| Gas ultrasonic flowmeter is a new type of electronic flowmeter with advantages of high precision measurement,no additional pressure drop,and self-diagnostic function and so on,which is more remarkable gradually in the field of natural gas measurement than mechanical flowmeter.However,the development of domestic flowmeter is late and measurement accuracy has obvious disadvantages,and foreign flowmeter occupy the major domestic gas market so that local enterprises develop slowly.Therefore,improving the measurement technology of flowmeter is the primary task at present.This paper is based on the time-difference method working principle,improving measurement precision of flowmeter by analyzing the impact of temperature,pressure and other factors on the ultrasonic gas flowmeter.Firstly,the paper describes the flowmeter measurement technology,working principle and the main error factors in detail.The improved time-difference method is used to eliminate the influence of the change of sound velocity on the flowmeter that caused by temperature and pressure fluctuations;the influence of mechanical factors,circuit factors and flow field factors on gas flow measurement will be analysed,and corresponding methods will be taken to reduce the error brought by these factors.Secondly,this paper analyses the attenuation characteristics of ultrasonic signals under different conditions,designs a multi-level threshold comparasion circuit to judge the size of the signal;and utilizes band-pass filter and automatic gain amplification technology to filter and dynamically amplify the signals;and measures the signal transmission and reception time with high-precision time measurement chip TDC-GP22.The whole design largely solves the error measurement problem caused by the signal amplitude attenuation.In addition,AGA10 sound velocity calculation model is introduced in the flowmeter,programming and optimizing it and a new model that meet the ultrasonic flowmeter CPU operating frequency and accuracy is proposed,can achieve the self-diagnosis function of the flowmeter by comparing with the measured speed of sound.Thirdly,the dynamic noise window is designed by estimating the signal propagation time,and combine it with the threshold comparison and zero-crossing detection technology to achieve the signal reception,which can shield the noise signal,to some extent.A new algorithm isproposed to remove and correct the wrong data,and echo method is used to measure the channel distance.Finally,the natural gas flow conversion process of ultrasonic flowmeter is introduced,the compressibility factor compensation model which can achieve the conversion between the working volume and standard volume is described in detail,and choose the high-precision AGA8-92 DC compressibility factor calculation models to improve the accuracy of flow conversion.Aiming at the shortcomings of large computation,this paper uses the approximate Newton method and the least squares method to optimize the algorithm,which can meet the requirements of measurement accuracy and real-time measurement of flowmeter.Then a general-purpose compressibility factor algorithm that can be suitable for different type natural gas is developed in MATLAB,it is only necessary to know that the molar fraction of the gas can calculate a high-precision simplified model of the compressibility factor associated only with temperature and pressure. |
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