Differential-pressure flow
A restriction or primary element creates a pressure difference related to flow under defined conditions. The relationship is generally nonlinear, and density or other compensation may be needed depending on the desired quantity and process.
Magnetic flowmeters
Electromagnetic flowmeters use Faraday's law to relate induced voltage to conductive-fluid velocity. They generally require sufficient electrical conductivity and a properly filled measurement section.
Coriolis flowmeters
Coriolis meters infer mass flow from motion of vibrating tubes. They can also provide density-related information and do not depend on the same straight-run assumptions as many velocity-based technologies, though application limits still matter.
Vortex flowmeters
Vortex meters detect vortices shed from a bluff body. Performance depends on flow regime and process conditions; they are used for various liquid, gas and steam applications when conditions are suitable.
Ultrasonic flowmeters
Ultrasonic technologies infer flow from acoustic transit time or Doppler effects. Clamp-on designs can be attractive where intrusion into the pipe is undesirable, but pipe, fluid and installation conditions influence performance.
Selection is an application decision
Consider fluid phase and properties, required quantity (mass or volume), rangeability, pressure loss, accuracy need, pipe size, installation space, maintenance access, temperature/pressure limits and lifecycle cost.