Doppler or Transit-Time?

The question most Ultrasonic Flow Meter Operators ask themselves is ….. “Should I be using a Doppler or Transit-Time Flowmeter?” Unless you know the answer, your only solution is to carry a Clamp On Doppler Flow Meter in one hand and a Portable Transit-Time Flow Meter in the other, and hope for the best when you arrive at the job site.

Most ultrasonic flowmeters use either “Doppler” or “Transit-Time” technology to measure the velocity of fluids inside pipes. Without getting into a bunch of scientific mumbo-jumbo and long boring algorithms, the following discussions will help you understand the difference between Doppler Flow Meters and Transit Time Flow Meters, but in reality, you don’t have to worry because almost every portable ultrasonic flow meter rented by FlowRental is capable of operating in both Doppler and Transit Time Modes!

Take a look for yourself in the video on the right .....

Doppler

All Ultrasonic Doppler Flow Meters bounce signals off small particles or bubbles in the moving fluid, resulting in a “Doppler Shift” in the returned signal frequency. The flowmeter measures this frequency shift then calculates the resulting speed of the reflective particles, then multiplies that value by the internal diameter of the pipe to produce volumetric flow. A great discussion on Doppler Shift is presented by our friends at Wikipedia.

The only disadvantage of buying a Doppler Flow Meter is that it absolutely will not work if you don’t have enough sonically reflective particles in your flow stream. A number of nefarious Flow Meter manufacturers claim their flowmeters “enhance” the capabilities of Doppler Shift by employing hocus-pocus algorithms, but the bottom line is, if you don’t have something to bounce the signals off, you absolutely cannot make measurements based upon Doppler Shift technology. Period.

Take a look for yourself in the video on the right .....

Doppler or Transit-Time?

Our most popular Ultrasonic Flow Meter rentals are 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 15 years, advancements in electronic filtering have helped some companies produce Transit-Time flow meters that can work in dirty liquids. FlowRental is one of those companies.

TTransit-Time is always more accurate and reliable than Doppler however Doppler tools may be less expensive.