The propagation time of ultrasonic in positive and negative directions of gas flow can be exactly calculated by using cross-correlation

and the time difference between the two directions can be further computed

and the velocity of gas flow can be also computed.However

because the calculation error of peak of discrete cross-correlation function depends on sample interval

decrease in interval is to increase in precision and certainly it is to increase by a long way in the quantity of calculation.In measuring instruments that are controlled by single chip computer

the method that needs huge calculation is usually not used.The key of the question is to find a relatively simple method.In the vicinity of peak of discrete cross-correlation function

a more accurate peak can be gained by utilizing parabola algorithm.The algorithm permits to have longer sample period and curtails calculating time from 3.7s to 0.9s in the same measurement precision.