Doppler flow meters are based on the Doppler principle. The transmitter of a Doppler flow meter projects an ultrasonic beam at a frequency of about 0.5 MHz into the flowing current and diverts the reflected frequency.
Principle of operation:
An ultrasonic wave is projected at an angle through the wall of the pipe to the liquid by a transmission glass on a transducer mounted outside the pipe. Some of the energy is reflected by bubbles or particles in the liquid and is returned through the wall of the pipe to a receiving crystal.
If the reflectors travel at the speed of the fluid, the frequency of the reflected wave travels according to the Doppler principle, in proportion to the velocity of the flow. Combining Snell’s law and the Doppler equation, the flow velocity can be determined as follows if V << C.
where Δf = Difference between transmitted and received frequency.
fo = frequency of transmission
Ɵ= angle of the transmitter and receiver crystal with respect to the pipe axis
Ct = Velocity of sound in the transducer
Construction of Doppler flowmeter
The transmitter glass and receiver glass are mounted separately on opposite sides of the pipe exterior. The alignment is maintained by a mounting assembly that maintains the relative positions of the transducers as shown
When the process stream contains a large amount of solids and the velocity of the solid particles near the wall is probably lower than the average in a single transducer installation (which causes large errors), it is recommended to use the two transducer approach . With this approach, the reflected ultrasonic radiation is received from parts of the flow stream.
- It does not impose additional resistance to flow or alter the flow pattern when the transducers are inserted into the wall of the pipe.
- Its speed / output ratio is linear.
- It has no moving parts.
- Its repeatability is of the order of 0.01%.