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Thermal Analysis - Fundamentals and Applications to Polymer Science Part 5

Tham khảo tài liệu 'thermal analysis - fundamentals and applications to polymer science part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Page 52 1.55 m SOOŨK 4Q00K 50ƠK. f Ỵ X2000K ioddkX.VJk wavenumber ftmiTi Wavenumber Ạirrì 1.1 spm 1 in 1Đ - 101 Figure 4. 7. A Intensity distribution of IR furnace as a function of wavenumber at various heater voltages I 50 II 75 III 100 of maximum heating voltage. B Intensity distribution of a black body emitter as a function of wavenumber at various emitter temperatures sample temperature. It is essential that the furnace in no way affects the microbalance mechanism. At high temperatures radiation and convection effects can disrupt the microbalance. Various geometries and configurations of microbalance coupled with the use of radiation shields and convection baffles are employed to combat these effects. The operation of the furnace is controlled by a temperature programmer which ensures a linear heating profile over a range of heating rates. A variety of electronic control circuits based on the PID proportional integral differential principle are used to provide linear heating profiles. The parameters of the PID circuit are normally set by the manufacturer for optimum performance under Page 53 0 LL__L_ I_I A__ 0.1 1-0 10 WflMenumbBrẠưn Figure 4.8. Transmittance of quartz as a function of wavenumber the standard operating conditions of a dry nitrogen atmosphere at 105 Pa and a heating rate of 10 K min. However when working well outside these conditions particularly at high heating rates or under isothermal conditions it is advisable to check the temperature profile of the furnace for overshooting which can significantly raise the sample temperature above the programmed value sometimes with disastrous results. Many instrument software packages allow the PID parameters to be altered to avoid this problem. Even with the best furnace and temperature programmer design there is a difference between the furnace temperature Tr and the sample temperature Ts . This temperature difference is influenced by a number of factors. A thermocouple placed anywhere within .