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Hybrid mass flowmeter
Języki publikacji
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Przedstawiono propozycję hybrydowego wykonania miernika przepływu. Zastosowanie hybrydowej technologii umożliwia wykonanie powtarzalnej i stabilnej struktury mostka termicznego. Płaska struktura mostka termicznego wygodna jest do analizy matematycznej. Umożliwia również realizację bezpośredniego konwertera przepływ - częstotliwość w oparciu o mostek termiczny i symetryczny termiczny modulator sigma, delta.
The subject of this paper is the presentation of the measuring device - the flat hybrid mass flowmeter. The flat hybrid mass flowmeter has many advantages in comparision with a standard capillary tube mass flowmeter. One the internal surface of the channel, heating and sensing resistors are located. All of them have a direct thermal contact with the flowing medium. The surface, on which the heat exchange is performed, is very large. It insures not only the precise heat transfer but the measurement of the temperature of the flowing medium as well. Between two ceramic plates a channel is formated, which guides the medium alongside three borders of the ceramic structure. The ceramic structure is fixed to the base at the fourth border. The base is connected with the thermal shield which surrounds the thermal bridge structure. The hybrid technology enables the embodiment of the flat stable structuure of the thermal bridge, where the thermal conductivity in every crical place could be precisely specified which is stable and repeatable in the manufacturing process. Also a reference source in the flat hybrid structure of the thermal bridge could be precisely defined. In calorimetric devices, where heat exchange is performed, the temperature profile alongside the measuring channel is of paramount importance. It not only influences the linearity but inertia of the measuring device as well. The flat structure of the thermal bridge deals very well with all these issues. Both the proper design and localisation on the ceramic structure of heating resistors as well as balancing resistors and fixing the proper local thermal conductivity between ceramic structure and thermal shield, improves linearity of the temperature profile in balancing areas and finally insures low inertia of the complete device. A finite element method has been applied in order to evaluate the temperature distribution on the ceramic structure of the thermal bridge. By comparision of two cases with different power density and topology it is shown, that the proper topology design ensures significantly more stable structure. In this case flow of the medium very little influences the change of temperature distribution and improves linearity and inertia of the complete device. The temperature profiles alongside the measuring channel are calculated for both cases and due to the flow of medium the changes of the temperature distribution on ceramic structure. For the A/D conversion, where balancing resistors of the thermal bridge are inherent components of the A/D converter, a symmetrical thermal sigma delta modulator has been applied. In the A/D converter a synchronous heat balancing is performed. Since balancing resistors serve as heat sources and temperature sensors, there is no time delay for heat transfer, so the clock frequency can be very high (in our case it is 20 kHz). In practice, the clock frequency is limited due to the ability of a comparator to handle reliably a very small signals. Since some components are common both to the thermal bridge and the A/D converter, the presented solution has the minimum number of components which directly influence the performance of the flowmeter. The flat structure of the thermal bridge is very convenient for simulation and mathematical analysis.The symmetrical thermal sigma delta modulator, which has been designed for the A/D conversion, fits very well to the flat thermal bridge and ensures the wide dynamic range of regulation and a fast response as well as high lineraity of output signals versus delivered power.
Wydawca
Rocznik
Tom
Strony
21--26
Opis fizyczny
Bibliogr. 8 poz., rys., wykr.
Twórcy
Bibliografia
Typ dokumentu
Bibliografia
Identyfikator YADDA
bwmeta1.element.baztech-article-BWA0-0009-0003
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