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The thermoelectric characteristics of N-type thermocouples are better than that of K-type thermocouples
2012-3-14 8:39:47


相比有哪些优缺点? What are the advantages and disadvantages of N- type thermocouples compared to K-type thermocouples ? Advantages of N-type thermocouple:-High temperature oxidation resistance, long-term stability. The K-type thermocouple nickel-chromium positive electrode is preferentially oxidized to cause non-uniform alloy composition and thermal electromotive force drift. The N-type thermocouple increases the content of Cr and Si, which changes the oxidation mode of the nickel-chromium alloy from internal oxidation to external Oxidation, which causes the oxidation reaction to proceed only on the surface;-Good low-temperature and short-term thermal cycling stability, and suppressed magnetic transition;-Strong resistance to nuclear radiation. The N-type thermocouple eliminates the easily-degradable elements Mn and Co in the K-type and further strengthens the resistance to neutron radiation.-In the range of 400 ~ 1300 ° C, the thermoelectric characteristics of the N-type thermocouple are better than the K-type. Disadvantages of N-type thermocouples:-N-type thermocouples are harder than K-type materials and more difficult to process;-Relatively expensive. The thermal expansion coefficient of N-type thermocouple is 15% lower than that of stainless steel. Therefore, the outer tube of N-type armored thermocouple should use NiCrSi / NiSi alloy; the non-linear error is large in the range of -200 ~ 400 ℃.


Advantages of N-type thermocouple:
1. High temperature oxidation resistance and long-term stability. 镍铬的正极中Cr、Si元素择优氧化引起合金成分不均匀及热电动势漂移等,在N型热电偶增加Cr、Si含量,使镍铬合金的氧化模式由内氧化转变为外氧化,致使氧化反应仅在表面进行.-低温短期热循环稳定性好,且抑制了磁性转变; 2、耐核辐射能力强。 The K-type thermocouple nickel-chromium positive electrode is preferentially oxidized to cause non-uniform alloy composition and thermal electromotive force drift. The N-type thermocouple increases the content of Cr and Si, which changes the oxidation mode of the nickel-chromium alloy from internal oxidation to external Oxidation causes the oxidation reaction to proceed only on the surface.-Low-temperature short-term thermal cycling stability is good, and magnetic transition is suppressed; 2. Strong resistance to nuclear radiation. The N-type thermocouple eliminates the easily-degradable elements Mn and Co in the K-type to further enhance the resistance to neutron radiation. 3. In the range of 400 ~ 1300 ° C, the thermoelectric characteristics of the N-type thermocouple are better than the K-type. .
Disadvantages of N-type thermocouple:
硬,较难加工;-价格相对较贵。 1. The material of N-type thermocouple is harder than K-type thermocouple , which is more difficult to process;-the price is relatively expensive. 2. The thermal expansion coefficient of the N-type thermocouple is 15% lower than that of stainless steel. Therefore, the outer sleeve of the N-type armored thermocouple should be made of NiCrSi / NiSi alloy; 3. The non-linear error is large in the range of -200 ~ 400 ℃.

How to choose the right thermal mounting sleeve?
The shape of the thermal installation sleeve is mainly determined by the temperature, pressure, density and flow rate of the medium and the required insertion length. ASME / ANSI PTC 19.3 fully stipulates this, and the casing strength analysis software can be used to calculate whether the casing design meets the process requirements. The thermowell installed on the site needs to calculate the strength of the thermowell. The strength of the protective sleeve is mainly affected by the following three points:
1. Vibration caused by flow; the liquid passing through the sheath tube generates a vortex with a certain frequency, which is called the vortex frequency, and the frequency is proportional to the flow velocity. If this frequency is close to or consistent with the natural frequency of the thermowell, resonance will occur, which will absorb a large amount of thermal energy, which will generate high stress and may damage the thermowell and the sensor in the bush. ASME technical standards require that the ratio of the vortex frequency to the natural frequency of the thermowell should be less than 0.8.
2. Stress caused by flow; fluid flow changes with flow velocity and density, and a force is applied to the thermowell. The pressure caused by this flow can be calculated.
3. Process pressure; the maximum static pressure that the thermowell can withstand can be calculated. "
There are three types of connection methods for general thermal installation sleeves: threaded, flanged and welded.