The Advantages of Ultrasonic Flow Meters

Today's advanced ultrasonic flow meters enable the accurate measurement of liquid flow across a wide variety of process control, water resource management, and energy consumption applications. To select the right ultrasonic flowmeter for your specific liquid application, you must first understand the four basic ways in which ultrasonic flowmeters are classified.

Measurement principle: Transit-time vs. Doppler

Transit-time flowmeters are based on the time difference between upstream and downstream sound propagation intervals, and provide one of the most reliable and time tested measurement principles available to date. This approach provides very good accuracy (±1-2%), and works well for clean flow applications or flow with minor particles. Applications include pure water, sea water, wash water, sewage, process liquids, oils, chemicals, and any homogeneous liquids which are capable of ultrasonic wave propagation.

Doppler flowmeters are based on the Doppler Effect. They work well with suspension flows where particle concentration is above 100ppm and particle size is larger than 100um, but less than 15% in concentration. Doppler is easier to make and less accurate (±5%), thus, it is cheaper than a transit-time flowmeter.

Device Portability: Handheld vs. Wall-mount

Handheld (or portable) flowmeters are effective for flow survey, HVAC, and other applications where mobility is a critical requirement. Some handheld flowmeters can be deployed as long-term measurement solutions when and if the environment is accommodating.

Wall-mount (or fixed installation) flowmeters are more suitable for applications such as process control and long-term continuous flow monitoring. The instrument enclosure is often weather-resistant or explosion-proof. These types of meters are usually cheaper than handheld devices because they are normally ordered in large quantities.

Transducer installation: Clamp-on vs. Wetted

Clamp-on flowmeters are non-intrusive, easy to install and easy to maintain. The transducers are mounted outside of a pipe, and don’t require the cutting or drilling of the actual pipe. Clamp-on transducers are nonintrusive and therefore cause no pollution or pressure drop in the liquid being measured.

There are two varieties of wetted flowmeters, insertion and flow cell (inline type). Insertion flowmeters require the pipes to be drilled to install the transducers. However, a number of manufacturers provide hot-tapping tools which allows for the transducer installation without depressurizing the pipe or shutting down the flow.

Flow cell, also called spool piece are installed in line with the pipe, through the use of special pipe fittings. The transducers are pre-installed on the flow cell, thus, installation errors are kept to a minimum. In general, wetted transducers provide better accuracy and long-term stability over clamp-on varieties, but usually cost more.

Transducer scheme: Single-path vs. Multi-Path

single-path flowmeters utilize one pair of transducers to form one ultrasonic path to intercept the flow in a pipe. It is suitable for small and medium size pipes. For larger pipe sizes, shorter straight pipe runs and greater accuracy, multi-path transducer installation is a good option. Multi-path flowmeters are traditionally more expensive than a single-path flowmeters, and are often implemented in applications such as custody transfer where accuracy is of paramount important.

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