Measurement Technology And System Design Of The Thermal Gas Flow

With the development of automation in industrial process, the demand for flow measurement increase.As a complicated technology,it need to study the measurement according to the fluid's variety, flow state and application.Thermal mass flow measurement is based on heat transfer principle. Although the measurement has some advantages such as low press loss, large dynamic range and high preicison in practical application, it still has some limits. Study should be done to it closer in various conditions, large pipeline and disoder flow field.Study on the thermal gas mass flow measurement was proposed in the dissertation. When single sensor was used, the measurements based on temperature compensation and temperature control were introduced in order to overcome the shortages of the conventional temperature compensation. With the increment of the pipeline ,the multi-sensor fusion measurement and instrument were designed to solve the single gas flow measurement. In the disorder flow field resulted from the irregular pipeline, the measurement based on GA and LS-SVM was presented to improve the the precison.The main work and contributions are as follows:1) As the compensation resistor is non-linear and the output voltage of the sensor is related with the sensing resistor,the measurement is not accurate. The single sensor measurement based on temperature compensation was proposed. The sensing element was a probe made up of sensing resistor and compensation resistor. Firstly, the operation current of the probe was determined by study of the temperature characteristics at different surrounding temperatures and various currents. Then, the relationship between the sensing resistor and the compensation resistor on the probe was analyzed. The new sensor circuit with the function of temperature compensation was developed to decrease the influence of the surrounding temperature. Finally, the measurement model of the flow measurment was established according to the data from the experiments. The research results showed that the measurement have good consistency and high accuracy.2) As the conventional constant temperature compensation employs an additional circuit, the compensation process is complex and cann't improve the result greatly.The single sensor measurement which used temperature control was proposed.The sensing element was platinum resistor PT20. Without the other compensation circuit, the constant temperature algorithm was designed to control the resistor at two different temperatures, and the gas flowrate was obtained from the output voltage of the sensor. The research results showed that the measurement can improve the accuracy.3) A multi-sensor measurement was introduced in order to solve the problem of the single sensor flow measurement. The four platinum resistors PT20 were distributed in pipeline according to the Log-linear method. The data elimination method was adopted based on the fluid flow distribution. The least weighted data fusion algorithm was designed and the mass flow was obtained. The temperature fusion corrected the mass flow using the math correcting method.4) As the disorder flow field resulted from the irregular pipeline influenced the flow measurement, the method based on LS-SVM was proposed for the gas mass flow measurement. Four thermal flow sensors were distributed in the pipeline in order to acquire the flowrate of different characteristic point. Through the calibration test, the voltages of the sensors were obtained at different mass flowrate. From these data, the gas mass flow model was established based on GA and LS-SVM. In the measurement process, the mass flow was computed using the model. The experimental reseach showed the mesurement was valid.5) Based on the multi-sensor mass flow measurement, the gas flow instrument using ARM was developed. The expansibility and reliability of instrument were considerated.