Conditioning of the standard platinum rhodium thermocouple
In the years of high temperature precious metal thermocouple verification work, accumulated a large amount of data, from which found some new standard thermocouple, especially the standard platinum rhodium 30 - platinum rhodium 6 thermocouple, in the test cycle only a few times, The value of the ultra-poor, and its thermoelectric properties changed a lot. Tables 1 and 2 present some representative data. Standard platinum-rhodium 10-platinum thermocouple because there is one electrode is pure platinum, thermoelectric performance is relatively stable, but its new system or in use also changes. The reason, there are the following aspects.
1 contamination of the standard thermocouple
(1) the new system of standard thermocouple, cold wire in the cold (cold, cold), due to roll and
wire drawing die wear and tear, resulting in a number of metal powder residue in the even wire
surface, pollution hot electrode. In addition, during the above-mentioned processing, the
lubricating oil and the coolant used in the above-mentioned processes are decomposed to produce
carbon, sulfur, iron and the like due to the temperature rise caused by the cold working, thereby
further polluting the hot electrodes. That is the standard thermocouple and the surrounding
environment of the material or the impurities in the chemical changes that may change the
composition of the thermocouple alloy, the standard thermocouple thermoelectric stability.
(2) to withstand the high temperature of the standard thermocouple, due to high-temperature steam
(uranium, carbon) through the insulation tube gap and even contact with the silk, the contact part
of the serious division of the metamorphosis.
(3) the precious metal thermocouples are tested, because the use of different environments, the hot
electrode surface may be contaminated by various impurities, if cleaning is not sufficient, when
using a high standard thermocouple test, it will be different Degree of pollution. Influence of
thermocouple stability of standard thermocouple with high grade.
Elimination of impurities on the standard thermocouple pollution caused by the impact of the
general approach to cleaning.
Whether the new system or the use of certified precious metal thermocouple, before the test, must
be strictly cleaned. The cleaning method is as follows:
(A) pickling thermocouple: pickling thermocouple is the use of nitric acid (sulfuric acid)
oxidation capacity, remove the new system, the use of the surface of the hot electrode organic and
metal impurities.
(B) borax cleaning thermocouple: the use of the thermocouple with borax cleaning, is to remove the
remaining in the hot electrode surface is difficult to dissolve in the acid metal impurities and
oxides. Completely remove the hot electrode surface contamination of impurities, both to eliminate
the impurities on the thermoelectric properties of the seized thermocouple, but also to prevent the
test on the high level of standard thermocouple pollution and corrosion.
In addition, the protection of standard thermocouple double-hole insulation pipe of the physical
and chemical stability is better, in the course of the thermocouple does not produce harmful gases
; air tightness (ie, small porosity) is better to prevent the external media penetration into the
protective tube So that the hot electrode damage and deterioration. Double-hole insulation pipe in
use before the use of "aqua regia" cleaning, high temperature baking and marked positive and
negative polarities of perforation. If the insulation tube cleaning is not complete, for the
standard platinum and rhodium 10 - platinum thermocouple, easy to contaminate the platinum pole,
so that platinum purity decreased, the value of thermal EMF decreased. Such as: standard platinum
and rhodium 10 - platinum thermocouple to wear unclean double insulation pipe, after the test
after annealing, at 1 084.62 ℃ temperature point of thermal electromotive force decreased to
varying degrees (2.9 ~ 5.9μV). See Table 1 for the data.
The influence of the segregation of the chemical composition of the hot electrode on the
thermoelectric stability of standard platinum - rhodium thermocouple
According to the rules, the new system of standard platinum and rhodium thermocouple, the thermal
stability of the assessment, but short-term stability, not fully eliminate the thermocouple
internal stress. Therefore, in the test cycle, although not often used, the inspection, there are
some standard platinum and rhodium thermocouple, there will still be out of tolerance. In
particular, the standard platinum-rhodium 30-platinum-rhodium 6 thermocouple, because the two
platinum electrode is platinum and rhodium alloy binary elements, and in the 600 ℃ ~ 800 ℃, 1 150
℃ ~ 1350 ℃ rhodium sublimation, evaporation Rhodium will make the diameter of the
thermoelectrode smaller, not only mutual pollution between the hot electrodes, but also the
content of rhodium in the solid solution segregation, resulting in a single thermoelectric force
changes, thereby changing its thermoelectric properties, affecting the thermocouple stability
Sexual, and even tolerance. Also found that some super-cycle is not often used standard
thermocouple, thermoelectromotive force also changes, resulting in errors, which is the aging
error. These are related to the material with the silk is not long-term stability of the assessment
.
The following is a set of standard platinum rhodium 10 platinum thermocouple uniformity
(temperature point 1 084 62 ℃) assessment of a set of data (Table 2). Two pairs of hot electrodes
each 8.04m, respectively, cut into a pair of 2.01m, according to the requirements of the standard,
made by the standard axial annealing 8 thermocouple, and then after radial annealing after the
test. The eight standard thermocouples were found to be inhomogeneous with a potential of 4 μV,
which is equivalent to 0.34 ° C. This is due to the segregation of the chemical composition of
the thermocouple.
The Effect of Internal Stress on Thermoelectric Stability of Standard Pt - Rh Thermocouple
The so-called internal stress is drawn or the use of standard thermocouple process, the hot electrode by external force, the internal resistance generated. It is an important factor that affects the thermoelectric stability of standard platinum-rhodium thermocouples. In the standard thermocouple wire cold manufacturing and verification of the operation process, are attached to varying degrees of internal stress, resulting in decreased thermoelectric properties, thermoelectric stability to the error. Due to the standard dual-wire bending or stretching caused by the cold, so there is internal thermal stress and lattice lattice unevenness. According to foreign literature, the new platinum and rhodium thermocouple in the range of 200 ℃ ~ 1 000 ℃ error of about 8 ℃, and in the same conditions, after axial, radial annealing error of about 1 ℃, you can see After the quenching and tempering treatment, platinum-rhodium thermocouple error decreases, thermoelectric properties improved.
Two ways to remove internal stress:
(1) in the air hanging wire annealing, so that the hot electrode axial heat evenly attached to the
surface of the hot electrode low melting point metal impurities can be fully volatile. Eliminate
the axial stress in use.
(2) furnace annealing, so that the thermal electrode radial uniform heating. Elimination due to
axial annealing, the thermal electrode radial temperature by the impact of ambient air is not
consistent, cooling speed, easy to obtain uniform organization, still a small amount of residual
radial stress and stress due to wear when the insulation tube.
Only after a reasonable and sufficient annealing to improve the thermal electrode of the
microstructure to eliminate the thermal stress in the electrode to improve the standard platinum
and rhodium thermocouple thermoelectric stability in order to make the standard platinum and
rhodium thermocouple to meet the technical requirements of the verification procedures to ensure
that Standard platinum and rhodium thermocouple in the process of passing the international
temperature scale, as the transfer of temperature value of the standard equipment.
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