Digital optical fiber point sensor for high-temperature measurement

The development and performance of a high-temperature optical fiber point sensor based on fluorescence decay time of a chromium-doped material are reported. The device exploits a novel digital signal processing scheme for decay time measurement. It is based on a modified phase-sensitive-detection technique with the phase locked to a fixed value and the modulation frequency tracking the measured decay time. The sensor was calibrated in the temperature range from -25 to 500°C with a 0.1°C resolution. Remarkable features are the system immunity to fluorescent intensity losses and the long-term stability, showing a maximum drift of 0.3°C over more than 600 h     

Thermal metrology after the introduction of the ITS-90

The adoption of the international temperature scale of 1990 (ITS-90) is producing significant innovation in thermal metrology. The temperature range of the ITS is wider than that of the previous editions. Also the scale precision and the accuracy of the thermodynamic temperature values forming its base have been improved. A short presentation of the ITS-90 will introduce its most important features, describing briefly the types of interpolating instruments and fixed points. To realize the ITS-90, many national laboratories will have to develop within their premises relatively new techniques of measurement such as gas thermometry to interpolate between fixed points from 3 K and 24.6 K and high-temperature platinum resistance thermometry. The highest precision is acheived in the temperature range –40°C to 30° C, where the ITS-90 combines the most reproducible fixed points with the interpolating instrument exhibiting the highest accuracy and the lowest nonuniqueness. Since a reproducibility better than 0.2 mK can be achieved everywhere in that range, except in the proximity of the triple point of water where it is lower than 0.1 mK (all estimates being at the 1 level), the size of the kelvin is reporoduced to within 7·10^–6 in the worst case, and to within 2·10^–7 at the triple point of water. Some practical problems may result from the dissemination of the iTS-90 to users of temperature measurements in the temperature range 420–1085°C. They are related to the construction and use of high-temperature platinum resistance thermometers (HTPRTs), to the practical exploitation of radiation thermometry, and to the availabiality of suitable transfer standards in this temperature range. Several practical realizations of the ITS-90 that are underway in various national laboratories will enable one to better estimate its precision. Such an operation will essentially be carried out through intercomparisons, which will be successful if highly-reproducible transfer standards like the sealed cells for fixed points will be made available. The basic level of the scale precision, i.e., its irreducible component, is given by the scale nonuniqueness. Some results are already available and some experiments have recently been proposed in order to provide further data in critical areas. Another important aspect is the scale smoothness and its effect on the measured thermal properties. With the introduction of the ITS-90 the projects for the determination of thermodynamic properties have not completely stopped. Radiometric determinations and noise thermometry are mainly carried out at temperatures between 660°C and 1085°C. Other experiments involving gas thermomery may lead to better determinations below 3 K. Thermal noise measurements are presently considered for the redetermination of the Boltzmann constant, k_B. Their uncertainty should be within ± 10ppm, still too high to propose such a method for the definition of the kelvin in terms of k_B and of the SI unit of energy. As regards the determination of thermophysical properties of matter, special importance is now attributed to the temperature dependence of the vapor pressure of sodium. Such an interest in connection with the use of pressure-controlled sodium heat pipes for temperature reference. In addition, some information is provided on methods for the determination of pulsed-energy excitation. There, advantage is taken from improvements in radiation thermometry.Translated from Izmeritel'naya Tekhnika, No. 12, pp. 50–58, December, 1993.     

A comparison of ideal and real moist air models for calculating humidity ratio and relative humidity in the 213.15 to 473.15 K range and up to a pressure of 1 MPa

This study evaluates how the ideal mixture model of moist air approximates a real mixture model when determining both humidity ratio and relative humidity for the 0.1- to 1-MPa pressure range and the -60 to 200°C temperature range. The relevant thermodynamic properties are calculated using, among others, a specific algorithm based on the relationships proposed by Hyland and Wexler corrected for the new ITS-90 temperature scale.