Master 5 gauge-related knowledge points per week – go from a beginner to an expert

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1. Some people believe that since the compensation wire is an extension of the thermocouple, the thermocouple potential depends only on the hot and cold ends, and not on the temperature at the point where the compensation wire connects to the thermocouple. Is this statement correct? This statement is incorrect. Generally, the performance of a compensation wire is the same as that of a thermocouple within a certain temperature range (0–100°C); beyond this range, an additional potential is generated at the junction between the compensation wire and the thermocouple. Therefore, it cannot be said generally that the thermocouple potential is independent of the temperature at the junction between the compensation wire and the thermocouple. 2. In DDZ-Ⅲ type instruments, why is a 4–20mA current signal used for communication between the field and the control room, while a 1–5V voltage signal is used within the control room? Answer: 4–20 mA current signals are suitable for long-distance transmission between the field and the control room, as they help to avoid errors caused by voltage drops along the transmission wires. 1–5 V voltage signals are appropriate for short-distance communication within the control room; instruments can be connected in parallel, which facilitates design, installation, and maintenance. 3. What impact does incorrect polarity of the compensation wire have on temperature measurement? How can one determine the scale number and polarity of a compensation wire when they are unclear? Answer: Since the thermoelectric properties of the compensation wire are similar to those of the thermocouple within a certain range, incorrect polarity connection will increase the cold-junction error of the thermocouple. If the scale and polarity of the compensation wire are unknown, twist the ends of the two wires together and submerge them in boiling water; connect the other end to the instrument that displays this scale. A reading of around 1000°C should be indicated, which shows that the scale values are correct. If the reading differs significantly, then the scale values are incorrect. When using the MV function of a multimeter for measurement, a “+” reading indicates that the red test lead is connected to the positive pole of the compensation wire; a “–” reading indicates that the red test lead is connected to the negative pole of the compensation wire. 4. In a thermocouple temperature measurement system, the operator reported that the readings on the measuring instrument were unstable – they appeared intermittently and varied in value. It was confirmed that there was nothing wrong with the measuring instrument itself; please analyze the cause of this issue. Answer: There may be several reasons as follows: ① Poor contact between the thermocouple electrodes and the terminal connections; ② Intermittent short circuits or intermittent grounding of the thermocouple; ③ The thermocouple is broken or partially broken, resulting in intermittent connections; ④ The thermocouple is not securely installed and thus moves around; ⑤ The compensation wire is grounded, has intermittent short circuits, or is open-circuited. 5. If the polarity of the compensation wire of a temperature measuring instrument is reversed relative to that of the thermocouple, and it is also connected reversely to the instrument’s input terminal, will this cause additional measurement errors? What is the approximate additional measurement error? Answer: It can cause additional measurement errors. The error value is related to the temperature difference across the two ends of the compensation wire. If the temperature difference is zero, the instrument’s reading has no additional error. If the temperature at the cold end of the thermocouple is higher than the temperature at the instrument’s input, the instrument’s reading will be two times lower than the actual value, by an amount equal to the difference in temperature. If the cold junction temperature of the thermocouple is lower than the temperature at the instrument’s input, the instrument will display a value that is twice the difference between these two temperatures.