engleski

Computer-Controlled Apparatus for Thermal Conductivity Measurement based on Transient Hot Wire Method

A fully automated computer-controlled transient hot wire apparatus is developed and tested, which allows continuous measurement of thermal conductivity of reacting solid, dry and wet powder and granular materials. Single, thin (d=76 μ m) platinum wire is used as both a constant power heater and temperature sensor. The wire is heated by placing a constant voltage across the Wheatstone bridge. The supplied voltage has an output range of 0– 25V and is stable to within ± 10μ V. Change in the wire electrical resistance is determined by measuring the unbalanced voltage of a precision Wheatstone bridge during heating period. This voltage is read by a computer via an 8 channel 10-bit A/D converter. Measuring apparatus is connected to a personal computer via RS232 protocol that provides the sampling rate of 15ms. Specially designed programs with a graphical user interface provide control of the automatic measuring apparatus and easy usage. The core of the measuring apparatus is a microcontroller PIC 16F877 which was programmed in MPLab by a direct RISC instructions and ICD-2 programmer/debugger. The mould containing the investigated sample is obtained in the following manner. The specimens are cast in tube containers (plastic or copper) with inner diameter 2R ~ 51 mm, length about 250 mm and thickness of about 1 mm. The Pt wire was placed in the center of the tube. A Pt wire supported by a tensioning system is placed in the tube, the frame being next to the inside surface of the tube. The tube is sealed and placed in the temperature controlled water bath (± 0.03°C). The results of the apparatus evaluation on reference materials indicated an accuracy of 3% and a precision of 0.7% (for 95% confidence).

Computer-controlled apparatus; Thermal conductivity; Transient hot wire method; Computer program; Graphical user interface.

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