very positive! For details on the training offered by Yokogawa University and more video materials, please contact Yokogawa Testing & Measurement. 　　 　　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 in conjunction with 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 boards (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 considerable 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 its tip. 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 have an advantage in terms of interference 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 rate 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. Interference issues in thermocouple temperature measurement. When using thermocouples for temperature measurement, electrical interference is a common problem that arises. On the one hand, in recent years, due to the widespread use of variable-frequency power supplies across various industries, testing environments have become increasingly demanding. On the other hand, the thermoelectric potential generated by temperature changes in thermocouples is very small; therefore, they are more susceptible to electrical interference compared to other sensors such as thermal resistors. 　　Figure 6 shows the equivalent circuit diagram of a common frequency converter; it can be seen that there are three types of interference sources present: the 50Hz commercial power supply, the high-frequency pulse interference generated by pulse width modulation (PWM) (with a high power density), and low-frequency frequency conversion interference. 　　 　　Figure 6: Equivalent circuit diagram of the inverter. The interference suppression functions provided by Yokogawa’s measuring instruments include: a. Integral A/D, b. First-order low-pass filtering, c. Channel-independent A/D modules. As the name implies, integral A/D involves performing an integration operation on the analog signal over a specific period of time before carrying out the A/D conversion. Appropriate integration time (measurement period) can be set for different power frequency (50Hz/60Hz) interference sources to achieve suppression effects. The schematic diagram of interference suppression using integral A/D is shown in Figure 7. Figure 7: Schematic diagram of interference suppression using integral A/D. When the frequency of the interference signal does not match the A/D integration period, the impact of the interference cannot be suppressed solely through integration. Therefore, the Yokogawa high-speed module provides filtering functionality; by adjusting the cutoff frequency of the first-order low-pass filter and combining it with A/D integration, it is possible to suppress the influence of high-frequency noise sources. However, filtering introduces a certain delay in the measurement results, which is why it is commonly used in steady-state temperature testing. The schematic diagram showing the suppression of interference through A/D integration combined with a first-order low-pass filter is shown in Figure 8. 　　 　　Figure 8 Schematic diagram of an integrated A/D converter combined with a first-order low-pass filter. Most data acquisition instruments available on the market today are of the scanning type, meaning that multiple channels share one A/D converter, with switching occurring in a time-division manner via relays. The greatest advantage of scanning type is cost savings, while the disadvantages are limited data acquisition speed and interference-prone channel switching. Yokogawa has introduced channel-independent A/D modules (as shown in Figure 9), with each measurement channel using its own A/D converter, which improves noise resistance and significantly enhances common-mode voltage tolerance. 　　 　　Figure 9 Schematic diagram of the independent A/D module circuit. In addition to the methods for suppressing interference provided by the recorder, users can employ various approaches during actual testing to reduce the impact of interference. For example, insulating tape can be used to isolate the object under test from the front end of the thermocouple ； Remove the potential difference generated by earth currents by equipotentializing the signal source and the recorder ； Additionally, using twisted shielded thermocouple wires can suppress electrostatic coupling interference during thermocouple wiring. 　　About Yokogawa University Yokogawa is renowned worldwide for its products and quality, and has also earned trust through its excellent service. Building on a century of technical expertise and combining 20 years of hands-on experience, Yokogawa University presents classic cases and demonstrates detailed procedures to offer you one-stop professional training in testing. You can choose different forms of participation in Yokohama National University’s learning programs: classroom training, on-site training, digital training, or online training, to meet your needs for learning anytime, anywhere. A systematic curriculum helps you gain a deeper understanding of the principles and techniques behind various types of tests. We are committed to growing together with our clients, making Yokogawa University a cradle for the next generation of experts in general testing and optical communication testing!