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 is a measure for the average velocity of the fluid along the path of the ultrasonic beam. By using the absolute transit times ${\displaystyle t_{up}}$ and ${\displaystyle t_{down}}$, both the averaged fluid velocity ${\displaystyle v}$ and the speed of sound ${\displaystyle c}$ can be calculated. Using these two transit times, the distance between receiving and transmitting transducers ${\displaystyle L}$ and the inclination angle ${\displaystyle \alpha }$, if we assume that sound has to go against the flow when going up and along the flow when returning down, then one can write the following equations from the definition of velocity:

${\displaystyle c-v\cos \alpha ={\frac {L}{t_{up}}}}$ and ${\displaystyle c+v\cos \alpha ={\frac {L}{t_{down}}}}$

By adding and subtracting the above equations we get,

${\displaystyle v={\frac {L}{2\;\cos \left(\alpha \right)}}\;{\frac {t_{up}-t_{down}}{t_{up}\;t_{down}}}}$ and ${\displaystyle c={\frac {L}{2}}\;{\frac {t_{up}+t_{down}}{t_{up}\;t_{down}}}}$

where ${\displaystyle v}$ is the average velocity of the fluid along the sound path and ${\displaystyle c}$ is the speed of sound.

Doppler shift flow meters

Another method in ultrasonic flow metering is the use of the Doppler shift that results from the reflection of an ultrasonic beam off sonically reflective materials, such as solid particles or entrained air bubbles in a flowing fluid, or the turbulence of the fluid itself, if the liquid is clean.

Doppler flowmeters are used for slurries, liquids with bubbles, gases with sound-reflecting particles.

This type of flow meter can also be used to measure the rate of blood flow, by passing an ultrasonic beam through the tissues, bouncing it off a reflective plate, then reversing the direction of the beam and repeating the measurement, the volume of blood flow can be estimated. The frequency of the transmitted beam is affected by the movement of blood in the vessel and by comparing the frequency of the upstream beam versus downstream the flow of blood through the vessel can be measured. The difference between the two frequencies is a measure of true volume flow. A wide-beam sensor can also be used to measure flow independent of the cross-sectional area of the blood vessel.

Open channel flow meters

In this case, the ultrasonic element is actually measuring the height of the water in the open channel; based on the geometry of the channel, the flow can be determined from the height. The ultrasonic sensor usually also has a temperature sensor with it because the speed of sound in air is affected by the temperature.