Measuring regime and the accuracy of the transient hot-ball method


Ľ. Kubičár1, U. Hammerschmidt2, D. Fidríkova1, P. Dieška3 and V. Vretenár1

1Institute of Physics SAS, Bratislava, Slovakia
2Physikalisch-Technische Bundesanstalt, Braunschweig, Germany
3Faculty of Electrical Engineering and Information Technology STU, Bratislava, Slovakia

Keywords: hot-ball method, accuracy
property: thermal conductivity
material: glycerol, distilled water

Present contribution discusses the principle of the hot ball sensor for measurement of the thermophysical parameters. Operation of the sensor is based on a ball that delivers heat in a stepwise regime to the surrounding material. The ball surface temperature is a measure of the thermophysical parameters of the surrounding material. Depending on evaluation technique the thermal conductivity or specific heat, thermal diffusivity and thermal conductivity from the temperature response can be determined. Both techniques strongly depend on a proper choice of the corresponding time interval of the temperature response. Simulations and the uncertainty analysis have been performed to analyze the hot ball working regime. Several theoretical models including the ideal – asymptotic model are considered. The role of the heat capacity of the hot ball sensor and the role of the connecting wires are analyzed. Constructions of the hot ball sensor are given and the corresponding data taken from distilled water and glycerol from the temperature range of 15oC up to 35oC are presented. Methodology of data evaluation based on approximate procedure and on the fitting of the analytical function over the experimental data is discussed. Due to irregularities of the ball a calibration method is proposed and its reliability is tested. Intercomparison of the theory, simulations and uncertainty analysis and the experimental data gives a detailed picture on the physics of the hot ball principle. Limitation of the listed models for data evaluation is presented.

References
  1. L. Kubičár, V. Vretenár, V. Štofanik, V. Boháč, Int. J. of Thermophysics, DOI 10.1007/s10765-008-0544-4

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