Selecting the Right Flowmeter for Your Liquid Application
Introduction
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.
Ultrasonic flowmeters have many advantages over conventional mechanical or magnetic flowmeters,
specifically:
• They don’t have moving parts and therefore require minimal, if any maintenance.• Unlike traditional flowmeters, they don’t introduce obstructive components into the pipe, and
therefore eliminate any interference to the flow profile and pressure. In fact, clamp on
ultrasonic flowmeters require zero pipe work and never come in contact with the liquid at all.
The following description will provide you with the necessary information on today’s common ultrasonic
flowmeter technologies and principles enabling you to select the right ultrasonic flowmeter for your
specific liquid application.
Basic Flowmeter ClassificationsLiquid ultrasonic flowmeters can be classified in four basic ways:
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 transittime flowmeter.
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 weatherresistant or explosion-proof. These types of meters are usually cheaper than handheld devices
because they are normally ordered in large quantities.
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.
Introduction
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.
Ultrasonic flowmeters have many advantages over conventional mechanical or magnetic flowmeters,
specifically:
• They don’t have moving parts and therefore require minimal, if any maintenance.• Unlike traditional flowmeters, they don’t introduce obstructive components into the pipe, and
therefore eliminate any interference to the flow profile and pressure. In fact, clamp on
ultrasonic flowmeters require zero pipe work and never come in contact with the liquid at all.
The following description will provide you with the necessary information on today’s common ultrasonic
flowmeter technologies and principles enabling you to select the right ultrasonic flowmeter for your
specific liquid application.
Basic Flowmeter ClassificationsLiquid ultrasonic flowmeters can be classified in four basic ways:
1. 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.
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 transittime flowmeter.
2. 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.
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 weatherresistant or explosion-proof. These types of meters are usually cheaper than handheld devices
because they are normally ordered in large quantities.
3. 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.
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.
4. 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.
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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