The main source of thermocouple unstable factors

In industrial production, temperature sensors is the core part of the temperature measuring link, wide variety. According to the divided into two types: contact and non-contact measurement way, according to the temperature measurement and signal types are divided into resistive material and thermoelectric emfs type two kinds big, namely we say normally thermal resistance and thermocouple. Thermocouple is one of the most widely used temperature measuring element, because its price is low, the temperature range is wide, the output signal is stable, resistant to bad environment and other factors, has been widely used in industrial production. However in the process of measuring thermocouple for measuring conditions and environmental restrictions and leads to the instability of the measured results. Such as pollution, oxidation, reduction, physical condition, embrittlement, irradiation is affect the thermocouple measurements, the major source of instability. Today, instrument and meter plant together with you to look over the main source of thermocouple unstable factors. Main source of thermocouple instability will affect the seebeck coefficient of thermocouple defiled. Thermocouple wire materials are occasionally under the protection of the environment atmosphere or tube impurities defiled, additional potential produced by different degrees of tainted is different also, this kind of additional potential will change the original indexing features, this was the cause of thermocouple is not stable value for a factor. For example, platinum rhodium - 10 Platinum thermocouple, when using the ceramic tube of iron impurities, platinum rhodium wire is iron defilement, affecting its thermoelectric properties; When used in high temperature and reducing atmosphere containing silicon, due to the reduction of silicon free silicon become a platinum silicon compounds with platinum rhodium wire, make I become fragile. Insulation porcelain tube used in the verification standard thermocouples require clean with aqua regia, high temperature baking and perforation of positive and negative polarity. If the hot electrode in common tube of positive and negative electrode wear the wrong, the original wear platinum rhodium hole in platinum, permeate to platinum pole and change the standard thermocouple thermoelectric properties. All of these conditions can affect the stability of the thermocouple. Hot electrode at high temperatures most volatile of thermocouple accidentally silk material is alloy material, due to the vapor pressure of each composition materials is different, so the degree of volatile is different also, after using some time at high temperature, alloy composition ratio is changed, this will result in thermoelectric power generation changes obviously. The instability of REDOX many thermocouple is caused by accidentally wire oxidation. Copper - Lala, iron - Lala, nickel chromium - Such as nickel and silicon thermocouple oxidizing reaction can be. If the hot electrode is homogeneous oxidation, the impact could be smaller; If has the preferential oxidation, its influence is very serious. In the low oxygen partial pressure ( Under the condition of oxygen) The chromium in the chrome, nickel electrode will produce a preferential oxidation and change the composition of accidentally silk. One of the most common factors embrittlement embrittlement is thermocouple scrap. Thermocouple thermal electrode due to defile, grain growth, and REDOX reaction and long-term factors such as recrystallization, at high temperature is the cause of thermal embrittlement electrode. Hot electrode used in the atomic reactor, neutron bombardment, in which some elements into other elements, changed the hot electrode composition. Such as armoured platinum rhodium thermocouple rhodium will change as the palladium, a small amount of platinum will be turned into gold, into a mercury again, change thermoelectric properties, thermoelectric potential smaller. And inexpensive metal thermocouple, some evolution of neutron radiation is not obvious, such as iron, nickel, chromium, nickel and aluminum ( Silicon) 。 But copper under the radiation caused exerted great composition change. Therefore, in the case of a neutron radiation, use a nickel chrome nickel aluminum thermocouple is relatively appropriate, nickel chromium silicon silicon (a nickel N) Thermocouple is better. Severe external force by bending or any other type of work hardening in the thermocouple wire in the formation of the internal stress can produce physical nonuniformity, careful operation can avoid most of the resulting inhomogeneity. Good thermocouple assembly after proper annealing can also be used to a certain extent reduce the inhomogeneity.
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