As we always say “Doppler does Dirty” A Doppler flowmeter bounces signals off small particles or bubbles in the moving fluid, creating a “Doppler Shift” in the returned signal frequency. The flowmeter measures the Doppler Shift then calculates the resulting speed of the reflective particles, multiplies that value by the internal diameter of the pipe to produce volumetric flow. The only disadvantage of buying a Doppler flowmeter is that it absolutely will not work if you don’t have enough sonically reflective particles in your flow stream (suspended solids). A number of manufacturers claim their flowmeters “enhance” the capabilities of Doppler Shift, but the bottom line is, if you don’t have something to bounce the signals off, you absolutely cannot make accurate Doppler measurements.
Our most popular flowmeter rentals are for Transit-Time flow meters (aka “Time of Flight” or “ΔT”), which depend upon the fluid being acoustically clean enough for signals to pass between two transducers. The time taken for signals to travel from one transducer to the other is compared against the time required for signals to travel in the opposite direction.
This difference in time (Transit-Time) is processed by a bunch of powerful algorithms to produce an accurate flow velocity. During the past 10 years, advancements in electronic filtering have helped some companies produce Transit-Time flow meters that can work in dirty liquids.
Transit-Time is always more accurate and reliable than Doppler however Doppler tools may be less expensive