10 Tips On How To Choose A Suitable Digital Flow Meter

Digital flow meters are very promising instruments for pipe flow measurement in process control,
flow survey, custody transfer, etc. Choosing a suitable digital flow meter requires a clear
appreciation of its important characteristics and an understanding of the qualities and features
available.

In the following article, we offer ten tips that will help customers to select the right digital flow
meter for liquid applications. For information on how those flow meters work, please visit the
‘How to Use Water Flow Transmitter’ page.

Tip 1: Know the main types of digital flow meters

Ultrasonic flow meters

Ultrasonic flow meters measure the traveling time (transit time models) or the frequency shifts
(Doppler models) of ultrasonic waves in a pre-configured acoustic field.
A transit-time model usually provides very good accuracy (±1%). It works well for both clean
flow and flow with minor particles.

A Doppler flow meter is based on the Doppler effect. It works well with suspension flow where
particle concentration is above 100ppm and particle size is larger than 100um, but less than 10%
in concentration. A Doppler model is easier to make and less accurate (±5%), thus it is cheaper
than a transit-time model.Another not so popular model is the cross-correlation based ultrasonic flow meter. It works better
for suspension flow or turbulent gas flow.The most commonly used ultrasonic flow meter is a transit-time based single-path flow meter due to its high accuracy and reasonable cost.

Magnetic flow meters

Magnetic flow meters, also known as electromagnetic flow meters or induction flow meters,
obtain the flow velocity by measuring the changes of induced voltage of the conductive fluid
passing across a controlled magnetic field.Positive displacement flow meters Positive displacement flow meters, also known as PD meters, measure volumes of fluid flowing through by counting repeatedly the filling and discharging of known fixed volumes.

Tip 2: Choose a digital flow meter by the media type

Media: The type of media (liquid, gas or slurry). Are there any particles in the liquid and what is
the viscosity of the liquid? Media conditions: pressure and temperature of media and whether media conditions are likely to
remain constant or vary.

Tip 3: Choose a digital flow meter by the measurement requirement

Flow range: min. and max. readings required of the media.Accuracy: How accurate should the measurement be?
In general, the accuracy for the multi-path models is higher than that of the single path models, flow cell accuracy is better than insertion type, and insertion type is better than clamp-on. However, if you have an on-site calibration facility, this would not be an issue.

Tip 4: Choose a digital flow meter based-on portability (handheld vs. wall-mount)

The handheld (or portable) flow meter is suitable for flow survey, HVAC and many
applications where mobility is a concern.
The wall-mount (or fixed installation) flow meter is more suitable for applications such as
process control and long-term continuous flow monitoring. The instrument enclosure is
normally weather-resistant or explosive-proof.

Tip5: Choose a digital flow meter based-on transducer installation method: clamp-on vs.
wetted (insertion and flow cell)

The clamp on transducers are mounted on the outside of a pipe, thus there is no need to cut the pipe or to drill holes into the pipe, and there is no direct contact thus no pollution to the liquid, no pressure drop, etc. The wetted flow meter comes with two varieties, insertion-type and flow cell-type. The insertion flow meter requires drilling holes into the pipe wall to install the transducers. Flow cell-type, also called spool piece-type, needs to be installed in line with the pipe, thus needs pipe fitting, etc. In general, a wetted transducer has better accuracy and better long-term stability than clamp-on type, but it might be more expensive.

Tip 6: Choose a digital flow meter based-on transducer scheme (single-path vs. multi-path)

Single-path flow meter utilizes 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 large pipe size and very high accuracy requirement, multi-path transducer installation may be considered. However, a multi-path flow meter is much more expensive than a single-path flow meter. It is usually used in custody transfer applications where accuracy is extremely important.

Tip 7: Choose a digital flow meter based-on liquid type

Does the liquid have particles more than 200ppm and particle size larger than 75um (the number may be different for different manufacturers)? If it has particles and the electrical conductivity of fluid is higher than 3 µS/cm, use a magnetic flow meter. If it has particles but is not electrical conductive, then use a Doppler type. Otherwise, consider a transit-time ultrasonic flow meter. Does the liquid temperature fall into the transducer temperature range?

Tip 8: Choose a digital flow meter based-on environment

Environmental considerations: special installation considerations such as hygienic installation, installing in an ATEX zone or requiring tamper proof readings Tip9: Choose a digital flow meter based-on pipe Pressure range in the pipe (if you select a wetted transducer)?Pipe size range? Pipe material? Lining? Enough straight pipe length? For best accuracy, it is normally recommended to have 15 times of pipe diameter straight pipe run at the measuring site. If there is a pump or valve on the upstream, the straight length should be increased.

Tip10: Other considerations

What output signals do you need? What functionalities do you want? Analog? Digital? RS232? Data logger? The financial budget.