Research And Optimization Design Of High Precision And Small Flow Ultrasonic Gas Flow Meter

Gas flow measurement plays an important role in chemical industry,medicine,environmental protection and daily life.Ultrasonic gas flow meter is widely used in the field of gas flow measurement because of its advantages such as non-contact,stability and reliability,high precision and simple to install.Due to the particularity of gas and the variability of measurement conditions,performance of the ultrasonic gas flow meter is not perfect enough,which needs to be studied and analyzed further,especially for small flow measurement which is still underdeveloped and used in a small scope.Therefore,it has theoretical and practical significance for the study of gas flow measurement and optimal design of the measurement system based on ultrasonic detection.In this thesis,a design scheme of gas flow meter based on ultrasonic detection that uses time difference is proposed.Firstly,the flow measurement mechanism is studied.A high precision detection method to measure the gas flow in a small tube diameter is designed.Starting from the uncertainty of measurement of ultrasonic gas flow meter,we point out and analyze concretely the key factors of measurement accuracy in detail in this thesis;A parallel single channel mounting method which can increase the ultrasonic sound propagation and reduce the ultrasonic attenuation coefficient is used;Under the premise of designing a theoretical calculation formula,the temperature and pressure compensation of the system is increased,and its detection accuracy is improved;Combining with theoretical analysis,we establish a ultrasonic signal conditioning system which is from the PIC micro-controller,ultrasonic transducer,switching circuit,amplifier and filter module,comparison module and finally returns to the micro-controller in this thesis;The NTC thermistor temperature measurement circuit and XGZP gas pressure sensor module are designed to measure temperature and pressure of gas medium;In the part of software design,we explain the gas flow measurement process of ultrasonic gas flow meter in detail,and focus on the method of ultrasonic propagation time measurement based on level comparison method.Through the serial communication,the measured data of ultrasonic propagation time,temperature and pressure data are uploaded to the upper computer to integrate them with the standard gas flow data.Using the least square surface fitting by MATLAB to obtain the correlation coefficient of empirical formula,then sending the coefficient to the lower computer system and backing it up to EEPROM;In the actual testing process,the removal coefficient is calculated,and the calculation results are verified by the host computer.After qualified verification,the digital filter is used to get the real gas flow value.In this thesis,concrete design scheme of the system and specific experimental scheme are given.The two-component mixed gas of oxygen and nitrogen is tested in the experiment,results of the experiment are compared with measurement results of standard gas flow meter.Flow measurement results are analyzed,the test and error correction scheme are put forward.The experimental results show that the ultrasonic gas flow meter designed in this thesis can run stably and meet the expected target,which can realize the high precision measurement of gas flow in the tube diameter.The maximum error of flow is less than 0.1L/min.