The reflected pulse is shifted according to the Doppler principle. Another consideration for proper operation is the requirement that flow be sufficient to keep particles in suspension. Typically 10-20 straight pipe diameters are needed upstream with 5 diameters downstream. Transducers are mounted on the outside of the pipe and are available in one or two transducer models. Ultrasonic meters are relatively inexpensive and reasonably accurate.
Proper installation is crucial because of the nature of the measurement technique.
Improper alignment of the transducers on either type of instrument will result in almost certain headaches. This type of flowmeter allows measurement without process interruption, however, and allow portability. In open channel flow measurement, weirs and flumes are used to produce known and repeatable characteristics in the process stream. Weirs, flumes and other devices develop a liquid head that is used to measure flow rate. These devices are particularly useful when large flows are to be measured or where liquids are handled in open channels or in pipelines that are not ordinarily full.