A requirement for reliable operation of a transit-time ultrasonic flowmeter is that the process fluid
be free from gas bubbles or solid particles which might scatter or obstruct the sound waves. Note that
this is precisely the opposite requirement of Doppler ultrasonic flowmeters, which require bubbles
or particles to reflect sound waves. These opposing requirements neatly distinguish applications
suitable for transit-time flowmeters from applications suitable for Doppler flowmeters, and also raise
the possibility of using transit-time ultrasonic flowmeters on gas flowstreams as well as on liquid
flowstreams