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Encapsulation method for small wireless measurement systems in high temperature environments
Keywords: wireless sensor, phase change, industrial process
This paper presents an encapsulation concept that enables the construction of small size wireless measurement systems that can operate in environments with ambient temperatures of up to 1200 C. The target is to be able to perform measurements of temperature and oxygen content in high temperature process flows, e.g. in the mining industry or power generation plants. One major challenge in designing measurement equipment for such environments is that electronics and batteries normally are limited to about 125 C. Common constraints include a maximum size of the system as well as a minimum operating life in the high temperature environment. To meet these constraints, thermal insulation is combined with water absorbed in a porous material in the core of the insulation. As the internal temperature of the device rises, the high specific heat of the water absorbs the bulk of the thermal energy conducted through the thermal insulation. Furthermore, as the temperature reaches the boiling point of the water, energy is absorbed as the water changes phase from fluid to gaseous. During this phase change, the internal temperature of the device is held constant at 100 C, allowing electronics and batteries to function. The optimal shape for this type of device is a sphere. The trade-off between the thickness of the insulating material and the volume left for the water was investigated using FEM simulations. These show that, for a number of insulating materials, and over a range of core and outer diameters, the optimum diameter for a core completely filling an insulating shell is about 67% of the outer diameter, independently of the actual outer diameter. The resulting simulated maximum operating time before the core temperature exceeds 100 C at an ambient temperature 1200 C is 21 minutes for a sphere with an outer diameter of 8 cm. If the outer diameter is increased to 20 cm the time increases to 125 minutes. Measurements were performed to verify the simulations. A sphere with an outer diameter of 8 cm was fabricated using moldable insulation material, and the core was filled with a core of porous material saturated with water. When subjected to an oven temperature of 1200 C the device held the core temperature at or below 101 C for a total of 25 minutes. The time to reach the boiling point of the water was 9 minutes. Thereafter, the temperature was held constant at 100 +/- 1 C for an additional 16 minutes whereafter a rapid rise in temperature took place once all water had evaporated. The results clearly show the feasibility of the proposed insulation concept. Future work includes measurements of radio environment in the targeted processes, as well as the complete integration of a wireless measurement system within the encapsulation.
Jonny Johansson, Associate Professor
Lule University of Technology
Lulea, Norrbotten

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