Yokogawa University: Interference issues in thermocouple temperature measurement

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　　Using thermocouples for temperature measurement often encounters the problem of electrical interference. 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 stringent. 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 1 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 1: Equivalent circuit diagram of the inverter. The interference suppression functions provided by Yokogawa’s measuring instruments include integration, first-order low-pass filtering, and channel-independent A/D modules. Integration A/D, as the name suggests, involves performing an integral 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 2. 　　　Figure 2: 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 effects of high-frequency interference 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 A/D integration combined with a first-order low-pass filter to suppress interference is shown in Figure 3. 　　 　　Figure 3 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 being done in a time-division manner using 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 4), with each measurement channel using its own A/D converter, which improves noise resistance and significantly enhances common-mode voltage tolerance. 　　 　　Figure 4 Schematic diagram of the independent A/D module circuit. In addition to the methods for suppressing interference provided by the recorder, users can also employ various approaches to reduce the impact of interference during actual testing. 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 stranded shielded thermocouple wires can suppress electrostatic coupling interference during thermocouple wiring.