[Daily question 20090330] Popularize instrument knowledge

1. The thermocouple output is unstable. Please give at least three possible reasons. aThermal electrode deteriorates ； b Improper installation position of thermocouple ； c There is dust on the surface of the protective tube. 2. Why is cold junction compensation needed when measuring thermocouple temperature? Since the graduation tables of various thermocouples are made when the reference end temperature is 0 degrees Celsius, in actual use, the reference end temperature of the thermocouple is often not 0 degrees Celsius and may be unstable. If no measures are taken, measurement errors will occur. 3. The thermal resistor measures temperature. When the thermal resistor is short-circuited, what will happen on the instrument display? What about when the thermal circuit is blocked? When short-circuited, a negative value is displayed ； When the circuit is open, the display instrument indicates infinity. 4. Why is differential regulation generally added to temperature regulation systems? Differential adjustment produces a control effect based on the changing trend of the deviation, which can improve the dynamic response speed of the system

1. The reasons for unstable thermocouple output may be poor terminal contact, rapid temperature changes, and unstable millivolt values ​​of some thermocouples in a certain temperature range. 2. This is determined by the temperature measurement principle of the thermocouple, because the millivolt signal it outputs is only related to the temperature difference between its two ends, so the cold end needs to be compensated. 3. The temperature is displayed to the maximum value when short-circuited and to the minimum value when open-circuited. 4. The temperature regulation loop is a large lag link, and differential action can be used to adjust in advance and partially overcome the lag.

1. The thermocouple output is unstable. Please give at least three possible reasons. answer: The wiring contact is not good, the thermocouple electrode will be broken continuously, external influence, the installation is not firm or vibration. 2. Why is cold junction compensation needed when measuring thermocouple temperature? answer: The size of the thermoelectric potential of a thermocouple is related to the temperature at both ends. Its temperature-thermoelectric potential relationship curve is graduated when the cold end temperature is 0 degrees. In practical applications, the cold end of the thermocouple is exposed in space and is affected by the ambient temperature. The cold end temperature cannot be maintained at 0 degrees, nor can it be maintained at a certain temperature. The potential of the thermocouple is determined by both the hot end temperature and the cold end temperature. So if the cold junction temperature changes freely, errors will occur. To eliminate this error, cold junction temperature compensation must be performed. 3. The thermal resistor measures temperature. When the thermal resistor is short-circuited, what will happen on the instrument display? What about when the thermal circuit is blocked? answer: Short circuit, the meter displays 0 degrees. Open circuit, the meter displays the maximum value. 4. Why is differential regulation generally added to temperature regulation systems? answer: In the temperature regulation system, the time constants of the temperature and components to be adjusted are relatively large, and differential regulation has a leading effect and can achieve better effects on the quality of regulation.

1. Reasons for unstable thermocouple output: The terminals have poor contact, the temperature changes rapidly, and some thermocouples with graduation numbers have unstable millivolt values ​​in a certain temperature range. 2. Because the millivolt signal output by the thermocouple is only related to the temperature difference between its two ends, the cold end must be compensated. 3. The temperature is displayed to the minimum value when there is a short circuit, and the temperature is displayed to the maximum value when the circuit is open. 4. The temperature regulation loop is a large lag link, and differential action can be used to adjust in advance and partially overcome the lag.

1. The thermocouple output is unstable. Please give at least three possible reasons. answer: The wiring contact is not good, the thermocouple electrode will be broken continuously, external influence, the installation is not firm or vibration. 2. Why is cold junction compensation needed when measuring thermocouple temperature? answer: The size of the thermoelectric potential of a thermocouple is related to the temperature at both ends. Its temperature-thermoelectric potential relationship curve is graduated when the cold end temperature is 0 degrees. In practical applications, the cold end of the thermocouple is exposed in space and is affected by the ambient temperature. The cold end temperature cannot be maintained at 0 degrees, nor can it be maintained at a certain temperature. The potential of the thermocouple is determined by both the hot end temperature and the cold end temperature. So if the cold junction temperature changes freely, errors will occur. To eliminate this error, cold junction temperature compensation must be performed. 3. The thermal resistor measures temperature. When the thermal resistor is short-circuited, what will happen on the instrument display? What about when the thermal circuit is blocked? answer: Short circuit, the meter displays 0 degrees. Open circuit, the meter displays the maximum value. 4. Why is differential regulation generally added to temperature regulation systems? answer: In the temperature regulation system, the time constants of the temperature and components to be adjusted are relatively large, and differential regulation has a leading effect and can achieve better effects on the quality of regulation.

1. 1) Poor contact between the thermocouple terminal and the hot electrode 2) The insulation of the thermocouple measurement line is damaged, causing continued short circuit or grounding 3) The thermocouple is not installed firmly or there is external vibration 4) The hot electrode will be broken but not broken 5) External interference (AC leakage, electromagnetic field induction, etc.) 2. After using the compensation wire, extend the cold end of the thermocouple from a place with higher temperature and instability to the operating room with lower temperature and more stability, but the cold end temperature is not 0°C. The temperature-thermoelectric potential relationship curve of various thermocouples commonly used in industry is obtained when the cold end temperature is maintained at 0°C. The instruments used with it are also calibrated based on this relationship curve. Since the temperature in the operating room is often higher than 0°C and is not constant, the thermoelectric potential generated by the thermocouple must be small at this time, and the measurement value also changes with the change of the cold end temperature, so the measurement results will cause errors. Therefore, when applying thermocouple temperature measurement, the cold end temperature must be maintained at 0°C, or certain corrections must be made to obtain accurate measurement results. 3. When the thermal resistor is short-circuited, the display instrument indicates a negative value. When the thermal circuit is blocked, the display instrument indicates infinity. 4. For systems with large lag, adding differential action can improve the quality of the regulating system. In this case, when the regulator senses the deviation, the interference has actually been acting for a period of time, so appropriately increasing the output will help overcome the impact of the interference. It is helpful to inhibit the continued growth of deviation. For the temperature regulation system, the hysteresis is large, so appropriate differential action should be added.

1. The thermocouple output is unstable. Please give at least three possible reasons. ①, the hot electrode is broken but not broken ②, the thermocouple terminal and the hot electrode are in poor contact ③, the thermocouple measurement line causes intermittent short circuit or grounding ④, external interference (electromagnetic induction, etc.) 2. Why is cold junction compensation needed when thermocouple temperature measurement? ①The size of the thermoelectric potential of a thermocouple is related to the temperature at both ends. Its temperature-thermoelectric potential relationship curve is graduated when the cold end temperature is 0°C. ②In practical applications, since the cold end of the thermocouple is exposed in space and is affected by the surrounding ambient temperature, the cold end temperature in temperature measurement cannot remain constant at 0°C, nor can it be fixed at a certain temperature. The thermocouple potential depends on both the hot end temperature and the cold end temperature. Therefore, if the cold end temperature changes freely, measurement errors will inevitably occur. In order to eliminate this error, cold junction temperature compensation must be performed. 3. The thermal resistor measures temperature. When the thermal resistor is short-circuited, what will happen on the instrument display? What about when the thermal circuit is blocked? When the thermal resistor is short-circuited, the display instrument indicates a negative value. When the thermal circuit is blocked, the display instrument indicates infinity. 4. Why is differential regulation generally added to temperature regulation systems? For systems with large hysteresis, increasing the differential effect can improve the quality of the regulating system. In this case, when the regulator senses the deviation, the interference has actually been acting for a period of time, so appropriately increasing the output will help overcome the impact of the interference. It is helpful to inhibit the continued growth of deviation. For the temperature regulation system, the hysteresis is large, so appropriate differential action should be added.