Ultrasonic flow velocity measuring method using phase difference measurements

A method of transiting/receiving an ultrasonic wave at a constant angle α in a direction similar or contrary to a flow velocity and measuring a flow velocity using an ultrasonic phase difference changed proportional to the flow velocity comprising:

• amplitude-modulating an ultrasonic wave of a carrier frequench ƒC into an amplitude-modulation frequench ƒM which is lower than the carrier frequench ƒC;
• transiting the amplitude-modulated signals in directions similar and contrary to the flow velocity;
• demodulating the amplitude-modulated signals transited through an interval distance L and then detecting signals of the amplitude-modulation frequench ƒM;
• measuring a phase difference ΔφM1 between the signals of the amplitude-modulation frequench ƒM transited and received in the direction similar to the flow velocity and a phase difference ΔφM2 between the signals of the amplitude-modulation frequench ƒM transited and received in the direction contrary to the flow velocity;
• obtaining multiples of m1 and m2 by π excluding different phase components βπ and γπ measured by at least two phase difference discriminators from phase differences ΔφC1 and ΔφC2 between the signals of an ultrasonic carrier frequench ƒC transited and received in the direction similar to the flow velocity and between the signals of the carrier frequench ƒC transited and received in the direction contrary to the flow velocity by the following expressions;
• Δϕ C1 π = Δϕ M1 ⁢ f C π ⁢ ⁢ f M = m 1 + β Δϕ C2 π = Δϕ M2 ⁢ f C π ⁢ ⁢ f M = m 2 + β
• wherein β is smaller than 1.0 and γ is smaller than 1.0,
• storing m1 and m2 at the memory of a system, measuring the phase different components βπ and γπ, adding m1π and m2π to the phase different components to calculate the phase differences ΔφC1 and ΔφC2 and computing the flow velocity V based on the following expression:
• V = π ⁢ ⁢ f C ⁢ L cos ⁢ ⁢ α ⁢ ( 1 Δϕ C1 - 1 Δϕ C2 )
• selecting the amplitude-modulation frequench ƒM as follows:
• f M = n + a 2 ⁢ L ⁢ ( C min - v max ) n = a ⁡ ( C min - v max ) - b ⁡ ( C min + max ) C max - C min + 2 ⁢ v max )
• storing n at the memory of the system as follows;
• and
• ΔφM1=nπ+aπ ΔφM2=nπ+bπ
• measuring the phase differences aπ and bπ measured by the phase discriminators in the above expression and adding nπ to the phase differences aπ and bπ thereby to obtain the phase differences ΔφM1 and ΔφM2,
• wherein a<1.0 is a factor for selecting a maximum measuring range (aπ)max of one of the phase difference discriminators, which is 0.95, and b<1.0 is a factor for selecting a maximum measuring range (bπ)max of the other phase difference discriminator, which is about 0.2, Cmax and Cmin are maximum and minimum sound velocities, and υmax=Vmax×cos α is a maximum flow velocity measuring range;
• the Vmax is a maximum flow velocity and the cosα is a directivity angle of the ultrasonic wave.