【Weekly 3 Questions, Question 14】2017.09.09
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The last edit to this post was made by xh8618 on 2017-9-9 at 13:10; by replying with bg9.png, you can see the answers to the previous question (a multiple-choice question): How to operate the three-valve instrument cluster? The sequence of opening is: open the positive pressure valve, close the balance valve, and open the negative pressure valve. The sequence for shutting down is: close the negative pressure valve, open the balance valve, and close the positive pressure valve. The topic for this issue is as follows (essay question): Why is cold-junction compensation necessary when using thermocouples for temperature measurement? The answer will be revealed in the next issue; by answering the question in that issue, you will be able to see the answer. Thank you! —————————————————————————————————————————————————————————————— Scoring criteria: 2 Wealth points for incorrect attempts; 3–6 points for *incomplete* answers; 8 Wealth points for correct answers with detailed explanations; 10–15 Wealth points for correct answers with additional *insufficient* explanations. Please score according to these criteria. Note: If there are editing records after answering, only those who participated will be credited points. For continuous malicious spamming, 2 points will be deducted each time, with no upper limit. Pure numbers, pure emojis, or random letters unrelated to the answer are all considered malicious spamming; points will start to be deducted after three warnings
The electromotive force of a thermocouple element depends on the temperature difference between the hot end (the measurement point) and the cold end (the detection circuit), that is, the relative temperature between them, rather than the absolute temperature of the measurement point; therefore, cold-junction compensation is necessary.
Cold-junction temperature compensation is necessary when using thermocouples for temperature measurement, as the electromotive force of a thermocouple is a function of the difference between the temperature at its hot junction and that at its cold junction. To ensure that the output electromotive force is a one-to-one function of the temperature being measured, it is essential to keep the cold-junction temperature constant； The thermoelectric potential given in the thermocouple calibration table is based on a cold junction temperature of 0°C; otherwise, errors will occur. Therefore, various measures are often employed to eliminate the effects of variations in cold forging temperature, such as the constant cold-end temperature method, cold-end temperature correction method, compensation wire method, and compensation bridge method. 1. Cold-end constant temperature method Generally, when calibrating thermocouples, the cold-end temperature is set at 0°C as a standard. Therefore, the cold end is often placed in an ice-water mixture to maintain its temperature at a constant 0°C. Under laboratory conditions, the cold end is usually placed in a test tube filled with insulating oil, and then that test tube is placed inside a thermal container filled with an ice-water mixture, so as to keep the cold end at 0°C. 2. Cold junction temperature correction method Since the temperature calibration table of thermocouples is established with the cold junction temperature at 0°C, and the measurement circuits or display instruments used in conjunction with them are calibrated based on this relationship curve, it is necessary to adjust the readings indicated by the instruments when the cold junction temperature is not 0°C. If the cold-end temperature is higher than 0°C but remains constant at t0°C, the measured thermoelectric potential will be lower than the calibration value of that thermocouple. To determine the actual temperature, the intermediate temperature method can be used, that is, correction is applied using the following formula: E(t,0) = E(t,t1) + E(t1,0). 3. Compensation wire method: In order to keep the cold-end temperature of the thermocouple constant (preferably at 0°C), the thermocouple can be made quite long so that the cold end is far away from the working end; it can then be placed together with the measuring instrument in an environment with a constant temperature or minimal temperature fluctuations. However, this method makes installation and use inconvenient, and it may consume a lot of valuable metal materials. Therefore, a connecting wire called a compensation wire is generally used to extend the cold end of the thermocouple. This type of wire possesses the same thermoelectric properties as the thermocouple it is connected to within a certain temperature range (0–150°C). If the thermocouple is made of an inexpensive metal, its own material can be used as a compensation wire to extend the cold junction to a location with a constant temperature.
The magnitude of the thermocouple potential is related to the temperatures at its two ends, and its temperature-thermocouple potential 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, it is not possible to keep the temperature at the cold end at 0°C or fixed at any specific value. The potential of a thermocouple depends both on the temperature at its hot end and on the temperature at its cold end. Therefore, if the temperature at the cold end varies freely, it will inevitably cause measurement errors. To eliminate this error, cold-end temperature compensation is necessary.
It is determined by the principle of temperature measurement: different temperature differences result in different thermoelectromotive forces. Additionally, variations in the position of the instrument and the temperature of the cold end of the thermocouple require cold-end compensation.
Since the magnitude of the thermoelectric potential of a thermocouple is the function of the difference between the temperature at the hot end and that at the cold end, in order to ensure that the output thermoelectric potential is a single-valued function of the temperature being measured, it is necessary to keep the temperature at the cold end constant； The thermoelectric potential given in the thermocouple calibration table is based on a cold junction temperature of 0°C; otherwise, errors will occur. Therefore, various measures are often employed to eliminate the effects of variations in cold forging temperature, such as the constant cold-end temperature method, cold-end temperature correction method, compensation wire method, and compensation bridge
Cold-junction temperature compensation is necessary when using thermocouples for temperature measurement, as the electromotive force of a thermocouple is a function of the difference between the temperature at its hot junction and that at its cold junction. To ensure that the output electromotive force is a one-to-one function of the temperature being measured, it is essential to keep the cold-junction temperature constant； The thermoelectric potential given in the thermocouple calibration table is based on a cold junction temperature of 0°C; otherwise, errors will occur. Therefore, various measures are often employed to eliminate the effects of variations in cold forging temperature, such as the constant cold-end temperature method, cold-end temperature correction method, compensation wire method, and compensation bridge method.
The thermoelectric potential t generated by a thermocouple is related to the temperature of its cold junction. Under normal circumstances, the cold junction temperature of a thermocouple is not constant; it changes with room temperature. As a result, t also changes with room temperature, which leads to measurement errors
Since the temperature at the cold end of the thermocouple is not constant, what the thermocouple actually measures is the temperature difference between the hot end and the cold end.
Cold-junction temperature compensation is necessary when using thermocouples for temperature measurement, as the electromotive force of a thermocouple is a function of the difference between the temperature at its hot junction and that at its cold junction. To ensure that the output electromotive force is a one-to-one function of the temperature being measured, it is essential to keep the cold-junction temperature constant； The thermoelectric potential given in the thermocouple calibration table is based on a cold junction temperature of 0°C; otherwise, errors will occur. Therefore, various measures are often employed to eliminate the effects of variations in cold forging temperature, such as the constant cold-end temperature method, cold-end temperature correction method, compensation wire method, and compensation bridge method.