J. Bojkovski

Methods of reducing the uncertainty of the self-heating correction of a standard platinum resistance thermometer in temperature measurements of the highest accuracy

Self-heating of resistance thermometers is a well-known phenomenon, which occurs when the measuring current additionally heats up the sensing element. In the paper, the self-heating of standard platinum resistance thermometers (SPRTs) is studied with special emphasis on the investigation and evaluation of factors which contribute to the uncertainty of the self-heating correction. The basic two-current method for self-heating correction is analysed and additional correction methods are proposed, based on the optimal selection of currents and the use of more than two different currents. Using the advanced methods we can decrease the uncertainty of the self-heating correction from 0.04 to 0.01 mK. This decrease may not be significant in routine SPRT calibrations, but it can present an improvement in measurements of the highest accuracy, such as intercomparisons.

Final Report on EUROMET.T-K7: Key comparison of water triple point cells

From 2002 to 2004 the Consultative Committee for Thermometry (CCT) carried out a Key Comparison (KC) of water triple point cells (CCT-K7). In 2005 the Regional Metrology Organization (RMO) EURAMET decided to initiate an additional KC of water triple point cells, EUROMET.T-K7, to extend the metrological equivalence to a larger number of national metrology institutes (NMIs) from the EURAMET region. The EUROMET.T-K7 comparison was carried out in 2006?2008 with the coordination of VSL and the participation of 24 EUROMET NMIs, 9 of them having participated in CCT-K7 as well (8 out of 9 used the same transfer cell they had used in CCT-K7). This report presents the results of this comparison, provides detailed information of the measurements performed by all participating laboratories and links the comparison results to the CCT-K7 comparison. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCT, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Measurement uncertainty, traceability and evaluation of test results in testing laboratories

The objective of this paper is to elaborate new elements related to metrological analysis in the field of testing, such as measurement uncertainty and traceability. Until now the international standard ISO 45001 did not explicitly require the uncertainty specifications in the area of testing to such an extent as the newly implemented standard ISO/IEC 17025. Therefore several additional steps should be performed in specifying the measurement and testing results, especially concerning performance of testing and other conformity assessment activities. The paper is focused on uncertainty analysis of a test procedure of the electrical safety of household appliances according to the European standard EN 60335-1. The traceability of measurements is presented. The example is chosen as a useful case study, as well as a very illustrative example, where many dilemmas could be highlighted. This particular case is relatively straightforward to evaluate due to the relative ease of traceability of electrical and thermal quantities. It is important that the measurement result and its uncertainty are correctly evaluated and on that basis the right conclusion of conformity or nonconformity with specifications is made. Therefore, knowledge and awareness of all facts about calibration, testing requirements and traceability are essential.

Automated system for measuring temperature profiles inside ITS-90 fixed-point cells

The defining fixed points of the International Temperature Scale of 1990 (ITS-90) are temperature reference points for temperature calibration. The measured temperature inside the fixed-point cell depends on thermometer immersion, since measurements are made below the surface of the fixed-point material and the additional effect of the hydrostatic pressure has to be taken into account. Also, the heat flux along the thermometer stem can affect the measured temperature. The paper presents a system that enables accurate and reproducible immersion profile measurements for evaluation of measurement sensitivity and adequacy of thermometer immersion. It makes immersion profile measurements possible, where a great number of repetitions and long measurement periods are required, and reduces the workload on the user for performing such measurements. The system is flexible and portable and was developed for application to existing equipment in the laboratory. Results of immersion profile measurements in a triple point of water fixed-point cell are presented.

Simultaneous calibration of a large number of thermocouples

The objective of this paper is to analyze major error sources in the process of simultaneous calibration of a large number of thermocouples (TCs). The main reason for the occurrence of additional error sources when a large number of TCs are simultaneously calibrated is due to additional inhomogeneity effects and characteristics of a particular measuring setup. The need for such calibrations is due to the fact that, often, a large number of relatively low-cost but traceably calibrated TCs are needed for monitoring purposes and measurements of temperature profiles in applications, such as evaluation of temperature and humidity chambers, wind channels, tunnel furnaces in steel plants, etc. The fact is that the overall uncertainty attributed to each particular TC during simultaneous calibration exceeds the uncertainty assigned to a TC during a calibration process of a single TC. Nevertheless, in most cases, the proposed solution is highly acceptable, especially in the area of testing, due to a significant decrease of calibration costs yet still meeting testing requirements.

Automation of reading liquid-in-glass thermometers

In this paper, the automation of calibration of liquid-in-glass thermometers (LiGTs) is discussed. Temperature readings are taken automatically with the use of a measuring system that consists of a standard calibration setup, a video camera and illumination, an image acquisition module and a computer that executes the algorithm for image processing and analysis. Once the reading of the temperature is extracted from the acquired image, this value can be passed to any other data processing program, which includes the comparison with the values from the reference thermometer and determination of correction and combined uncertainty. Data achieved in this way can be directly used to prepare the calibration certificate.

Magnetic effects on thermocouples

Thermometers in laboratory environment and industrial applications are often subject to extraneous, usually unwanted and uncontrolled magnetic fields. Magnetic field influence can be minimized, but cannot be fully cancelled out. Even more, in most cases, there is no awareness of the existence of magnetic fields, let alone their effect on measurement instrumentation. In the past, sensitivity to high dc magnetic fields has been investigated in cryogenics and at high temperatures. More recently, the magnetic effect on weak dc magnetic fields was presented. The goal of this paper was to analyse and empirically and experimentally prove the magnetic sensitivity of thermocouples exposed to low magnetic fields: both dc and ac. Precision and uniform alternating and direct magnetic flux densities were generated by means of permanent magnets and power amplifiers with air-cored coils. The magnetic effect on ferromagnetic and non-ferromagnetic thermocouples at liquid-nitrogen-boiling point (−196 ◦ C), ice point (0 ◦ C), in water (17 ◦ C) and at melting point of gallium fixed point cell (29.7646 ◦ C) was investigated. Magnetic-field-dependent temperature errors of up to 700 mK (at 5.3 mT: dc) and up to 1 ◦ C (at 10 mT: ac 50 Hz magnetic fields) were detected. From the results, it can be concluded that, ideally for temperature measurements of the highest accuracy in the above-cryogenic temperature range, magnetic sensitivity should be estimated and taken into account either as the correction of an error and/or as an additional source of measurement uncertainty. Special consideration should be given to thermocouple orientation relative to the magnetic field direction, influence of metal enclosures and magnetization effects on ferromagnetic components of thermocouples.

Automated system for evaluation of climatic chambers

In this paper an automated system for the evaluation of climatic chambers is presented. Besides automation, a basic emphasis is given to the reliability of operation, flexibility and the possibility of achieving a minimal uncertainty for the given application. The system is developed to completely satisfy all standards and requirements concerning evaluation and/or calibration of climatic chambers in the laboratory as well as on site.

Automation of a precision temperature calibration laboratory

The objective of the paper is to discuss the automation of a precision calibration laboratory in the view of achieving maximal quality of metrological performance. Besides automation of measurement equipment for ease of operation, communication and data processing, basic emphasis is given to reliability of operation, flexibility, and possibility of achieving minimal uncertainty for given measurement equipment. Several software design guidelines are discussed in the case of automation of a high-precision temperature calibration laboratory.

Acceptable traceability of a distributed national metrological system

Traceability of national standards to the international level, as well as the quality of the dissemination of these values to lower level working measurement standards has to meet primarily the requirements of a particular country, but equally importantly has to comply with strict and explicit rules of international accreditation bodies. In this respect vertical, hierarchically organised traceability links are usually properly evaluated in terms of added uncertainties, but horizontal connections, forming closed loops could be overlooked. It is essential to assure minimal influence from one physical quantity to another by proper uncertainty ratios. In many countries, especially in small and developing countries in transition period from one economic system to another, national metrology systems, as a key technology for development, production and technology transfer are often organised as distributed systems of laboratories; holders of national standards, as opposed to centralised systems, which are traceable to the international level, i.e. laboratories, where primary standards are realised. It is usually also not foreseen to realise SI units according to the definitions but on the highest level of transfer technology and disseminated with minimal additional uncertainties.