A. Ultrasonic Flowmeter Classification

Liquid ultrasonic flowmeters can be classified in many ways:

(1) Based-on principle: Transit-time vs. Doppler

A transit-time flowmeter is based on the time difference between upstream and downstream sound propagation intervals. It usually provides very good accuracy (±1%). The former works well for clean flow 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.

A Doppler flowmeter 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. Doppler is easier to make and less accurate (±5%), thus, it is cheaper than transit-time flowmeter.

Another not so popular flowmeter is cross-correlation based ultrasonic flowmeter. It works better for suspension flow or turbulent gas flow.

(2) Based-on portability: Handheld vs. Wall-mount

Handheld (or portable) flowmeer is very good for flow survey, HVAC and many applications where mobility is a concern. Another trend of using handheld flowmeter instead of wall-mount flowmeter is to save cost, since some handheld flowmeters (such as STUF-200H) can also be deployed as long-term measurement device if the environment is accommodating.

Wall-mount (or fixed installation) flowmeter 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. Wall-mount is normally cheaper than handheld because it is usually ordered in large quantity. 

(3) Based-on transducer installation: Clamp-on vs. Wetted (Insertion and Flow Cell)

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

Wetted flowmeter comes with two varieties, insertion and flow cell. Insertion flowmeter requires to dill holes on the pipe wall to install the transducers. Flow cell, also called spool-piece, needs to be installed in line with the pipe, thus needs pipe fitting, etc. The transducers are pre-installed on the flow cell, thus, installation error has been reduced to minimum. In general, wetted transducer has better accuracy and better long-term stability than clamp-on type, but it might be more expensive. Comparing with conventional flowmeters, wetted type does not have moving parts, no interference to flow profile, no pressure drop and needs very little maintenance.

(4) Based-on transducer scheme: Single-path vs. Multi-Path

Single-path flowmeter 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 needs, multi-path transducer installation may be considered. However, a multi-path flowmeter is much more expensive than a single-path flowmeter. It is usually used in custody transfer where accuracy is extremely important.

B.Ultrasonic Flowmeter Selection Guide

The most commonly used ultrasonic flowmer is transit-time based single-path flowmeter due to its high accuracy and reasonable cost. Depending on the applications, the selection of the right type of flowmeter may vary. You may need to answer the following questions before making the final decision:

• Is the pipe full?
  All ultrasonic flowmeters require that the pipe is full of liquids. There are some work around with partially-full pipe, but by no means you will get good results.

• Does the liquid have particles more than 200ppm and particle size larger than 75um (the number may be different for different manufacturers)?
  If yes, use Doppler. Otherwise, consider transit-time.

• Does the liquid temperature fall into the transducer temperature range?

• Press range in the pipe (if you select wetted transducer)?

• High accurate the measurement should be?
  In general, multi-path is higher than single path, flow cell is better than insertion, and insertion is better than clamp-on. However, if you have on-site calibration facility, the story will change.

• Do you want to easily move the flowmeter around?

• What output signals you need? What functionalities you want?
  Analog? Digital? RS232? Data logger?

• What are the safety requirements?

• Pipe size range? Pipe material? Lining?

• Flow rate range?

• Enough straight pipe length?
  In order to guarantee the result 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.

• The financial budget.

The above questions seem overwhelming if this is the first time you are looking for a ultrasonic flowmter. Please do not be scared. Most likely, a simple clamp-on transit time handheld flowmeter or wall-mount flowmeter will suit your application. If not, please feel free to contact us for detailed suggestions.

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