A Doppler flow meter transmits an acoustic signal into the flow stream at a specific frequency. This signal reflects off particulate, either solids or gas bubbles. The reflected signal is detected by the receive transducer. The flow meter compares the transmit & receive frequencies & computes the frequency shift. This frequency shift is proportional to the liquid's flow velocity & is known as the Doppler's effect.
Doppler Flow Meters are only used for liquids containing gas bubbles or particulates. Some of the applications where Doppler has advantage over Transmit time are Treated Waste Water, Raw Water, Slurries, Sewage, Viscous Liquids, Acid, Pulp Stock, Food Products, Aerated Water, Cooling Water etc.
A simple analogy to explain the transit time flow measurement principle is the example of a boat flowing in the river. A boat crossing a stream at an angle to the stream's current will take less time to cross when going with the current than going against it. This time difference is proportional to the velocity of the stream's current.
To relate this to a transit time flow meter, simply replace the boat with an acoustic signal. A upstream transducer (T1) sends a signal to a downstream transducer (T2) that in turn sends back a signal. When there is no flow then time required to travel from T1 to T2 as well as time required from T2 to T1 remains the same. However when there is a flow, the effect of flowing velocity on the acoustic signal is to assist the signal in the up to downstream direction & hinder the signal in the down to upstream direction.
This creates the time difference by which the liquid's flowing velocity & ultimately the flow rate is determined. This meter can be used on clean liquid applications only like clean water, Chemicals, potable water, DM Water, petroleum products.