| From the Project of Natural Gas Transmission from West to East China started in 2000,to the Goal to Gas Switch Policy advocating changing energy of high pollution to natural gas in recent ten years,gas flow measurement is necessary in more and more occasions.There is no flow baffle,no pressure loss,no contacting detection in gas ultrasonic flowmeter,meanwhile it is of wide measurement rage and high measurement accuracy.Profiting from these unique advantages,gas ultrasonic flowmeter is on rapid development,and it is a new kind of gas flowmeter of enormous potential.Gas ultrasonic totalizer develops from gas ultrasonic flowmeter.It contains all functions of flowmeter,and mainly differs from flowmeter in volume conversion and other function supplement circuits.Time-difference gas ultrasonic flowmeter is chosen to be researched in this paper.On the aspect of ultrasonic signal propagating time detection,a method that use digital signal processing instead of analog signal comparison is proposed.In the digital signal field the method is computing the ultrasonic signal propagating time in downstream and upstream using Gaussian Function to fit the envelop of the receiving signal in its rising part in replacement of direct Threshold Method to improve accuracy.Decrease the correlation method of computing propagating time-difference in frequency domain other than time domain.Aiming at the problem that the zero error and zero drift of time-difference gas ultrasonic flowmeter would be too high,a method that match the input impedance of sensor in transmitting state and the output impedance of the same sensor in receiving state in the channel is proposed.The zero drift of propagating time-difference is restrained below ±3 ns from-10 ℃ to40℃,conforming with grade 1.0 Ultrasonic Flowmeter National Standard.As flow velocity distribution is divided into laminar flow and turbulent flow,the general flow velocity correction coefficient K is derivated.In accordance with AGA8-92 DC the compressibility factor computing model,a simplified algorithm is analyzed and concluded.Take the constituents proportion ofAmerican Gulf Coast natural gas as an example,by controlling single variable of the acknowledged relationship among temperature,pressure and compressibility,a four pieces piecewise function solving compressibility Z is fitted out.At last the totalizer is equipped with GPRS communication model,4-20 mA model,485 communication model to rich and improve its function. |
