Mười phút để hiểu cách chọn 10 loại lưu lượng kế?

Repeatability
Definition: The consistency or closeness of measurement results when the same quantity is measured multiple times under identical conditions. Repeatability reflects the short-term stability of the measuring device or system.
For example, if you use a flow meter to measure the same flow rate (e.g., 10 liters per second) and repeat the measurement 10 times under the same conditions, a flow meter with high repeatability will yield results very close to each other, such as 9.98, 9.99, 10.01 liters. Higher repeatability means less data dispersion.

Measurement Accuracy
Definition: The closeness of the measured value to the true value (or reference standard). Measurement accuracy reflects the correctness of the measuring device.
If you aim to measure a liquid flow of 10 liters per second, ideally, the flow meter should display 10.0 liters per second. If it shows 9.8 liters per second, it indicates lower measurement accuracy. The higher the measurement accuracy, the closer the measured value is to the true value of 10.0 liters per second.

When you see the accuracy specifications of a flow meter, pay attention to whether it's expressed as a percentage of Full Scale (FS) or Reading (Rd or RD).

1) What is Full Scale (FS)?
Using FS to express error means the error is calculated based on the maximum range of the scale. The absolute error remains constant, but the relative error percentage changes with the flow rate.

For example: Suppose a flow meter has a range of 3–300 LPM with an accuracy of ±3.0% FS.
Then, Absolute Error = 300 LPM × 3.0% = 9 LPM

If the displayed flow is 250 LPM, the actual flow could be 250 LPM ±9 LPM (241–259 LPM). The relative error is 9 LPM / 250 LPM = ±3.6%.

If the displayed flow is 100 LPM, the actual flow could be 100 LPM ±9 LPM (91–109 LPM). The relative error is ±9%.

The advantage of FS is that it's simple to calculate since the absolute error doesn't change. It's suitable for indicating errors at high flow rates. However, at lower flow rates, the absolute error constitutes a larger percentage of the flow rate, leading to poorer measurement accuracy.

 

2) What is Reading (Rd or RD)?
When using Rd to express error, the error is calculated based on the actual measured value. The percentage remains constant regardless of where the flow rate is within the range.

For example: Suppose a flow meter has an accuracy of ±0.3% Rd.

When the actual measured value is 100 LPM, the absolute error is 100 LPM × 0.3% = 0.3 LPM.

When the actual measured value is 50 LPM, the absolute error is 50 LPM × 0.3% = 0.15 LPM.

The advantage of Rd is that the error varies with the measured value, offering more precise measurements at low flow rates and stable relative errors. It's suitable for applications with varying flow rates, maintaining accuracy across different ranges. The downside is that calculations are more complex, requiring dynamic computation for each reading. At high flow rates, the error might increase, affecting measurement accuracy.

➤ Flow Meter Accuracy Reference Table:
Based on data from manufacturers like Emerson, Endress+Hauser, and Doweston, using Full Scale (FS) error as the standard:

• High Accuracy: FS ≤ ±0.5%, suitable for laboratory measurements or high-precision industrial applications.
• Medium Accuracy: FS between ±0.5% and ±2%, suitable for most industrial applications and daily measurement needs.
• Low Accuracy: FS > ±2%, suitable for applications with large flow variations or lower accuracy requirements.

(This table is for preliminary selection reference only. Due to varying characteristics of flow meters from different manufacturers, it is recommended to refer to catalogs or consult manufacturers to confirm if the flow meter's accuracy meets your usage conditions.)

Whether the equipment can be shut down, whether pipe cutting is necessary, and the installation location must be considered when selecting a flow meter:

• Determine if the equipment can be shut down and pipes cut for flow meter installation: When the pipeline cannot be stopped or cut, it's recommended to use non-intrusive flow meters like ultrasonic flow meters, although installing without cutting the pipeline usually requires a higher budget.
• Select the connection method based on the pipe material: Choosing an appropriate connection method can enhance the sealing of the flow meter. Metal pipes (such as stainless steel, copper) usually require welding or flange connections, while plastic pipes (such as PVC, PPH) often use adhesive bonding or threaded connections.
• Pay attention to corrosion resistance requirements: When the fluid is corrosive, you need to choose flow meter materials with good chemical resistance, such as engineering plastics (PVC, PPH, PVDF, PFA).
• Note whether the installation position is horizontal or vertical?

When purchasing a flow meter, besides the price of the flow meter itself, you also need to consider installation, maintenance, and operating costs. For example, electronic flow meters have a long service life but may require periodic calibration.
Additionally, consider whether you have special certification requirements for the flow meter, especially in industrial environments with high safety and explosion-proof demands. Special certifications include:
CE: The European Union's safety, health, and environmental standards; all equipment sold in the EU must comply with these regulations.  
UL: U.S. safety certification, mainly for electrical safety and fire protection, widely used in the North American market.  
ATEX: European explosion-proof certification for explosive environments, applicable to high-risk areas like chemical and petroleum industries.  
Measurement Certification: Certification for flow meter accuracy and measurement compliance to ensure the accuracy of commercial measurements.

➤ Flow Meter Cost Reference Table:
Based on price ranges of flow meters found on Amazon and Alibaba, using DN25 (1 inch) as the standard:

• High Cost: Above 15,000 TWD
• Medium Cost: 5,000~15,000 TWD
• Low Cost: Below 5,000 TWD

(This table is for initial selection reference only. Due to varying characteristics of flow meters from different manufacturers, it is recommended to consult catalogs or contact manufacturers to confirm whether the flow meter price meets your cost considerations.)

In some cases, there is a need for flow data storage and transmission. You should ensure that the signal transmission method is compatible with other equipment, such as Pulse, 4-20mA, RS485, I/O LINK, etc.
For example, recent "Energy Management Act" regulations require equipment in buildings and workplaces to submit annual energy reports. This necessitates flow meters with data collection capabilities. When purchasing flow meters, pay attention to whether the communication interfaces and protocols are compatible.
Learn more:
• Ultrasonic Flow Meters Simplify Energy Reporting
• What Is Analog Current Output?
• What Is RS485?
• What Is a Switch Signal?

(This table is for initial selection reference only. Due to varying characteristics of flow meters from different manufacturers, it is recommended to refer to catalogs or consult manufacturers to confirm whether the flow meter price meets your cost considerations.)

We hope the above content helps you select the right flow meter. With 30 years of professional experience in the fluid field, LORRIC is dedicated to providing customers with reliable products and services. We offer a variety of flow meters for purchase. You can click on the flow meter names in the table to visit each product page for more details or leave a message. If you have other purchasing or technical questions, feel free to contact us!
Learn more:
• Understand Different Types of Flow Meters in 5 Minutes