The Key Technology Of Ultrasonic Gas Meter Small Flow Measurement

Ultrasonic gas meter is a non-contact measuring instrument that works by using acoustic principles.It has been widely promoted with no pressure loss,wide application range and high measurement accuracy.It is a promising gas measuring instrument.Ultrasonic flow detection technology was earlier used in liquid measurement.The application of ultrasonic gas flowmeter was about 1990 s,but domestic has only started to use ultrasonic gas flowmeter widely in recent years,and there is a certain gap between the independent research and development capabilities in this field and foreign advanced levels.The main difficulty is reflected in the improvement of measurement accuracy.The accuracy of ultrasonic gas detection is limited,especially in small flow measurement.It is difficult to break through the accuracy,resulting in the final gas flow measurement accuracy cannot be improved.Therefore,it is necessary to explore the factors affecting the measurement accuracy of ultrasonic gas flow meters.In this thesis,the measurement principle of the ultrasonic gas flow meter is described in detail,and the calculation method in measuring the flow rate by the time difference method is introduced,and the problem of zero error caused by the mismatch of ultrasonic transducers is analyzed.In addition,in the actual measurement of the analyzed gas flow rate,the time difference method calculates the linear flow velocity of the gas flow,which needs to be converted into the actual surface flow velocity by the correction factor for the final flow rate calculation.The accurate correction factor needs to consider many factors.Such as the specific design of the flow channel,the flow state of the flow field,which includes the influence of factors such as temperature,pressure and humidity in the flow field.This thesis mainly uses CFD flow field simulation software to conduct experiments.The rectangular flow channel is designed as a measuring tube.A multi-layer rectangular flow channel design scheme is realized by adding a rectifier.When the gas passes through the narrow flow channel,the rectification is completed.The experimental results show that when the number of rectifiers is 3,the change of the correction coefficient is the most gentle,and it shows a good linear change in the laminar flow state,which is suitable for obtaining a more accurate fitting equation,improving the measurement accuracy.Further experiments also show that changes in the distance between the rectifiers and the length of the rectifiers will cause varying degrees of correction coefficients.Selecting the appropriate number of rectifiers and the length of the rectifiers will improve the measurement accuracy.Based on the narrow flow channel,the simulation calculation research is carried out on the influence of the relative humidity change on the gas flow rate,and the measurement error change caused by the relative humidity change is verified in the actual measurement of the narrow flow channel ultrasonic gas meter.In order to solve the change of the adhesive force of the rectifier under the change of relative humidity,a nano ceramic fluorine coating with hydrophobic and oleophobic properties is used on the surface of the rectifier.After changing the material properties of the contact wall,the measurement results obtained can greatly reduce the fluctuation of the error and avoid the situation that the measurement accuracy changes with the relative humidity.Finally,theoretical analysis of reducing the zero-point error by matching the transducer parameters is performed,and the zero point error caused by the mismatch of the transducer is measured on the established experimental platform.The problem of zero drift caused by temperature changes is explored,providing more practical research for further improving the accuracy of ultrasonic flowmeters.