As one of the commonly used temperature sensors, the thermocouple (including K index, index, T index, etc.) temperature measuring element, its long-term stability is a very important technical index, especially for coal-fired power plants. This is especially true for companies that are more difficult to repair. In layman's terms: the so-called long-term stability of the product, it reflects how long it can be used after it is installed on the device without changing its accuracy. For users, the longer the long-term stability it is good.
Therefore, how to make the thermocouple temperature measurement element have a longer service life is an important issue that every thermocouple manufacturer must pay attention to and solve well; and determine the long-term stability of the thermocouple temperature measurement element. The most important factor, we believe, is the selection of materials and the formulation and implementation of processes in turn.
What is thermocouple stability? How to check the stability of thermocouples?
Stability is an important indicator to measure the performance of thermocouples. This article introduces the concept of thermocouple stability, analyzes the main causes that affect the stability of thermocouples, and shares methods and specific requirements for testing thermocouple stability.
What is thermocouple stability?
Thermocouple stability refers to the degree to which the thermoelectric characteristics of the thermocouple change with the use time and use conditions under a certain temperature.
Under specified conditions, a large change in thermoelectric characteristics indicates poor stability, and a small change indicates good stability. The stability of thermocouple will directly affect the accuracy of thermocouple measurement. Therefore, stability is an important indicator to measure the performance of thermocouple.
Main factors affecting the stability of thermocouples
1. In a high temperature environment, various harmful media will contaminate and corrode the hot electrode;
2. The hot electrode will be oxidized and volatilized when used under high temperature conditions;
3. Under long-term high temperature conditions, the hot electrode may recrystallize, which will cause the hot electrode to become brittle;
4. In the case of neutron radiation, some thermoelectrodes will cause composition changes;
5. The thermocouple deformed by external force will produce stress and change the thermoelectric characteristics.
The verification regulations stipulate that newly produced standard thermocouples shall undergo stability testing.
The stability test of the thermocouple is: put the thermocouple in the annealing furnace, anneal twice at the specified temperature, take it out after the first annealing, measure its thermoelectric potential at the specified temperature, and then put it back Put it into the annealing furnace and anneal it for the second time according to the above method. After taking it out, measure its thermoelectric potential at the same specified temperature point to E end. The thermoelectric potential difference measured after two annealing (E start-E end) is the stability of the thermocouple, which should meet the requirements of "Standard Thermocouple Wire Stability Tolerance Table".
The main source of thermocouple instability
Contamination will affect the Seebeck coefficient of the thermocouple. Thermocouple wire materials are often contaminated by environmental atmosphere or protective tube impurities, and the additional potential generated by different degrees of contamination is also different. This additional potential will change the original indexing characteristics, which is a factor that causes the thermocouple to display unstable values. . For example, platinum rhodium 10-platinum thermocouple, when the ceramic tube used contains iron impurities, the platinum rhodium wire will affect its thermoelectric characteristics after being contaminated by iron; when used in a high-temperature reducing atmosphere containing silicon, due to silicon It is reduced to free silicon and combined with platinum rhodium wire to synthesize platinum silicon compound, making the couple wire brittle. The insulating porcelain tubes used in the verification of standard thermocouples are required to be cleaned with aqua regia, baked at high temperature, and specify the perforation polarity of the positive and negative electrodes. If the positive and negative electrodes of the thermoelectrode are misplaced in a common tube, the platinum in the platinum rhodium hole will penetrate into the platinum electrode and change the thermoelectric characteristics of the standard thermocouple. All the above conditions will affect the stability of the thermocouple.
2. The hot electrode volatilizes at high temperature
The wire materials of thermocouples are mostly alloy materials. Because the vapor pressure of each component material is different, the degree of volatilization is also different. After a certain period of use at high temperature, the alloy composition ratio will change, which will cause thermoelectric potential. obvious change.
The instability of many thermocouples is caused by the oxidation of the couple wire. Thermocouples such as copper-constantan, iron-constantan, and nickel-chromium-nickel silicon can all undergo oxidation reactions. If the thermoelectrode is uniformly oxidized, the effect may be smaller; if it is preferentially oxidized, the effect is very serious. In low oxygen partial pressure (that is, in the case of hypoxia), the chromium in the nickel-chromium electrode will produce preferential oxidation and change the composition of the couple wire.
Embrittlement is the most common factor in the rejection of thermocouples. The hot electrode of the thermocouple is the cause of embrittlement of the hot electrode due to factors such as contamination, grain growth, oxidation-reduction reaction and long-term recrystallization at high temperature. Thermoelectrodes are used in atomic reactors and are bombarded by neutrons. One of the elements is transformed into other elements, changing the composition of the thermoelectrode. For example, the rhodium of the armored platinum-rhodium thermocouple will be converted into palladium, and a small amount of platinum will firstly turn into gold and then into mercury, which will change the thermoelectric characteristics and reduce the thermoelectric potential. The low-cost metal thermocouples, such as iron, nickel-chromium, and nickel-aluminum (silicon), are not obvious to the neutron radiation transmutation. However, the transmutation of copper under radiation causes a great change in composition. Therefore, in the case of neutron radiation, it is more appropriate to use a nickel-chromium-nickel-aluminum thermocouple, and a nickel-chromium-silicon-nickel-silicon (N-type) thermocouple is better.
5. Under external force
The internal stress caused by severe bending or any other type of work hardening in the thermocouple wire may produce physical inhomogeneities. Careful operation can avoid most of the inhomogeneities caused by this. Proper annealing of the assembled thermocouple can also reduce this inhomogeneity to some extent.