A flow meter is an instrument that indicates the measured flow rate and/or the total volume of fluid within a selected time interval. Simply put, a flow meter is used to measure the flow rate of fluid in a pipe or open channel. In many cases, the flow meter’s measurement accuracy may deteriorate. In such cases, the flow meter needs to be calibrated. This article will provide detailed information on flow meter calibration.
What is Flow Meter Calibration?
Flow meter calibration is performed according to the verification procedures for various flow meters promulgated by the National Bureau of Metrology. Except for standard throttling devices, which do not require laboratory verification, all other flow meters are almost always calibrated upon shipment. Flow meters should also be calibrated regularly during use.
The main calibration methods for liquid flow meters include the volumetric method, the mass method, the standard volume tube method, and the standard flow meter comparison method. The main calibration methods for gas flow meters include the sonic nozzle method, the servo-type standard flow meter comparison method, and the bell jar method.
What is the Standard for Flow Calibration?
Internationally, flow meter standards cover a wide range of areas. It includes fluid flow measurement, chemical equipment, and industrial automation systems. These standards provide unified technical specifications for the design, manufacture, and use of flow meters.
Flow meter measurement standards are a complex system. They encompass accuracy requirements, verification and calibration methods, and national and international standards. Only by strictly adhering to these standards can flow meter measurement results be accurate and reliable.
For example:
International Electrotechnical Commission (IEC) 61075-1 standard specifies that the initial calibration interval for industrial flow meters should not exceed 12 months. And calibration must be performed by a qualified metrology laboratory. The American Petroleum Institute (API) MPMS Chapter 3.1.2 further specifies this requirement: In high-temperature and high-pressure environments, the calibration interval should be shortened to 6 months. And each calibration must include a pressure pulsation test (API RP 754, 2020).
Why Calibrate Flow Meters?
After prolonged use, flow meters tend to lose accuracy. Flow meter measurement errors can be caused by a variety of factors, including:
- Flow meter aging
- Internal surface deposits
- Contamination
- Excessive exposure to chemicals
- Differences in fluid properties
- External influences
- Temperature
- Pressure
- Improper use
All of these factors can lead to inaccurate flow meter results. It is important to note that while all of these factors are cause for concern, in many cases, changes to the flow meter’s surface go unnoticed. Flow meter calibration not only improves product quality but also helps save energy and optimize resource utilization. Therefore, regular maintenance and calibration of flow meters is essential.
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How to Calibrate a Flow Meter?
Flow meter calibration is crucial for ensuring flow meter measurement accuracy. Proper calibration methods can improve flow meter measurement accuracy and ensure production quality. Flow meters are calibrated before leaving the factory. However, due to aging and environmental influences, flow meters require periodic recalibration. Different flow meters require different calibration methods depending on their characteristics. Our main calibration methods include the standard device method, static volume method, dynamic volume method, mass method, pressure differential method, and calorimetric method.
The standard device method applies to all flow meter calibrations. We can use a standard flow meter of known accuracy and a standard fluid (water) as a reference against which to compare the flow meter to be calibrated.
The static volumetric method is primarily suitable for calibrating low-flow liquid flow meters. The volume of fluid flowing through the flow meter over a certain period of time is measured and compared with the actual volume of fluid flowing into the container.
The dynamic volumetric method is suitable for calibrating high-flow liquid flow meters. The volume of fluid flowing through the flow meter over a certain period of time is typically measured while the fluid is flowing.
The mass method is suitable for calibrating mass flow meters. The mass of fluid flowing through the flow meter over a certain period of time is measured and compared with the actual mass of fluid flowing into the container.
The differential pressure method is suitable for calibrating differential pressure flow meters. The pressure differential generated when the fluid flows through the flow meter is measured and compared with the theoretically calculated pressure differential.
The calorimetric method is suitable for calibrating thermal flow meters. The heat change generated when the fluid flows through the flow meter is measured and compared with the theoretically calculated heat change.
Flow meter Calibration Steps:
The following steps can be used to calibrate a flow meter. You can refer to them.
First, we need to determine the calibration requirements. Depending on the flow meter types, we’ll determine the applicable calibration standards and procedures. General calibration standards include ISO, ASTM, or national/industry-specific standards. We also need to define the flow range to be calibrated, ensuring that the calibration points cover the actual flow range.
It’s also key to prepare the calibration equipment in advance. Select a calibrated, highly accurate flow meter as a reference standard. We should prepare different flow standards based on the calibration method. Common devices include volume tubes, piston flow meters, standard scales (for mass flow meters), or static volumetric devices. Common tools include pressure gauges, thermometers, and control valves to measure and control calibration conditions. As the saying goes, “A good worker must first sharpen his tools.” Preparing suitable tools ensures a smooth calibration process.
When beginning a calibration, there are several steps to consider. We need to ensure that the flow meter and calibration device are properly installed. All connections are leak-proof, and there is no residue in the piping. We must also maintain stable environmental conditions, such as temperature and pressure. These factors can affect flow measurement. Allow the fluid to circulate for a period of time. A more accurate calibration can be achieved when the system reaches a stable state. At different flow rate points, record the flow meter readings and those of the reference device. These readings should cover the full flow meter range. To ensure reliable results, measure each flow rate point at least twice or more and calculate the average.
This is where data analysis comes in. Compare the flow meter readings with those of the reference device and calculate the relative or absolute error. Then, we determine whether the accuracy is met according to the standard range. Generally, the flow meter’s error should be within the allowable range.
After calibration is complete, we need to document the calibration process, measurement data, calculation methods, error analysis, and conclusions. Finally, we need to produce a formal calibration report. This report should be archived for future inspections and quality audits.
The final step is adjustment and maintenance. If the error exceeds the allowable range, the flow meter requires adjustment or repair. A regular calibration should be established based on frequency of use and accuracy.
Through these steps, we can ensure the accuracy and long-term reliability of the flow meter. It provides reliable data support for production processes, energy management, and environmental monitoring.
Gas Flow Meter Calibration:
A gas flow meter can decrease in measurement accuracy over time. So, they need to be calibrated regularly. The following is an introduction to gas flow meter calibration. It is crucial to perform calibration according to the instructions in the gas flow meter’s manufacturer’s instruction manual.
First, verify that the gas flow meter is operating properly. Refer to the gas flow meter’s calibration certificate or factory inspection sheet. Verify that key parameters affecting measurement accuracy are correctly entered into the converter. For pipelines with stopped flow, check the flow meter’s zero flow rate.
For remote calibration, the gas flow meter must be removed and sent to a secondary or primary flow standard for calibration. This new calibration data is then entered into the existing data acquisition system. While remote verification may provide higher accuracy, it is expensive and labor-intensive. It can also affect the gas flow meter’s operating status and disrupt production schedules.
How to Calibrate a Mass Flow Meter?
Based on the calibration of a Sino-Inst mass flow meter, we have summarized the following steps for your reference. Before calibration:
1. The flow meter is guaranteed to be full. Medium-filled flow meter
2. To ensure that the internal medium does not flow. The valves at both ends of the flow meter are closed.
3. Click on the flow meter panel.
- ↑ Switch to the DP interface.
- Observe the change value of DP. Changes of no more than 10 digits are qualified.
- Satisfy the above 3 points, and you can calibrate the zero point.
- Long-press the SET button to enter the user menu
- Default password 0000
- Click SET to turn the page until the zero point calibration page
- Click the → arrow to move the cursor to Y
- Click SET to confirm the cursor moves up
- Then move the cursor to Yes
- Click SET to complete the zero-point test.
- The zero point test time is estimated to be about 30 seconds.
- Click ↑ to switch to the DP interface
- Observe the change range of DP.
- The average value of the change is guaranteed to be within ±5, that is, the zero point calibration is successful.
If there is a deviation, the operation can be repeated.
The following video shows the detailed operation process. The calibration process may vary between manufacturers. Therefore, it is recommended to consult with relevant professionals when calibrating.
How often should a flow meter be calibrated?
Common flow meter calibration intervals:
To ensure the accuracy of measurement results, flow meters require regular metrological calibration. The following is a detailed description of calibration intervals.
1. Coriolis mass flow meters: Those with an accuracy class of 0.5 or better generally require no more than one year. Those with an accuracy class of 0.5 or below typically require no more than two years.
2. Other mass flow meters: Those used for trade settlement generally require no more than one year. Other flow meters typically require no more than two years.
3. Ultrasonic flow meters: Generally require no more than two years. For insertion-type flow meters, if they have self-diagnostic functions and can retain alarm records. Calibration can also be performed every six years, with annual on-site inspections.
4. Electromagnetic flow meters: Accuracy class 0.2 or better requires a one-year calibration interval. The calibration cycle for flow meters with an accuracy grade lower than 0.2 and those using referenced errors is two years.
5. Turbine flow meters: A common calibration cycle is two years. Accuracy grades 0.5 and higher require calibration once a year.
6. Vortex flow meters: Generally, no more than two years.
7. Differential pressure flow meters: The calibration cycle for throttling devices using the geometric test method is generally no more than two years. Standard nozzles, long-diameter nozzles, classic Venturi tubes, and Venturi nozzles used to measure single-phase clean fluids may be extended based on usage, but generally no more than four years.
8. Gas positive displacement flow meters: The calibration cycle for flow meters with accuracy grades 0.2 and 0.5 is two years. For other grades, it is three years. For periodically calibrated flow meters, if the indication error calculated using the flow meter coefficient specified in the last calibration certificate exceeds the maximum allowable error. And the linearity error of the calibration result meets the requirements, the calibration cycle is one year.
9. Float flow meters: Generally, no more than two years.
10. Liquid positive displacement flow meter: Flow meters used for trade settlements and frequently used flow meters with a calibration rating greater than 0.5 are typically calibrated every six months.
What is the main purpose of flow meter calibration?
The main purpose of flow meter calibration is to improve the meter's measurement accuracy. Regular calibration ensures accurate measurement results and also extends the meter's service life.
How do you test a flow meter?
Flow meter testing primarily involves the following aspects.
An accuracy test involves flowing a medium at a certain flow rate from the bottom chamber of a standard flow meter through the flow meter under test. By comparing the readings of the standard flow meter and the flow meter under test, the accuracy of the flow meter under test is calculated.
A repeatability test is an important indicator for evaluating the repeatability of flow meter measurement results. Usually, the same flow rate is measured multiple times (at least 10 times) under the same operating conditions. The changes in the test results are compared to assess the flow meter's repeatability.
A stability test involves measuring flow rates over different time periods. To determine whether the test results are stable. The test results should be consistent within the tolerance range.
All in all, flow meter calibration is a complex process. Each flow meter has its own calibration method. Please follow the product manual for the specific calibration process. If necessary, you can also consult the supplier or a relevant calibration agency.
Sino-Inst is a professional flow meter supplier. If you have any flow meter-related technical questions. Please feel free to contact us. Our professional technical engineers are available 24 hours a day to solve your questions free of charge.
