Flow Transmitter Query

Well, it depends upon the type of flow transmitter in use. So many installations continue to use the differential-pressure-across-the-orifice-plate type of transmitter, where other techniques are readily available and perhaps easier to use. What's wrong with using variable area? Or mag-flow? Or ultrasonic time-of-flight? Or vortex-shedding? Or mass flow? Or pelton wheel? The square root extraction is avoided in all of these.