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When we think of productive, efficient industrial output, many drivers may spring to mind: innovative thinking, strong research and development and of course a workforce to drive the end result. What may not come to mind is reliable temperature measurement; but, put simply, for many industrial processes this is fundamental to ensure success and efficiency.
This success crosses many diverse fields of industry ranging from aerospace to nuclear fuel production, power generation, iron, steel, glass and ceramics’ manufacture. The reliable measurement of temperatures above 1,000° C in these industries is vital to ensure competitive high quality products and services. In addition to optimal production, the aforementioned industries have other goals. These include reducing carbon emissions, improving energy efficiency, and minimising waste in the manufacturing process. There’s one way of attaining all these aspirations – improving high temperature measurement.
There is a clear case for better temperature measurement to improve industrial output. Improving industrial temperature measurement is in itself an industrial task – a chance for collaboration on a truly large scale. This is how the research encompassed in the high temperature metrology for industrial applications (HiTeMS) project came to life.
The project
HiTeMS is a three year, 15 partner research project that commenced in September 2011. Alongside collaboration from numerous prestigious European research institutes, key to the success of the project is involvement from industry itself. Private sector involvement will provide access to testing facilities for many of the new sensors and sensing technologies to be developed by the HiTeMS partners. Fundamentally, the aim of this research project is to further the development of methods and techniques that will significantly improve the practice of industrial measurement of up to 2,500° C and beyond, both in non-contact thermometry and contact thermometry. Special emphasis will be given to facilitating in-situ traceability, in accordance with the International Temperature Scale of 1990 (ITS-90).
HiTeMS will address problems in contact thermometry and non-contact thermometry.Firstly, the contact thermometry research will focus on the lifetime assessment of base metal and drift measurements of base and noble metal thermocouples; self-validation or in-situ validation for temperature sensors reaching at least 2,000° C and the determination of reference functions for high temperature, non-standard thermocouples.
Secondly, the non-contact thermometry research will focus on emissivity and reflected radiation, with the key target of achieving the previously mentioned in-situ traceability. This strand of the project will also improve corrections for varying window or path transmission, to approximately 2,500° C and aims to perform real time traceable temperature measurement for improved laser materials processing.
The HiTeMS project will address both contact thermometry and non-contact thermometry challenges via six technical work packages (WP) and one addressing knowledge transfer.
Improvements in contact thermometry
Regarding the first part of HiTeMS, the research will address the improvements needed in industrial thermometry focusing on temperatures to at least 2,000° C. It is envisaged that industrial high temperature contact thermometry will be advanced through the following work: rigorous traceable evaluation of sensor lifetime and drift characteristics (WP2); development of self validating sensors to above 2,000° C (WP3) and the development of a distributed European Union (EU) capability for the determination of non-standard high temperature thermocouple reference functions (WP6).
Significant improvements are needed in high temperature contact thermometry to benefit industry. For example, the current sensor of choice at high temperatures is the tungsten-rhenium (W-Re) thermocouple. These typically have manufacturing tolerances of more than one percent of measured temperature, such as 20° C at 2,000° C, and are known to drift by large amounts in short periods of time. This is illustrated in Figure one above, where the thermocouple electromotive force (emf) output at the Ru-C melting temperature was measured repeatedly with a single W-Re thermocouple.
Testing of contact thermometry sensors
The focus of WP2 is to establish a rigorous and standardised means of determining the lifetime characteristics of base metal thermocouples and the drift characteristics of base and noble metal thermocouples. In this field, some work has already been done; however, this project aims to go to higher temperatures and do more extensive studies, particularly with regards to lifetime versus wire diameter, and to ensure that the results are firmly grounded in traceability to ITS-90.
The research will commence by establishing the lifetime and drift testing capability. Fundamentally, temperature traceability will be assured in the facility by radiation thermometry above 1,500° C and below that temperature with ultra-stable novel Pt/Pd robust calibrated thermocouples. The lifetime will then be assessed of a number of base metal (Type K and N) thermocouples with different sheaths and wire diameters ranging from one millimetre to three millimetres, at 1,300° C.
These will be supplied in a mineral insulated, metal sheathed (MIMS) format by a number of companies from around the EU. The drift characteristics of base metal (Type K and N) thermocouples will be assessed at 1,000° C and 1,100° C for continuous use and 1,100° C and 1,300° C for short term use. The drift characteristics of noble metal thermocouples, comprising bare wire format, with alumina insulation, will be assessed at 1,100° C and 1,720° C for continuous use and also at 1,600° C and 1,820° C for short term use. In terms of size ranges, thermocouples with different sheaths and wire diameters will be tested for performance.
The culmination of this work will be dissemination of the drift testing capability to the industrial partner and to selected collaborator facilities. To ensure wide uptake of the methods developed in this WP, a standard procedure for lifetime and drift assessment of thermocouples will be developed and made available as a guide through the European Association of National Metrology Institutes (Euramet).
Sensor validation
The focus of WP3 is to develop in-situ, self-validating, contact thermometry sensors and traceable measurement methods for temperatures higher than 2,000° C in vacuum and inert atmospheres, and up to 1,800° C in oxidising atmospheres. The in-situ aspect is vital because the temperature sensor of choice at these temperatures both drifts significantly and becomes brittle, therefore cannot be removed for recalibration. Linked to this part of the project is a material capability study.
This research aims to identify the most suitable materials for the process of self-validating thermocouple construction. The main focus of our research is to test the compatibility of insulators, sheath materials and graphite at three key temperatures: 1,700° C, 2,000° C and 2,300° C. Once this compatibility test is complete, miniature HTFPs will be constructed and these will then be tested with thermocouples to monitor performance, both in vacuum and inert atmospheric conditions.
The miniature HTFP design is shown in Figure two. The second part of this activity will be an alternative method of self-validation. This part of the research will see electrical noise temperature sensors integrated with thermocouples to allow determination of thermocouple drift. Electrical noise thermometry (ENT) is a primary thermometry method which is not generally used in industry. This is because ENT cannot be used directly for industrial process control because it is intrinsically slow. It can also be used to periodically check the performance of the thermocouple and with an uncertainty of around 0.1 percent, is sufficient to correct for thermocouple drifts in use.
At the end of this chapter of research, we aim to demonstrate self-validation for the first time in an industrial environment.
Referencing non-standard thermocouples
The final WP6 within the improvements in contact thermometry research aims to establish an EU-wide capability, to determine low-uncertainty reference functions of non-standard high temperature thermocouples. The reason for this work is that several useful thermocouple types exist, but their use is restricted due to poor quality reference functions with substantial uncertainties.
What is unique in this part of the project is that HTFPs will be used for the first time to establish a thermocouple reference function. To put this into context, HTFPs have been used regularly in past research, but not to support calibrations or determination of the aforementioned reference function. The desired HTFPs for this research are under construction and traceable temperatures will be allocated by radiation thermometry. The thermocouple chosen for this research (Ir-60%Rh/Ir) will be able to function in an oxidising atmosphere of 2,000° C, with specific industrial uses.
The chosen thermocouple wire of 0.5 millimetre diameter has been assembled into two metre long thermocouples using twin bore hafnia insulation. The whole thermocouple was inserted into a tantalum sheath flushed with argon to provide support for and protection of the thermocouple wires. The sheath will avoid damaging the thermocouple wires through contact with graphite during use. The reference function will then be determined by calibration with HTFPs [Pt-C (1738° C) and Ru-C (1953° C)] to nominally 2000° C, and by comparison with radiation thermometry to nominally 1600° C.
The homogeneity of the thermocouple and its development over time is likely to be a very important uncertainty factor. This will be investigated through homogeneity measurements both before and after reference function determination. Once WP6 research reaches a successful conclusion, a capability for non-standard thermocouple reference function determination for EU-wide use will be available. This will open the door for new types of thermocouples that are not being considered for industrial use due to a lack of suitable reference points and functions.
Improvements in non-contact thermometry
The remainder of the HiTeMS workpackages will address problems associated with high temperature non-contact thermometry. It is envisaged that by using a problem solving approach, improvements will be developed by several methods. Firstly, emphasis will be placed on correcting for the major problems of emissivity and reflected radiation (WP1), with the overall aim of achieving low uncertainty in-situ traceability to ITS-90. Secondly, the use of in-situ HTFPs will improve industrial process control through windows or by fibre optics, by quantifying changes in transmission up to and above 2,500° C (WP4).
Finally, an industrial implementation of the latter will be demonstrated (WP5) in the context of laser hardening. This coupled with the use of newly developed facilities for determining the emissivity of metal surfaces at high temperatures, will for the first time in this challenging process achieve real time, traceable, non-contact surface temperature measurement of more than 1,000° C.
Traceable and accurate measurement
The aim of WP1 activity is to dramatically reduce uncertainties of industrial non-contact surface temperature measurement, while retaining traceability to ITS-90. It is anticipated that two problems will be tackled: the influence of reflected radiation and the unknown emissivity. This will be achieved by determining, within the measurement field, locations where the temperature is known.
This information, coupled with a simple model, will allow corrections to be determined for emissivity and reflected radiation. An additional challenge to this strand of the research is the fact that the output of a radiation thermometer is dependent on its target size. The knock on effect for industrial thermometers is that viewing large radiating materials leads to a large and unknown uncertainty in the temperature measurement.
This problem will be addressed focusing on the effects on radiation thermometers, line scanners and thermal imagers. To establish the necessary temperature reference points, three approaches will be used: • Ultra-violet, multi-wavelength radiation thermometry • Traceable gold cup radiation thermometry • Active, two-colour radiation thermometry
By using this three pronged approach, reliability of measurement will be guaranteed, facilitating low uncertainty temperature measurement.
Correcting non-contact thermometry techniques
The fundamental aim of WP4 is to develop methods to correct for well known drift sources in non-contact thermometry, focusing on temperatures ranging from 1,500° C to 2,500° C. Although the source of such drifts are well known either through unpredictable output changes of the non-contact thermometer or transmission path fluctuations, due to, for example, contamination of viewing window, there has been, to date, no totally reliable way of correcting for these problems.
This project will ensure that tools are available so that industry can correct for such problems. For the project, HTFPs will be built, operating at temperatures up to 2,475° C. In addition, an algorithm will be developed to enable real time correction for the radiation thermometer output change, facilitating optimum process control. If the desired outcome is achieved this will be a great step forward, because no universally applicable solution currently exists for this problem.
Exploring exotic thermal processes
WP5 focuses on the development of dependable traceable industrial temperature measurement methods, specifically looking at thermal processing by laser hardening. These measurements are very challenging due to the rapid heating, the unknown surface emissivity, the small target sizes, and contamination of windows used for viewing and controlling the process.
These measurement challenges will be solved by the following two approaches. Firstly, the development and integration of a reliable HTFP into the industrial process will be made. This will enable corrections for any instability in the control pyrometer and also for corrections of the changing transmission of the window and fibre-optic bundle, used to transmit the light from the industrial process to the control pyrometer. Secondly, the research will determine the emissivity of surfaces at appropriate temperatures, of typical materials that are hardened by industrial laser processing.
Once complete this work will facilitate accurate in-situ non-contact temperature measurement, both causing a step change improvement in process control and introducing proper traceability in a major industrial sector.
Enabling knowledge transfer
The seventh and final work package will focus on the important role of the project’s knowledge transfer capability. Key to this knowledge dissemination is a stakeholder committee, currently comprised of nearly 70 members across the EU. This stakeholder committee is open to anyone with an interest in improving high temperature measurement. Members are drawn from a wide variety of stakeholders: industry, research and universities. Due to the magnitude of this project it was only feasible thanks to the cooperation of numerous institutes and industrial partners. While regrettably there are too many to give full credit to in this article, the full listings are available at http://projects.npl.co.uk/hitems/partners.html
In summary
The HiTeMS project is anticipated to bring great advances in both contact and non-contact thermometry. As highlighted in this article, we’ll break research ground in a number of areas, such as correcting non-contact thermometry measurement and mitigation of contact thermometry sensor drifts at high temperatures. The ultimate aim is to provide better ways for high temperature industries such as iron, steel, aerospace and nuclear fuel to undertake high temperature measurements with greater confidence, thereby improving their processes, products and competitiveness. The objective is to disseminate the outcomes this project as widely as possible for the benefit of industry.
References
1. The ITS-90 is the temperature scale that is in use throughout the world. Its objective is to establish a scale that is precise and reproducible and a close approximation to thermodynamic temperature. More information can be found at: www.bipm.org/en/publications/its-90.html.
Published: 13th Jun 2012 in AWE International
