Doppler ultrasonic flowmeters operate on the Doppler shift
principal , whereby the transmitted frequency is altered linearly
by being reflected from particles and bubbles in the fluid.
The net result is a frequency shift between transmitter and
receiver frequencies that can be directly related to the flow
velocity. If the pipe internal diameter is known, the volumetric
flow rate can be calculated. Doppler meters require a minimum
amount of solid particles or air in the line to achieve measurements.
If all scatterers moved at th velocity
VD,
then the flow velocity calculated from the Doppler-shifted
frequency would be
where:
| VD= |
flow velocity |
| c = |
sound speed in fluid |
| fD= |
Doppler frequency difference (difference between transmitted
and received frequency; note that fD
is proportional to the Mach number VD
/c) |
| f0 = |
transmitted frequency, and |
| θ = |
angle between sound beam and pipe axis |
