Flow meter selection instruction

 

Thank you for considering Longrun Industrial Instrument Co., Ltd products. This short article is intended as an instruction to help us provide you with the best product for your application at the least cost.

 

It is important to remember that there is no universal flow meter that will be appropriate for every application. The basis of good flow meter selection is a clear understanding of the requirements of the particular application. We have listed several points below to help customers know more about flow meters and select the right flow meters for liquid application.

 

1.      Main types of 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(less than 20000ppm). 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. 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. Electromagnetic flow meters can measure difficult and corrosive liquids and slurries, and they can measure flow in both directions with equal accuracy. Electromagnetic flow meters have relatively high power consumption and can only be used for electrical conductive fluids as water. 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.

 

2.      Liquid type

Does the liquid have particles more than 100ppm and particle size larger than 100 micron? If it has particles and the electrical conductivity of fluid is higher than 5 µs/cm, use a magnetic flow meter. If it has particles but is not electrical conductive, then use a Doppler type. The transit-time ultrasonic flow meter works well for clean flow with minor particles or air bubbles no more than 20000ppm.

 

3.      Pipe

What is the size of the pipe? Is the pipe always full? Normally, ultrasonic flow meters, Doppler ultrasonic flow meter and magnetic flow meters all require always- full pipes. So, if the pipes do not full, a specialized non-full type flow meter would be used.

For best accuracy, it is normally recommended to have 15 times of pipe diameter straight pipe run at the measuring site. (10 times of pipe diameter in upstream pipe and 5 times of pipe diameter in downstream pipe). If there is a pump or valve on the upstream, the straight length should be increased.

 

4. Portability  

The handheld or portable flow meter is suitable for flow survey, HVAC and many applications where mobility is a concern. The handheld or portable flow meter is usually powered by a rechargeable battery and is packed in a case, which makes it easily be carried by hand. 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.

 

5. Transducer installation method

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 flange connected type, needs to be installed in line with the pipe, thus needs pipe fitting, etc. In general, a wetted transducer or flow cell transducer has better accuracy and better long-term stability than clamp-on type, but it might be more expensive.

 

6. Other considerations

It may include specific requirement for analog input/output, data logger, GSM/GPRS wireless data transmission module, the financial budget and so on.

 

In order to offer the best solution in short time, we wish to get these factors include liquid type (is it clean or not?), pipe situation (material, diameter, is it full or not?), portability (fixed type or handheld type), transducer type (clamp on, insertion wetted or flow cell), etc.

Anyway, no matter what problems you meet regarding flow meters, you can contact us at any time by email, telephone or fax.