Stability Studies of a New Design Au/Pt Thermocouple Without a Strain Relieving Coil

The performance of a simple, new design Au/Pt thermocouple developed by NMIA is assessed. This thermocouple is proposed as a more accurate replacement, over the temperature range from 0 to 1,000°C, for the commonly used Type R and S industrial transfer standards, in a robust form familiar to industrial calibration laboratories. Due to the significantly different thermal expansions of the Au and Pt thermoelements, reported designs of the Au/Pt thermocouple incorporate a strain-relieving coil or bridge at the thermocouple junction. As the strain relieving coil is mechanically delicate, these thermocouples are usually mounted in a protective quartz tube assembly, like a standard platinum resistance thermometer (SPRT). Although providing uncertainties at the mK level, they are more delicate than the commonly used Type R and S thermocouples. A new and simple design of the Au/Pt thermocouple was developed in which the differential thermal expansion between Au and Pt is accommodated in the thermocouple leads, facilitated by a special head design. The resulting thermocouple has the appearance and robustness of the traditional Type R and S thermocouples, while retaining stability better than 10 mK up to 961°C. Three thermocouples of this design were calibrated at fixed points and by comparison to SPRTs in a stirred salt bath. In order to assess possible impurity migration, strain effects, and mechanical robustness, sequences of heat treatment up to a total of 500 h together with over 50 thermal cycles from 900°C to ambient were performed. The effect of these treatments on the calibration was assessed, demonstrating the sensors to be robust and stable to better than 10 mK. The effects on the measured inhomogeneity of the thermocouple were assessed using the NMIA thermocouple scanning bath.     

Life Expectancy Study of Small Diameter Type E, K, and N Mineral-Insulated Thermocouples Above 1000???C in Air

Very little if any current data is available for the life expectancy of very small diameter thermocouples operating at high temperatures, greater than 1000???C. Over the past 10?years significant changes in the supply stream of the materials used to manufacture base metal thermocouples have occurred. In many industrial applications, small diameter thermocouples are the only solution for high-temperature measurements. This study has been undertaken to assess the performance of small diameter magnesium oxide insulated metal sheathed thermocouple sensors at or above 1000???C. Three different American Society of Testing and Materials (ASTM) standard letter designation thermocouple types have been included in this study, Types E, K, and N. Inconel 600 and 316 stainless, for three different sizes, 0.5?mm, 1.0?mm, and 1.5?mm, have been tested for different thermocouple types. Each group of sensors was placed in an equalization block in air for a maximum of 500?h or until failure. The performance of 0.5?mm diameter thermocouples varies widely depending on the thermocouple type, sheath material, and test temperature. Larger diameter Type K and N thermocouples show very little drift up to 500?h at 1100???C. The data for each test was collected at 10?s intervals for the entire duration of the test. Data for the sensor drift and subsequent failure are presented.     

薄膜热电偶热电特性分析与试验

薄膜热电偶具有体积小、操作简便、准确度高等优点,在精确测温领域显出了极大优势。采用磁控溅射法生产的热电偶在材料组合上几乎是无限的。本文对Ni CrNi Si薄膜热电偶、Ni Cr SiNi Si Mg薄膜热电偶静态下的重复性、使用寿命、温度上限等进行对比、分析,以便在使用中可以更好地"因地制宜"。与大连某高校制作的薄膜热电偶进行对比试验,验证制作工艺对薄膜热电偶性能的重要影响。薄膜热电偶作为一种新型热电偶,在发动机叶片等特殊部件表面测温领域具有广泛的应用前景。     

Comparative Study of Pt/Pd and Pt–Rh/Pt Thermocouples

The Pt/Pd thermocouple has demonstrated superior thermoelectric drift and homogeneity performance over conventional Pt–Rh/Pt thermocouples. Here, we present a systematic comparison of the drift and homogeneity performance of Pt/Pd and Type R thermocouples by ageing the thermocouples at 1350 °C for a total of 500 h and measuring the performance at regular intervals during this time. The thermocouples studied were one Pt/Pd thermocouple, one Type R thermocouple and one ‘special’ Type R thermocouple which was given the same preparatory annealing treatment as the Pt/Pd thermocouple prior to use. The thermoelectric stability of each thermocouple was measured at the freezing point of Ag (961.78 °C) and the melting point of Co–C eutectic (1324.29 °C). The thermoelectric homogeneity of the thermocouples was also measured. Two difference methods were used by withdrawing the thermocouple from the Ag cell and by moving a localized heat source along the thermocouple. The long-term drift of the Pt/Pd thermocouple was around 50 mK (Ag) and 65 mK (Co–C) after the first 100 h ageing at 1350 °C, followed by a further 25 mK (Ag) and 35 mK (Co–C) over the subsequent 400 h ageing. This drift performance and inhomogeneity were an order of magnitude lower than for the two Type R thermocouples. The Type R thermocouple which was given the ‘special’ preparatory treatment was about 50 % more stable than the conventional Type R thermocouple.     

Thermocouples with Built-In Self-testing

The aim of this paper is to create a method of built-in self-testing of thermocouples in situ. This aim is achieved by using the equivalent operating time of the thermocouple. The method does not require replacement of thermocouples from their operating place as it could be done during the operation of the thermocouples. The only necessary condition, that makes self-testing possible, is a constant measuring junction temperature during the procedure of self-testing. The determined equivalent operating time allows finding the place of a given thermocouple in the thermocouple’s drift model as well as in the model of thermoelectric inhomogeneity.     

Seebeck Changes Due to Residual Cold-Work and Reversible Effects in Type K Bare-Wire Thermocouples

Type K thermocouples are the most commonly used thermocouple for industrial measurements because of their low cost, wide temperature range, and durability. As with all base-metal thermocouples, Type K is made to match a mathematical temperature-to-emf relationship and not a prescribed alloy formulation. Because different manufacturers use varying alloy formulations and manufacturing techniques, different Type K thermocouples exhibit a range of drift and hysteresis characteristics, largely due to ordering effects in the positive (K+) thermoelement. In this study, these effects are assessed in detail for temperatures below \(700\, {^{\circ }}\hbox {C}\) in the Type K wires from nine manufacturers. A linear gradient furnace and a high-resolution homogeneity scanner combined with the judicious use of annealing processes allow measurements that separately identify the effects of cold-work, ordering, and oxidation to be made. The results show most K+ alloys develop significant errors, but the magnitudes of the contributions of each process vary substantially between the different K+ wires. In practical applications, the measurement uncertainties achievable with Type K therefore depend not only on the wire formulation but also on the temperature, period of exposure, and, most importantly, the thermal treatments prior to use.     

铠装贵金属热电偶测量端位置的确定方法

热电偶测量端位置的准确定位对于热电偶的校准和使用有着重要影响,特别对于铠装贵金属热电偶来说,由于无法通过肉眼准确识别保护管内部热电偶测量端的位置,因此在热电偶检定和使用过程中一般将其外保护管顶部作为测量端所在位置。然而通过现场试验发现,铠装贵金属热电偶测量端一般并不位于保护管顶部,本文提供了一种准确定位铠装贵金属热电偶测量端的方法。     

A gold-platinum thermocouple

Summary Since the homogeneity of gold is much higher than that of platinorhodium, the gold-platinum thermocouples provide considerably more accurate temperature measurements than platinorhodium-platinum thermocouples. The high degree of homogeneity of gold is especially valuable when small temperature differences are measured. Another important advantage of the gold-platinum thermocouples as compared with the platinorhodium-platinum ones is their higher thermal emf and lower electrical resistance. The defects of the new thermocouple are its high thermal conductivity, tendency to acquire plastic strains and the low melting point of gold as compared with platinorhodium. These defects are absent in another thermocouple made of pure metals, the rhodium-platinum thermocouple, which we propose to analyse in the future.     

A Slimline Integrated Self-Validating Thermocouple: Initial Results

NPL, in collaboration with CCPI Europe, have designed a slimline integrated self-validating (“inseva”) thermocouple with the same external form factor as conventional thermocouples, with the aim of making them suitable as direct replacements for existing thermocouples in process. Type S thermocouples have been manufactured in recrystallized alumina-sheathed assemblies, with Cu and Co–C reference ingots, with an outer diameter of 7 mm. The new slimline inseva thermocouple is, in principle, suitable for use in the same positions and conditions as the conventional thermocouple which it replaces. This paper reports the initial reference ingot melt and freeze plateaus successfully observed using the first inseva thermocouples, and demonstrates observation of furnace sensitivity and ramp rate sensitivity of the plateau temperatures.     

Thermoelectric uniformity of chromel,copel, alumel and copper wire at low temperatures

Conclusions As a result of these tests comparative data have been obtained on the thermoelectric uniformity at low temperatures of Soviet-made thermocouple wire. According to these data the ambiguity of the thermocouple calibration characteristic due only to local irregularities of electrodes may attain ±5° for chromel-alumel, ±2.5° for chromel-copel, and ±0.5° for copper-copel thermocouples, at temperatures approaching –200° C.Therefore, copper and copel are the most suitable materials for high-precision thermocouples operating at low temperatures.     

高精度金-铂热电偶的制作和热电稳定性研究

金-铂热电偶是采用纯金和纯铂电极制成,长期使用过程中是否能够保持温度与热电势关系稳定是其准确测温的关键.首先,对热电极丝材进行剪切、清洗、退火并焊接组装成热电偶;然后,再对制作成的热电偶进行退火实验,热电偶在其使用上限965℃附近累计退火时间700 h,在银凝固点的稳定性达到±0.02℃.在制造热电偶过程中,采取了传统...     

Optimizing Heat Treatment of Gas Turbine Blades with a Co–C Fixed Point for Improved In-service Thermocouples

Improvement of energy efficiency of jet aircraft is achieved by operating gas turbine engines at higher temperatures. To facilitate this, gas turbine engine manufacturers are continuously developing new alloys for hot-zone turbine blades that will withstand the increased in-service temperatures. A critical part of the manufacture of these blades is heat treatment to ensure that they attain the necessary metallurgical characteristics. Current heat-treatment temperature-control requirements are at the limit of what is achievable with conventional thermocouple calibrations. A project that will allow thermocouple manufacturer CCPI Europe Ltd. to realize uncertainties of ± 1°C, or better, in the calibration of its noble metal thermocouples is described. This will be realized through implementing a Co–C eutectic fixed point in CCPI’s calibration chain. As this melts at 1,324°C, very close to the heat-treatment temperatures required, low uncertainties will be obtained. This should yield an increase in effectiveness of the heat-treatment process performed by Bodycote Heat Treatments Ltd., allowing them to respond effectively to the increasingly stringent demands of engine manufacturers. Outside the current project, there is a strong requirement by industry for lower uncertainties at and above 1,300°C. Successful implementation of the current fixed point in an industrial setting is likely to result in rapid take-up by other companies, probably through the supply of ultra-low uncertainty thermocouples, looking to improve their high-temperature processes.     

Evaluating the Inhomogeneity of Thermocouples Using a Pressure-Controlled Water Heat Pipe

There exists various research reports concerning the evaluation methods for the measurement uncertainty due to inhomogeneity of thermocouples; however, the universal method is still waiting to be established. This article considers the evaluation methods for the measurement uncertainty due to inhomogeneity of thermocouples based on comparison between results of two measurement methods. The first method is to estimate the uncertainty from the immersion characteristics of a thermocouple within a fixed-point furnace during its realization. The second method is to estimate the uncertainty from the immersion characteristics of a thermocouple within a heat-pipe furnace with a long uniform region. A pressure-controlled water heat-pipe furnace with an immersion depth of 1000?mm is developed to enable this work. It overcomes the technical difficulties that existed in applying conventional sealed heat pipes to such applications. From the immersion characteristics of a thermocouple measured by the above two methods, we have introduced three measurement parameters. Estimating the measurement uncertainty due to the inhomogeneity from our measurement results as examples is discussed.     

在液槽中测试热电偶不均匀性方法探索

介绍了通过测量热电偶在液体槽中250 ℃时的浸没实验特性评价热电偶均匀性的方法。构建的实验装置在液槽上的一个狭小的空间内可产生较大的温度梯度场,热电偶电极通过该梯度场进行扫描得到了在不同位置的热电势值。对一组新制和使用一段时间的铂铑10-铂热电偶进行测试,发现实验数据具有较好的重复性,新制作的热电偶热电势的均匀性明显优于使用一段时间的热电偶。     

Construction and Investigation of Pt/Pd Thermocouples in the Framework of the Euromet Project 857

The objective of the EUROMET Project 857 in the field of thermometry, “High-temperature fixed points for improved thermocouple calibrations,” is the development of robust high-temperature fixed points based on metal-carbon eutectic alloys for the calibration of thermocouples above 1,084°C. This paper describes the construction and investigation of Pt/Pd thermocouples to be used to compare different cobalt–carbon (Co–C) fixed-point cells constructed by the three participants of this project. A set of three Pt/Pd thermocouples was prepared by PTB and NPL and a set of four by LNE. Their metrological performances in terms of thermoelectric stability and homogeneity were assessed in different ways. The results of these investigations, as well as the results of first measurements of local Co–C eutectic fixed-point cells by using Pt/Pd thermocouples, are presented.     

钨铼热电偶在空气中的热电动势稳定性及其特性研究

为了解决钨铼热电偶在实际使用中遇到的问题，本文探讨了钨铼热电偶在空气中的高温稳定性规律。研究结果发现：钨铼热电偶由于某一极完全氧化而导致材料性质发生了质变，从而引发了与之相应的热电性质的根本改变；首次揭示出钨铼热电偶两极的稳定性在不同温度下是不同的；发现了钨铼热电偶的体电阻与热电势具有同时突变的特性，从而，可以通过测量电阻变化对其寿命进行预报。本项研究结果为钨铼热电偶的生产及合理使用提供了理论依据。     

An automated thermocouple calibration system

An automated thermocouple calibration system (ATCS) was developed for the unattended calibration of type-K thermocouples. This system operates from room temperature to 650°C and has been used for calibration of thermocouples in an eight-zone furnace system which may employ as many as 60 thermocouples simultaneously. It is highly efficient, allowing for the calibration of large numbers of thermocouples in significantly less time than required for manual calibrations. The system consists of a personal computer, a data acquisition/control unit, and a laboratory calibration furnace. The calibration furnace is a microprocessor-controlled multipurpose temperature calibrator with an accuracy of ±0.7°C. The computer software is menu-based for flexibility and ease of use. The user needs no programming experience to operate the system     

Miniature Fixed-Point Cell Approaches for {{\varvec{In\,Situ}}} Monitoring of Thermocouple Stability

In the framework of the European Metrology Research Project ENG08 “MetroFission,” LNE-Cnam and NPL have undertaken cooperative research into the development of temperature measurement solutions for the next generation of nuclear fission power plants. Currently, in-pile temperature monitoring is usually performed with nickel-based (Type K or N) thermocouples. When these thermocouples are exposed to a neutron flux, the thermoelements transmute, leading to large and unknown drifts in output. In addition, it is impossible to routinely recalibrate the thermocouples after irradiation for obvious reasons of safety. To alleviate this problem, both LNE-Cnam and NPL have developed, via differing approaches, in situ calibration methods for the thermocouples. The self-validating thermocouple methodologies are based on the principle of a miniature fixed-point cell to be co-located with the thermocouple measurement junction in use. The drift of the thermocouple can be monitored and corrected for by regular determination of the output at the phase transition of the fixed-point material: in effect performing regular in situ calibration checks. The two institutes have constructed miniature fixed-point cells for use at three different temperatures; the freezing point of silver \((961.78\,^{\circ }\mathrm{C}\) ; LNE-Cnam), the freezing point of copper \((1084.62\,^{\circ }\mathrm{C}\) ; LNE-Cnam and NPL), and the melting point of Fe–C ( \({\sim }1154\,^{\circ }\mathrm{C}\) ; NPL). This paper introduces the construction and validation of the miniature fixed-point cells prior to use, to ensure traceability to the ITS-90. A comparison of the performance of the two cell designs is discussed, where typical industrial Type N thermocouples have been used for measurement of the fixed-point cells. Such initial measurements demonstrate the feasibility of each of these two approaches.     

Thermoelectric behaviour of amorphous magnetic alloys

The Thermoelectric emfs of thermocouples formed by amorphous METGLAS 2826 (Fe₄₀Ni₄₀P₁₄B₆) and METGLAS 2826B (Fe₂₉Ni₄₉P₁₄B₆Si₂) with standard thermocouple wires like copper, chromel, alumel, etc., were measured as a function of temperature between −196° C and 30° C to assess their suitability as thermoelectric temperature sensors. Thermoelectric emfs generated by METGLAS 2826/Cu and METGLAS 2826B/Cu thermocouples at −196° C are about an order of magnitude smaller when compared to thermal emfs of a standard copper/constantan thermocouple at the same temperature.     

Theoretical Study of the Conditions of Maximum Manifestation of the Error Due to Inhomogeneity of Thermocouple Legs

The method of theoretical analysis of temperature ranges for the maximum manifestation of the error due to acquired thermoelectric inhomogeneity of thermocouple legs is proposed in this paper. The drift function of the reference function of a type K thermocouples in a ceramic insulation, that consisted of 1.2 mm diameter thermoelements after their exposure to 800 \(^{\circ }\)C for 10 000 h in an oxidizing atmosphere (air), is analyzed. The method takes into account various operating conditions to determine the optimal conditions for studying inhomogeneous thermocouples. The method can be applied for other types of thermocouples when taking into account their specific characteristics and the conditions that they have been exposed to.