An ultrasonic flow meter is a type of flow meter that measures the velocity of a fluid with ultrasound to calculate volume flow. Using ultrasonic transducers, the flow meter can measure the average velocity along the path of an emitted beam of ultrasound, by averaging the difference in measured transit time between the pulses of ultrasound propagating into and against the direction of the flow or by measuring the frequency shift from the Doppler effect. Ultrasonic flow meters are affected by the acoustic properties of the fluid and can be impacted by temperature, density, viscosity and suspended particulates depending on the exact flow meter. They vary greatly in purchase price but are often inexpensive to use and maintain because they do not use moving parts, unlike mechanical flow meters.
Means of operation There are three different types of ultrasonic flow meters. Transmission (or contrapropagating transit-time) flow meters can be distinguished into in-line (intrusive, wetted) and clamp-on (non-intrusive) varieties. Ultrasonic flow meters that use the Doppler shift are called reflection or Doppler flow meters. The third type is the open-channel flow meter.
Principle
Time transit flow meter Ultrasonic flow meters measure the difference between the transit time of ultrasonic pulses propagating with and against the flow direction. This time difference (TTFM = Transit Time Flow Measurement) is a measure for the average velocity of the fluid along the path of the ultrasonic beam. By using the absolute transit times t u p {\displaystyle t_{up}} and t d o w n {\displaystyle t_{down}} , both the averaged fluid velocity v {\displaystyle v} and the speed of sound c {\displaystyle c} can be calculated. Using these two transit times, the distance between receiving and transmitting transducers L {\displaystyle L} and the inclination angle α {\displaystyle \alpha } , if we assume that sound has to go against the flow when going upstream and along the flow when going downstream, then one can write the following two equations from the definition of velocity:
c − v cos α = L t u p {\displaystyle c-v\cos \alpha ={\frac {L}{t_{up}}}} and
c + v cos α = L t d o w n {\displaystyle c+v\cos \alpha ={\frac {L}{t_{down}}}}
By adding and subtracting the above equations can solve for v {\displaystyle v} and c {\displaystyle c} ,
v = L 2 cos ( α ) t u p − t d o w n t u p t d o w n {\displaystyle v={\frac {L}{2\;\cos \left(\alpha \right)}}\;{\frac {t_{up}-t_{down}}{t_{up}\;t_{down}}}} and
c = L 2 t u p + t d o w n t u p t d o w n {\displaystyle c={\frac {L}{2}}\;{\frac {t_{up}+t_{down}}{t_{up}\;t_{down}}}}
where v {\displaystyle v} is the average velocity of the fluid along the sound path and c {\displaystyle c} is the speed of sound.
… excerpt ends here. Continue reading the full article.
