Basic knowledge of temperature measuring instruments

Thread Content

​ 1. How does the reading of the display instrument change when a thermocouple or compensation wire is short-circuited? (Synthesis, urea, water vapor, aromatics) Shows the temperature value approximately at the short-circuit location. 2. What are the common temperature scales? The commonly used temperature scales are the Celsius scale (℃), the Fahrenheit scale (°F), and the Kelvin scale (K). 3. What are the conditions for a thermocouple to generate a thermoelectric potential? The two thermoelectric electrode materials are different, and the temperatures at the two junctions are different. 4. Why is cold-junction temperature compensation necessary for thermocouples? The magnitude of the thermocouple’s electromotive force is related to the temperatures at its two ends, and its temperature-electromotive force curve is calibrated when the temperature at the cold end is 0°C. In practical applications, since the cold end of the thermocouple is exposed to the environment and thus affected by the surrounding temperature, its temperature cannot be kept at 0°C or fixed at a constant value. The potential of the thermocouple depends both on the temperature of its hot end and that of its cold end. To eliminate this error, cold-end temperature compensation must be performed. 5. If the compensation wire of a temperature measuring instrument is connected in the reverse polarity to the thermocouple, and also connected in the reverse manner to the instrument’s input terminal, will this cause additional measurement errors? What is the approximate additional error? It can cause additional measurement errors. The error value is related to the temperatures at both ends of the compensation wire. If the temperature is zero, the instrument’s reading has no additional error. If the cold junction temperature of the thermocouple is higher than the temperature at the instrument’s input, the reading on the instrument will be lower than the actual value by twice the temperature difference across the compensation wire. For example, if the actual temperature is 100°C, the cold junction temperature is 25°C, and the temperature at the instrument’s input is 15°C, then the instrument’s reading will be around 80°C. If the cold junction temperature of the thermocouple is lower than the temperature at the instrument’s input, the reading on the instrument will be higher than the actual value by twice the temperature difference across the compensation wire. For example, if the actual temperature is 100°C, the cold junction temperature is 15°C, and the temperature at the instrument’s input is 25°C, then the instrument’s reading will be around 120°C. 6. What determines the thermoelectric properties of a thermocouple? The thermoelectric properties of a thermocouple are determined by the chemical composition and physical properties of the electrode materials. The magnitude of the thermoelectromotive force depends on the materials that make up the thermocouple as well as the temperatures at its ends, and it is not affected by the thickness or length of the thermocouple wire. 7. How to simply determine the polarity of a thermocouple (taking type J as an example)? 1) Visually observe that the end with rust is the positive pole, indicating it is iron. 2) If it can be attracted by a magnet, then it is iron and serves as the positive pole; the other end is the negative pole.