Stability of Tungsten�CRhenium Thermocouples in the Range from 0?��C to 1500?��C

The effect of exposure up to 1500 °C on emf values of type C (95 % tungsten 5 % rhenium vs. 74 % tungsten 26 % rhenium) thermocouples were evaluated. Three thermocouples consisting of thermocouple wires of 0.5 mm diameter, twin-bore beryllia tubes, and tantalum sheaths were prepared. After three type C thermocouples were calibrated in the range from 0 °C to 1550 °C, which confirmed insignificant difference among them, the drifts of two among them were measured at the palladium?Ccarbon (Pd?CC) eutectic point (1492 °C). They indicated a similar tendency, where the emf of thermocouples increased rapidly within the first 30 h, and after that, decreased gradually. To investigate the mechanism of the drift, the inhomogeneities of thermocouples were examined at 160 °C using a water heat-pipe furnace during the drift measurements at the Pd?CC eutectic point. It was found that the increase of emf within the first 30 h exposure at around 1500 °C was caused by the emf change due to inhomogeneity above 700 °C, and after that, the decrease of emf was caused by that around 1400 °C.     

Inhomogeneity Measurements of Long Thermocouples using a Short Movable Heating Zone

The thermoelectric inhomogeneity of wires is one of the main components of the measurement uncertainty when using thermocouples. During calibration, it is therefore important to determine how much the inhomogeneities affect the measurement result. Thermoelectric inhomogeneity is normally assessed by gradual, or step-wise, insertion of a thermocouple into a furnace or liquid bath. With this type of equipment, the length that can be scanned is typically limited to about half a meter. To assess thermoelectric inhomogeneity over greater lengths, it is necessary to adopt a different technique. Therefore, an apparatus with a short, movable heating zone has been set up and evaluated. The apparatus produces a short, well-defined heating zone that is moved along the thermocouple while both the measuring and reference junctions are kept at 0 °C. Heating is done by means of a hot-air fan that produces a temperature-controlled heating zone up to 700 °C. Two directionally controllable cold-air fans, one on each side of the heating zone, make it possible to vary the slopes of both temperature gradients of the heating zone. Two temperature gradients influence the thermocouple when performing measurements of inhomogeneity with this setup. The results are, therefore, not directly comparable to the results from measurements taken in a bath or furnace, where only one temperature gradient is present. The resulting curve obtained with the two gradients is approximately equivalent to the derivative of the curve obtained with one gradient. It is possible to convert the two-gradient curve to a single-gradient curve by numerical integration, as shown in this work. Comparisons with inhomogeneity measurements obtained using salt baths show good agreement with the calculated results.     

Development and Realization of Iron–Carbon Eutectic Fixed Point at NPLI

The concept of metal–carbon eutectic temperature fixed point has been introduced in 1999 and is extensively being investigated by thermometry researchers to cover the high-temperature range above copper fixed point. Metal–carbon eutectic fixed points also helped to provide direct traceability with reduced associated uncertainty in the high temperature range for thermometry and radiometry applications. In view of this, CSIR-National Physical Laboratory, India (NPLI) has developed iron–carbon (Fe–C, 1153 °C) eutectic fixed point cell in the graphite crucible and realized by using the noble metal thermocouples. The preparation parameters such as design and fabrication of a graphite crucible, Fe:C eutectic composition and filling procedure, furnace profile, melting and freezing plateau measurements, heat flux immersion, inhomogeneity, etc. have been optimized and presented in this paper. The measurement uncertainty of the Fe–C eutectic cell realized with Type-S thermocouple was estimated to be 3.04 μV (0.25 °C) at coverage factor k = 2.     

Drift as a Function of Temperature in Platinum–Rhodium-Alloyed Thermoelements

Platinum–rhodium-alloyed thermocouples are the most commonly used high-temperature reference thermometer in national measurement institutes, second tier laboratories and industry. Despite the common use of these thermocouples, there is still a great deal that is not known about the drift processes that occur in them. Drift processes in these alloys are known to be made up of three main components: crystallographic changes, rhodium oxidation and migration, and contamination. Through careful use, contamination can be largely avoided; however, the other two processes often cannot. Research on drift in the different platinum–rhodium alloys is important because the largest uncertainty component during calibration of these thermocouple types is due to inhomogeneity, and the same mechanisms responsible for inhomogeneity are responsible for the drift. This study investigates the drift processes as a function of temperature and time for the 5 %, 13 %, 17 %, 20 %, 30 % and 40 % Rh alloys when paired with pure platinum. The use of a linear gradient furnace and high-resolution homogeneity scanner has enabled identification of drift characteristics in the temperature range \(100 \,^{\circ }\)C to \(950 \,^{\circ }\)C, where the bulk of reversible drift occurs. The experiments have quantified the drift rates and magnitude for thermoelements given two commonly used annealing procedures: the high-temperature quench anneal and the low-temperature vacancy anneal. Consequently, this study provides users of platinum–rhodium thermoelements with guidance on what levels of drift they should expect and exposure times before re-annealing is required. It also shows that a Pt–Rh alloy of 20 % Rh is by far the most stable and has properties comparable to the Pt–Pd thermocouple.     

基于高温黑体炉的钨铼热电偶校准方法研究

基于中国计量科学研究院的高温黑体炉设计了一种适用于钨铼偶等高温热电偶的校准方法。优化设计的均温块测温孔轴向均匀性20mm范围内小于0.5℃,优选的测温孔与中心孔的辐射温度差异可达到小于0.5℃。经铂铑10-铂热电偶验证了基于高温黑体炉的校准方法,在800~1300℃与S型热电偶标准热电势间差异小于0.5℃,不确定度评估为0.8~1.5℃,k=2。在800~1900℃范围内,测试了多只不同来源的C型钨铼偶热电势并考核了偶丝校准前后的均匀性,实验结果表明,钨铼偶丝与国际标准钨铼偶热电势的差异基本保持在1%以内,校准不确定度为3.7~13.0℃,相对不确定度为0.7%t (t为温度),k=2。     

Comparison of Co�CC and Pd�CC Eutectic-Point Cells for Thermocouple Calibration Between LNE-Cnam and NMIJ

Evaluation of the melting temperatures of two Co?CC and two Pd?CC eutectic-point cells were performed using three Pt/Pd thermocouples constructed at LNE-Cnam and NMIJ. During the comparison of cells, Pt/Pd thermocouples were evaluated at the Ag fixed point, at which their drifts and inhomogeneity represented differences within 0.03 °C and 0.14 °C, respectively. One Co?CC cell and one Pd?CC cell were designed and constructed at LNE-Cnam, while one Co?CC cell and one Pd?CC cell were designed and constructed at NMIJ. In spite of differences in cell design and in materials sources, melting points of Co?CC and Pd?CC eutectic-point cells realized using LNE-Cnam high-temperature furnace agreed within approximately 0.02 °C and 0.01 °C, respectively. Furthermore, the realizations of the Pd?CC eutectic point at LNE-Cnam and NMIJ agreed within a temperature equivalent of approximately 0.11 °C. The uncertainty of the realization was estimated to be approximately 0.14 °C (k = 2), with a major contribution from the inhomogeneity of the thermocouple.     

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。     

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.     

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.     

真空炉温度均匀性测量的数值模拟研究

热电偶是检测真空炉温度均匀性的常用计量器具,热电偶的几何特性及发射率对炉温均匀性检测的影响目前少有研究。本文利用数值模拟方法研究了热电偶发射率和直径对真空炉温度均匀性检测的影响,结果表明直径和发射率对常用规格热电偶稳态温度影响量级相当。开展零维模型分析,结果表明热电偶温度对直径和发射率相对变化率的敏感性相等,验证了数值模拟结论的准确性,为真空炉温度计量提供了理论指导与技术借鉴。     

Evaluating Uncertainties in Interpolations Between Calibration Data for Thermocouples

Two methods for evaluating thermocouple calibration uncertainty over the temperature range of the calibration are presented, when the thermocouple is calibrated at only a few temperatures. The evaluation of the uncertainty at fixed-point temperatures is well established, but it is often not clear how the uncertainty arising from interpolation between fixed points can be determined. We present a conventional method, based on that described in the “Guide to the expression of uncertainty in measurement” (GUM), and a numerically-based Monte Carlo method, for quantifying the calibration uncertainty arising from the use of an interpolating polynomial defined by calibration data. The two methods are compared and found to be in excellent agreement, but the Monte Carlo method is, in general, more flexible, e.g., when measurements are described by non-normal distributions.     

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.     

Thermoelectric Characteristic of High-Temperature Thermocouples W5%RE/W20%RE

In the temperature range (900 to 2800) K, there has been confirmed compliance with the existing national standards for thermocouple wires W5% Re/W20%Re (type A) produced in Russia. The homogeneity within a lot of wires was evaluated by measuring the emf deviations from the corresponding reference function of thermocouples constructed from the front and the rear sections of paired coils of wires. The diameter of the wires amounted to 0.35 mm and 0.5 mm. Stability indicators were thermal emf changes after annealing for 2 hours at 1773 K. It was found that the inhomogeneity of thermoelements did not exceed (4 to 5) K for paired wire coils with a thermoelectric stability within a temperature equivalent of (1.0 to 1.5) K. EMF deviations from the reference table values for the thermocouples investigated did not exceed 1 % in the temperature range of (900 to 2773) K. Such deviations meet the requirements of the new draft of IEC standards 60584-1 and 2. Thermocouples were calibrated in four laboratories by comparison with various standard temperature gauges (type B thermocouple, radiation pyrometer, standard specimens of thermoelements). Measurements were carried out under vacuum, argon, and hydrogen. Depending on the calibration method, the expanded uncertainty of the measurements at 1773 K varied from (2.8 to 8) K.     

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...     

Assessment of Temperature Measurements Using Thermocouples at NIS-Egypt

Thermocouples are increasingly used in industry and research. For many industrial heating processes, particularly those carried out at high temperatures, a thermocouple is the most convenient and simple instrument for temperature measurement. In some instances, it is the only feasible method. The aim of this study is to select and recommend the best thermocouples from both base and noble metals to users in industrial and scientific institutions. Different types of thermocouples and calibration methods are described. From this work, the Nicrosil–Nisil thermocouple has been proposed as the best base metal thermocouple and the Au/Pt thermocouple is the most recommended as a substandard up to 1,000 °C.     

基于LabVIEW的热电偶动态特性测试系统

随着航空技术的发展,对热电偶的动态特性评价提出了更高的要求。针对现有热电偶采集方案不能连续采集以及无法实时获取温度值的问题,设计了一套基于LabVIEW的热电偶动态特性测试系统,并与示波器采集方案进行数据比对。经试验验证,该测试系统可靠性高,自动化程度高,有效地提高了工作效率,为温度传感器相关性能的评价提供了支撑。     

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。     

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.     

Consideration of the Nonlinearity of the Calibration Characteristic of an Ordinary Thermometer when Measuring Temperature Drops with Differential Thermocouples

The formula for calculating the temperature drop using differential thermocouples is analyzed. A more accurate formula for calculating the drop is derived, which describes the relation between the measured temperature drop, the mean temperature of the measurement and the nonlinearity of the calibration characteristic of an ordinary thermocouple. A comparative estimate of the calculation of the drop from the known formula and the proposed formula when using an ordinary TPP10 thermocouple is made.     

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