A technique for expanding the thermal indication capabilities of standard thermal paper for studying the f eld distribution in a microwave chamber

In this work we have reviewed the methods for studying the fi eld power density distribution inside microwave heat treatment units and identifi ed shortcomings of these methods. The paper proposes a technique for extending the thermal indication capabilities of standard thermal paper and on its basis we give an example of a study of temperature distribution when a three-layer thermal indicator assembly is heated in a microwave beam-type chamber of about 600 W power. Temperature variations of the optical density of standard thermal paper in the range of 25–400 °C have been obtained experimentally by refl ectance densitometry. Based on the analysis of the temperature dependence of the change in the optical refl ectance, six regions of the standard fax thermal paper conversion were identifi ed: temperature ranges of 25–70; 70–100; 100–150; 150–210; 210–290; 290–400 °С and optical density of 0.06–0.07; 0.08–0.90; 0.91–0.99; 0.71–0.91; 0.21–0.70; 0.20–0.38 B respectively. In the fi rst region the optical density of the thermal paper does not change relative to the initial surface, in the second region there is an initial increase in the optical density. In the third and fourth areas, the maximum optical density is reached due to darkening of the leucodye, while in the third area a “smooth” surface of the thermal paper is observed, and in the fourth area a “velvety” surface of the thermal paper due to formation of microcrystals and clots of the thermosensitive layer material. In the fi fth region, there is a sharp decrease in optical density as a result of discoloration of the leucodye, and in the sixth region, there is a secondary increase in optical density due to carbonization of the paper backing. The results obtained can be used to design microwave equipment as well as to optimize microwave treatment conditions for materials and articles for food, chemical, electronic and other industries. The developed method can also be used in other areas, where it is necessary to make local measurements of the heterogeneity of temperature fi eld distribution when it is impossible to use other methods of thermometry.

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