Inline transit-time ultrasonic
Sensors compare sound travel in different directions through the water. The difference is used to calculate flow. Inline versions have a defined measuring body.
Advantages: no measuring rotor, electronic diagnostics in many models, and products suited to remote consumption monitoring. Some have wide certified measuring ranges.
Limitations: bubbles, deposits and poor acoustic conditions can affect measurement. Electronics still require power. A long battery-life claim depends on the actual measuring, logging and communications configuration.
Before buying: compare the certified flow range, number of acoustic paths, approved orientation, air/empty-pipe behavior, battery conditions and supported outputs. “No moving parts” does not mean no inspection or eventual replacement.
Transit-time and Doppler ultrasonic methods are different; their fluid requirements should not be confused. Kamstrup ultrasonic and electromagnetic comparison
Clamp-on ultrasonic
Sensors mount outside an existing pipe. This can avoid cutting the line and keeps the sensors out of contact with the water. Honeywell clamp-on overview
Advantages: attractive for surveys, troubleshooting and retrofits where shutdowns or wetted components are undesirable.
Limitations: performance depends on the installation, including actual pipe dimensions, lining, acoustic transmission and sensor coupling. A convenient temporary reading is not automatically an accepted billing measurement.
Before buying: supply outside diameter, wall thickness, material, lining, temperature, fluid properties and available straight pipe. Ask the supplier to assess the site and demonstrate signal quality. These are explicit inputs in the Siemens ultrasonic application data sheet.
Doppler ultrasonic
Doppler instruments use reflections from suitable suspended particles or bubbles. They may be useful in applications where those reflectors are present consistently.
They are not a universal choice for clean drinking water, and “ultrasonic” on a quotation is insufficient. Ask whether the offered instrument is transit-time or Doppler and what fluid conditions it needs. Kamstrup explanation of ultrasonic principles