(1) Transit-time vs. Doppler

Liquid ultrasonic flowmeters can be classified in many ways:

A transit-time flowmeter is based on the time difference between upstream and downstream sound propagation intervals. It usually provides very good accuracy (1% or better). The instrument works well for clean flow or flow with the small amounts of air bubbles or suspended solids found in most industrial environments. Applications include pure water, sea water, wash water, sewage, process liquids, oils, chemicals, and any  homogeneous liquids which are capable of ultrasonic wave propagation.

A Doppler flowmeter is based on the Doppler effect. It works well for flows where the suspended particle concentration is above 100ppm and the particle size is larger than 100 microns, but less than 10% in concentration. Doppler is easier to make and less accurate (5%), thus, it is cheaper than the transit-time flowmeters.

(2) Portable vs. Dedicated

Portable flowmeters are very good for flow surveys, HVAC and many applications where mobility is a concern. Another trend of using portable flowmeters instead of dedicated flowmeters is to save cost, since some portable flowmeters can also be deployed as long-term measurement devices if they are protected from the environment.

Dedicated (or fixed installation) flowmeters are more suitable for applications such as process control and long-term continuous flow monitoring. The instrument enclosure is normally weather-resistant or explosive-proof. Dedicated flowmeters are normally less expensive than portables because they are usually ordered in larger quantities. 

(3) Transducer types: Clamp-on vs. Wetted (Insertion and Inline)

Clamp-on transducers are non-intrusive, easy to install and easy to maintain. The transducers are mounted out side of the pipe, thus, there is no need to cut the pipe or to drill holes into the pipe, and there is no direct contact with the liquid and no pressure drop, etc.

Wetted transducers come in two varieties, insertion and inline types. Insertion transducers require the drilling of holes into the pipe wall to install the transducers. The inline type, also called spool-piece, is installed directly into the pipe, normally with flanged fittings, etc. The transducers are pre-installed in the spool piece, thus, installation error is reduced to a minimum. In general, wetted transducers have better accuracy and better long-term stability than clamp-on type, but it are more expensive. Compared to conventional flowmeters, wetted type ultrasonic transducers have no moving parts, no interference to flow profile, no pressure drop and need very little maintenance.