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Over the medium temperature range (from $$156\,^{\circ }\mathrm{C}$$ to $$1000\,^{\circ }\mathrm{C}$$ ), radiation thermometry is usually established within a national metrology institute (NMI) by means of variable temperature blackbody radiation sources, whose temperature is determined using a platinum resistance thermometer or thermocouple, calibrated in terms of the International Temperature Scale of 1990 (ITS-90), positioned in close proximity to the back radiating surface of the blackbody. It is also reasonably common to establish a scale using a suitable radiation thermometer, such as an indium gallium arsenide (InGaAs) detector-based narrow band radiation thermometer, calibrated using a number of fixed-point blackbody sources from the indium (In) to silver (Ag) (or copper (Cu)) points, with the calibration results fitted using a parameterized Planckian interpolation function. During 2007 and 2008, two InGaAs-based radiation thermometers were circulated around seven NMIs within the European Association of National Metrology Institutes (EURAMET) region in order to undertake a comparison of parameters necessary for radiation thermometry over the medium temperature range. Measurements were made of the size-of-source effect and gain (range) ratios of the two thermometers along with an assessment of the effect of changes in the ambient temperature and humidity on the thermometer output. The thermometers were also calibrated using fixed-point and/or variable temperature blackbody sources at each institute. A brief overview of the results obtained by this project is presented in this paper.

A comparison of a Pt/Pd calibration above the Ag point between the INRIM and NMC was arranged with the aims of evaluating measurement systems and exploiting the potential of the Pt/Pd thermocouples. Two commercial Pt/Pd thermocouples were used as transfer thermometers. A calibration method using a blackbody cavity as a transfer source and a radiation thermometer as a reference thermometer was adopted in both institutes. The T 90 carried by the radiation thermometers is established by an extrapolation technique for INRIM and by scale realization according to ITS-90 definition for NMC and, therefore, this exercise is also a useful comparison of different approaches to disseminate T 90 above the Ag point. The comparison results are presented and analyzed.

The calibration of platinum-based thermocouples from 420 °C to 1,100 ˚C is currently carried out at INRIM making use of two different apparatus: for temperatures below 930 ˚C, a potassium gas-controlled heat pipe (GCHP) is used, whereas a metal-block furnace is adopted for higher temperatures. The standard uncertainty of the reference temperature obtained in the lower temperature range is almost one order of magnitude better than in the higher temperature range. A sealed copper cell was investigated to see if it could be used to calibrate thermocouples above 930 ˚C with a lower uncertainty than our current procedures allowed. The cell was characterized with Type S and Pt/Pd thermocouples and with an HTPRT. The freezing plateaux were flat within 0.01 ˚C and lasted up to 1 h with a repeatability of 0.02 ˚C. The temperature of the cell was determined with a standard uncertainty of 0.04 ˚C. Hence, the copper cell was found to be superior to the comparator furnace for the calibration of platinum-based thermocouples because of the significant decrease in the uncertainty that it provides. An analysis was also carried out on the calibration of Pt/Pd thermocouples, and it was found that the combined use of the potassium GCHP and the Cu fixed-point cell is adequate to exploit the potential of these sensors in the range from 420 °C to 1,084 °C. A comparison with a fixed-point calibration was also made which gave rise to agreement within 0.07 ˚C between the two approaches.

The high-temperature extension of the fixed-point technique for primary calibration of precision infrared (IR) thermometers was investigated both through mathematical simulations and laboratory investigations. Simulations were performed with Co–C (1,324°C) and Pd–C (1, 492°C) eutectic fixed points, and a precision IR thermometer was calibrated from the In point (156.5985°C) up to the Co–C point. Mathematical simulations suggested the possibility of directly deriving the transition temperature of the Co–C and Pd–C points by extrapolating the calibration derived from fixed-point measurements from In to the Cu point. Both temperatures, as a result of the low uncertainty associated with the In–Cu calibration and the high number of fixed points involved in the calibration process, can be derived with an uncertainty of 0.11°C for Co–C and 0.18°C for Pd–C. A transition temperature of 1,324.3°C for Co–C was determined from the experimental verification, a value higher than, but compatible with, the one proposed by the thermometry community for inclusion as a secondary reference point for ITS-90 dissemination, i.e., 1,324.0°C.

Below the freezing point of silver, radiation thermometers are generally calibrated by implementing the multi-point interpolation method using blackbody measurements at three or more calibration points, rather than the ITS-90 extrapolation technique. The interpolation method eliminates the need to measure the spectral responsivity and provides greater accuracy at the longer wavelengths required below the silver point. This article identifies all the sources of uncertainty associated with the interpolation method, in particular, those related to the reference blackbody temperatures (either variable-temperature or fixed-point blackbodies) and to the measured thermometer signals at these points. Estimates are given of the ‘normal’ and ‘best’ uncertainties currently achievable. A model of the thermometer response is used to propagate all the uncertainties at the reference points and provide a total uncertainty at any temperature within the calibration range. The multi-point method has the effect of constraining the total uncertainty over this range, unlike the ITS-90 technique for which the uncertainties propagate as T 2. This article is a joint effort of the working group on radiation thermometry of the Consultative Committee for Thermometry (CCT), summarizing the knowledge and experience of all experts in this field.

At INRIM, different Co–C fixed-point cells have been constructed and investigated. Two cells of different design and volume and filled with highly pure cobalt (99.998%) were used to extend the fixed-point calibration of five Pt/Pd thermocouples that had been previously calibrated at the triple point of water and at the fixed points of In, Sn, Zn, Al, and Ag. The calibration at the Cu point was also added during this exercise. Because a previous calibration from 962 °C up to 1,500°C against the local standard radiation thermometer was available, a comparison was possible with the Co–C fixed-point calibration. Agreement within 0.10 °C was found when the value of 1,324.0 °C, the same value proposed for the Co–C point to be included as a secondary reference point of the ITS-90, was assumed.

Five Pt/Pd thermocouples, constructed and calibrated at IMGC at fixed points in the temperature range from 0 °C to the Ag point, were calibrated by comparison with the local primary standard radiation thermometer with the aim of replacing the presently used Pt/Pt–Rh alloy thermocouples as secondary reference standards up to 1500 °C. To fully exploit accuracy of Pt/Pd thermocouples, high-level calibration techniques need to be adopted. For this purpose, a new high-temperature three-zone furnace was arranged and characterized in order to obtain the best axial uniformity and a specially designed blackbody cavity was used as a transfer source for calibrating the thermocouples in the temperature range from 962 °C up to 1500 °C. At the end of the comparison measurements, additional calibrations at the Ag fixed point were made, in order to check the stability of the thermocouples' signals. A comparison between experimental results and the reference function is presented and an extrapolation of the fixed-point calibration data is analysed.

The primary purpose of the comparison was to establish the equivalence of the calibration of infrared radiation thermometers within Europe in the temperature range 800 °C to 2000 °C. Seven EU national metrological institutes (NMIs) participated. The equivalence of calibrations was determined by circulating a transfer standard infrared radiation thermometer between the participating laboratories. The calibrations obtained were compared with the reference calibration performed by the co-ordinating laboratory (National Physical Laboratory). It was found that, after correction for the size-of-source effect, the calibrations of the participating laboratories nearly all agree to better than 0,1 % of temperature.

A precision infrared thermometer was constructed and was used to study an absolute calibration technique between the freezing points of tin (232°C) and copper (1085°C). This technique is based onto a calibration of the thermometer at four freezing points (Sn, Zn, Al and Cu) and on the assumption of an empirical equation to relate the output signal of the thermometer to temperature. It was found that the thermometer can be calibrated in terms of ITS-90 temperatures with an uncertainty at a 99% confidence level of ±0.04°C. This paper reports on the past achievements as well as on the developments under way that are aimed to a simplification of the design and operation of the thermometer. These developments include the replacement of the Si PbS detector previously used with an InGaAs photodiode and the design of a much simpler optical system.

Radiation temperature scales were realized with a precision infrared thermometer at IMGC, NPL and UME using fixed-point blackbodies at the freezing points of indium, tin, zinc, aluminium and silver. From the fixed-point calibration in each laboratory, continuous scales were obtained with a four-coefficient interpolation equation. The differences between the three scales were well within 0.1 degreesC at all temperatures between the indium and silver points. Calibrations of the same thermometer performed at NPL and UME using a more traditional calibration scheme based on the comparison with contact thermometers showed differences of up to more than 0.4 degreesC. This suggested that the fixed-point scheme, that has been used for the first time in an international comparison, should be more reliable for accurate calibrations of precision infrared thermometers.

Cylindrical blackbody cavities are analysed in view of their use as transfer sources for comparing pyrometers. Attention is given to the effect of temperature gradients on the comparison accuracy. It is shown that temperature gradients have a limited effect even when the working wavelengths of the pyrometers to be compared are largely different.