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Conditioning of the standard platinum rhodium thermocouple
Dec 24, 2017

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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