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THE NATIONAL STANDARD OF THERMODYNAMIC TEMPERATURE

Activities of the Laboratory of Metrology and Quality

The Laboratory of Metrology and Quality operates within the framework of the Faculty of Electrical Engineering of the Ljubljana University. It was established in 1954 under the name of "Laboratory of Electrothermics", and has been operating as the Laboratory of Metrology and Quality since 1992. While the Laboratory was developing, and because of the growing importance of quality assurance, it also started dealing with quality control. More than 700 investigations for the domestic and foreign companies have been conducted by the Laboratory, comprising more than 12,000 pages. Within the Faculty, the Laboratory also performs teaching and scientific research.

The Laboratory is a holder of the national standard of temperature, and in addition, it is a calibration laboratory for temperature and humidity at the highest level in Slovenia. The Laboratory was the first in Slovenia, and among the first in Europe, to successfully pass an audit under the new standard for calibration and testing laboratories, SIST EN ISO/IEC 17025.

 

Ensuring traceability of the national standard of thermodynamic temperature to the international level

At the moment, the Laboratory of Metrology and Quality is ensuring traceability of the national standard of thermodynamic temperature in the primary laboratory of the Nederlands Meetinstituut - the Van Swinden Laboratorium - the Netherlands, whose fixpoints are in compliance with the International Temperature Scale, ITS-90. The checking of stability of the national standards (platinum resistance thermometers) is done in three-month intervals by measuring their resistance value in the triple point of water, which is the fixpoint of the International Temperature Scale, ITS-90. The resistance value in the triple point of water is also measured before and after transport of the national standards.

Dissemination of the value of the national standard to the lower hierarchical levels in Slovenia

The dissemination of the value by means of national standards is conducted following the accredited procedures for comparison calibration, to the laboratory's working standards, which are then used for calibration of customers' standards. The national standards can exceptionally be used for calibrations, e.g. when uncertainties close to the best measuring capabilities of the laboratory must be met, or in inter-laboratory comparisons.

Scientific research work in the field of the quantity of thermodynamic temperature

The scientific research work of the Laboratory of Metrology and Quality consists mainly of two fundamental tasks: establishing the primary level for temperature, and reducing measurement uncertainties in value dissemination, as well as increasing reliability. The long-term orientation of the Laboratory is the realisation of the complete fixpoint scale of ITS-90, ranging from -189 °C to 1553 °C, which would enable the Laboratory, after having made international inter-laboratory comparisons, to switch to the primary level for the given temperature range. For the time being, the LMK is covering at the primary level the range between -189 °C and 420 °C (accredited capacities) or 962 °C (the freezing points of aluminium and silver are not yet accredited).

In order to reduce measurement uncertainties in value dissemination, and to increase reliability, the Laboratory is developing analytical and computer-aided methods of maintenance and control of both group and individual measurement standards. Automation of calibration will greatly contribute to the assurance of the highest possible reliability of the results presented.

Calibration equipment

The measurement standards system is based on two sets of standard thermometers, i.e. a set of three 25.5 ( standard platinum resistance thermometers, and a set of three R-type thermocouples. The platinum thermometers cover the temperature range between -189 and 660 °C, their measuring principle being based on the measurement of resistance as a function of temperature. Resistance is measured either by means of an AC resistance bridge with incorporated standard resistors, or by means of a dc-resistance bridge with external standard resistors. The 660 to 1500 °C range is covered by thermocouples whose measuring principle is based on the measurement of thermal voltage as a function of temperature. Thermal voltage is measured by means of a nanovoltmeter.

Because of the nature of comparison calibrations, the dissemination of value to working standards is only made possible in precision calibration baths and furnaces, which have good stability and a homogeneous temperature field, with regard to the range covered by individual calibration bath or furnace. The Laboratory possesses altogether 12 baths and furnaces, with the ranges overlapping, but covering together a temperature range of -98 to 1500 oC. The standard platinum resistance thermometers are calibrated in fixpoints between -189 and 660 oC, while the standard thermocouples are calibrated in fixpoints within a range between 660 and 1553 oC.