I have accumulated a large amount of data in the high-temperature precious metal thermocouple verification work for many years, and found some new standard thermocouples, especially the standard platinum-iridium 30-platinum-ruthenium 6 thermocouple, which was used only several times during the verification period. The indication value is out of tolerance, and its thermoelectric performance has changed a lot. Tables 1 and 2 list some representative data. The standard platinum-iridium 10-platinum thermocouple has a thermoelectric performance that is relatively stable because one electrode is pure platinum, but it also changes in its new or used state. The reasons for this are as follows.
1 Impurity contamination of standard thermocouples
(1) The new standard thermocouple, when the cold wire is cold-worked (cold-rolled, cold-drawn), due to the wear of the roll and the wire drawing die, some metal powder remains on the surface of the wire, contaminating the hot electrode. In addition, in the above processing, the lubricating oil and the coolant used, due to the temperature rise caused by the cold working, cause the oil to decompose to generate carbon, sulfur, iron, etc., further contaminating the hot electrode. That is to say, the standard thermocouple chemically changes with the materials of the surrounding environment or the impurities therein, which may change the composition of the thermocouple alloy and affect the thermoelectric stability of the standard thermocouple.
(2) For standard thermocouples that are subjected to high temperatures, since the high-temperature vapor (uranium, carbon) is in contact with the filaments through the gap of the insulating tube, severe deformation of these contact portions occurs.
(3) The precious metal thermocouples to be tested may be contaminated by various impurities due to their use in different environments. If the cleaning is not sufficient, it will be different when it is checked with a high-grade standard thermocouple. The degree is polluted. Affects the thermoelectric stability of high-grade standard thermocouples.
To eliminate the effects of impurities on standard thermocouple contamination, cleaning is generally used.
Whether it is a new or a precious metal thermocouple that has been certified, it must be rigorously cleaned prior to verification. The cleaning method is as follows:
(a) Pickling thermocouple: The pickling thermocouple utilizes the oxidizing ability of nitric acid (sulfuric acid) to remove organic and metallic impurities from the surface of the newly used hot electrode.
(b) Borax cleaning thermocouple: The chrome cleaning in use of the thermocouple is to remove metal impurities and oxides which are hard to dissolve in acid on the surface of the hot electrode. Thoroughly remove impurities contaminated on the surface of the hot electrode, which not only eliminates the influence of impurities on the thermoelectric properties of the tested thermocouple, but also prevents contamination and corrosion of the high-grade standard thermocouple in the verification.
In addition, the physical and chemical stability of the two-hole insulating tube protecting the standard thermocouple is good, and no gas harmful to the thermocouple is generated during use; the airtightness (ie, the porosity is small) is good, and the external medium is prevented from penetrating into the protective tube. The hot electrode is damaged and deteriorated. The double-hole insulated tube should be cleaned with “Aqua regia†before use, and baked at high temperature and marked with the polarity of the perforation of the positive and negative electrodes. If the insulating tube is not thoroughly cleaned, it is highly susceptible to contamination of the platinum electrode for the standard platinum-iridium 10-platinum thermocouple, which lowers the purity of the platinum electrode and lowers the thermoelectromotive force value. For example, a standard platinum-iridium 10-platinum thermocouple is put on an unclean double-hole insulating tube. After annealing, the thermal electromotive force at a temperature of 1 084·62 °C is reduced to a different extent (2·9~5·9 μV). See the data listed in Table 1.
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2 The effect of segregation of the chemical composition of the hot electrode on the thermoelectric stability of the standard platinum-rhodium thermocouple
According to the requirements of the regulations, the new standard platinum-iridium thermocouples, the thermoelectric stability assessed, are only short-term stability and do not fully eliminate the internal stress of the thermocouple. Therefore, during the verification period, although not frequently used, there are some standard platinum-iridium thermocouples at the time of inspection, and there will still be an out-of-tolerance. In particular, the standard platinum rhodium 30-platinum rhodium 6 thermocouple, because the two hot electrodes are platinum-bismuth alloy binary elements, and sublimed between 600 ° C ~ 800 ° C, 1 150 ° C ~ 1 350 ° C, evaporation铑 will make the diameter of the hot electrode thinner, not only the mutual contamination between the hot electrodes, but also the segregation of the content of bismuth in the solid solution, causing the unipolar thermoelectromotive force to change, thereby changing its thermoelectric characteristics and affecting the stability of the thermocouple. Sex, even super bad. In addition, it has also been found that some standard thermocouples that are not frequently used in super cycles have also changed the thermoelectromotive force, causing errors, which are aging errors. These are related to the lack of long-term stability assessment of the material of the filament.
The following is a set of data for the new standard platinum rhodium 10-platinum thermocouple uniformity (temperature point 1 084·62 ° C) (Table 2). The two pairs of hot electrodes are each 8·04m, which are respectively cut into 2·01m pairs. According to the requirements of the regulations, 8 sets of standard thermocouples are made by axial annealing, and then subjected to radial annealing for verification. It was found that the eight standard thermocouples were inhomogeneous with each other and could reach 4 μV, which is equivalent to 0·34 ° C, which is caused by the segregation of the chemical composition of the hot electrode.
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