Here's a brief explanation of the term “Doppler” as it applies to Ultrasonic Flow Meters, and why you may not need an Ultrasonic Doppler flowmeter after all ……
Not long ago, all ultrasonic flowmeters were called “Doppler Meters” because they all used a technology called “Doppler Shift” to measure the movement of small reflective particles or air bubbles in passing liquids. Simply stated, a single frequency is emitted by an acoustic tranducer, through the pipe wall and into the passing fluid. That signal is bounced off passing particles or bubbles and the reflected signal is "Shifted" in frequency by an amount equal to the speed of the passing particle or bubble.
Unfortunately, Doppler Flow Meters were not very accurate or reliable in their day, and they quickly gained a bad reputation because they only worked on fluids with just the right amount of particles or bubbles. The same holds true today …. if your liquid is too clean, or too dirty, Ultrasonic Doppler Flow Meters simply can’t work. Regardless of the Flow Meter Manufacturer, or the fancy terms they apply to their technology, they still depend on exactly the right amount of acoustically reflective materials to make ANY measurements. Without these reflectors, Ultrasonic Doppler Flow Meters employing Doppler Shift Technology simply can't and don't work.
Many believe that raw sewage is a Doppler Flow Meter application, but even though most sewage is a really nasty liquid with numerous large "lumps" passing by, it's still >99% water. In most sewage applications, there simply isn't enough particles or aeration to make successful Ultrasonic Doppler measurements, so a Doppler Flow Meter may not be your best choice.
Above is an animation of how a Doppler Flow Meter reflects it's signal off "stuff" in the passing fluid.
During recent years, as electronics have advanced, so has ultrasonic flow measuring technology. No longer must you depend upon the exact ratio of water and flowing particles, because along came a technology called “Transit-Time” (or Time of Flight or Delta T) to solve the problem.
Transit Time Ultrasonic Flow Meters will measure almost any liquid, and are much more accurate and reliable than outdated Ultrasonic Doppler Flow Meters. Rather than measuring the speed of particles and bubbles like a typical Doppler Flow Meter, Transit Time Ultrasonic Flow Meters measure the speed of sound between two transducers mounted on the outside of the pipe. As long as the two transducers can acoustically "see" each other, your measurements will be much more accurate. Here's an animation of a Transit Time Ultrasonic Flow Meter:
FlowRental maintains a large selection of Doppler and Transit Time Ultrasonic Flow Meters for immediate overnight delivery to your job site. These "Dual Mode" meters are capable of measuring both gas and fluid flow in either Doppler Mode or Transit-Time Mode. If your fluid is filled with particulate or bubbles, then switch to "Doppler Mode", or if your liquid is clean and bubble free, chances are that Transit-Time will produce the most accurate readings possible.
So, when someone says they need a "Doppler Flow Meter", what they really mean is they need a "FlowRental Dual Mode Ultrasonic Flowmeter" capable of measuring in both a Doppler Ultrasonic Flow Meter mode and a Transit Time Ultrasonic Flow Meter mode, both in the same box. You can measure ultra-dirty fluids and ultra-pure fluids with the same Dual Mode Ultrasonic Flow Meter from FlowRental.