Research On Thermal Gas Mass Flowmeter And Compensation Arithmetics

It is necessary and important to measure the flow rates of gas mixtures in many industrial areas. But the flow rate of gas mixures, such as coal gas and natural gas et al, are difficult to be obtained because gas mixtures have some special characteristics such as large variation range, dirt, variable components, and variable temperature and pressure. The advantages and disadvantages of some conventional flowmeters used in gas mixtures were analyzed and compared, and the thermal mass flowmeter (TMF) was choosed to be the subject of the dissertation and used to measure the flow rates of gas mixtures. According to the principle of the TMF, the component and temperature variations of gas mixtures change the output of TMF, and bring large error. The dissertation studied the the temperature and component compensation arithmetics for TMF used in gas mixtures, which can eliminate the influences on the output of TMF caused by temperature or component variations. The compensation arithmetics under investigation improve the metering performance of TMF, and enlarge the serviceable ranges and fields.TMF is based on hot-wire anemometry, so the feedback control theory and transfer function method were used to study the hot-wire anemometry thoroughly. The principle of TMF was also studied in the dissertation, and the model of TMF was build and used to analyze its characteristics and performances by digital simulation. Some probe structures were studied, and the influences result from the errors of probe manufactureing and installation and the influence caused by vortex were also analyzed in detail.In order to correct the output of TMF containing temperature influence, several compensation arithmetics were analyzed in the dissertation including analytic compensation arithmetic, compensation arithmetic using two sensing elements, compensation arithmetic using single sensing element and automatic compensation by measuring bridge. And a new automatic compensation bridge was designed in the dissertation base on the conventional bridges, which breaks through some limits for temperature compensation circuitry.The influence caused by component variations of gas mixture was analyzed using the governing equations and experiment data of TMF. After studying the existent component compensation arithmetics based on analysis of heat transfer equation and based on experiment and empirical equations, the dissertation proposed new component compensation arithmetic for TMF based on the combination of the analysis of the physical property parameters and empirical equations. The key techniques for implementation the omponent compensation arithmetic were also introduced.A TMF prototype has been developed to validate the new automatic compensation bridge and the component compensation arithmetic proposed in the dissertation. The principle of the automatic temperature compensation circuitry, the function blocks and the implementation techniques were analyzed. The prototype was installed in the blowing experiment table to obtain the characteristic curves of the sensor, and the temperature compensation circuitry is also validated using this experiment table. It was proved that this compensation circuitry is correct and applicable. Then, the prototype was used in a gas company to measure the flow rate of coal gas. The field experiment result validates the effectiveness of the component compensation arithmetic proposed in the dissertation, and it also shows that there is tiny error in the compensation arithmetic and it needs further study.The temperature and component compensation arithmetics proposed in the dissertation will help to the design and popularization of TMF. The compensation arithmetics can reduce the cost and maintenance of TMF and enlarge the adaptability of TMF in gas mixtures with variable components and temperature.