Reciprocal Circuit Design To Reduce Zero Error For Ultrasonic Gas Metering

Ultrasonic transit time flow meters for gas(USMs)has advantages of high measuring ratio,high accuracy with low maintenance requirement.At no-flow conditions,the up and downstream transit times of such meters are not usually be the same thus causing zero-flow error problem.The main reason is the variations between transducers.The existence of zero-flow error makes it essential for USMs to be "dry calibration",bringing more production cost.The zero-flowrate may also be influenced when the characteristics of transducers altered due to temperature change.Zero-flow error will lead to false flow detection and influence the measurement stability at low velocities.In the present paper,reciprocal transmitting and receiving circuits are designed to reduce the zero-flow error:(1)Theoretical analysis and circuit design.Electroacoustic reciprocity theory and reciprocal operation of USMs are analyzed in this section.Based on the theory,three transmitting and two receiving circuits are shown in the paper.(2)Simulation of reciprocal circuits.The transducers and ultrasonic signal transmitting process are modelled with reciprocal circuits in PSpice to validate overall effectiveness.Different reciprocal methods are tested in different impedance to achieve low zero error and better signal.Maximum value of zero error is also calculated.(3)A new method of signal detection is carried out base on threshold and zero crossing detection with ratio characteristics.Time control circuits with MSP430,time measuring with TDC-GP2,wave peak acquisition and peak holding circuits are designed.(4)Using reciprocal circuits to solve zero-flow error.Practical experiments are done with these circuits in different transducers pair,temperature,pressure and impedance show clear improvement in reducing zero-flow error.Calibration results show that USMs based on reciprocal circuits can achieve 1.0%measurement accuracy.