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:
1. Correct measurement errors caused by instrument drift and wear to ensure measurement accuracy
2. Comply with metrology regulations; flow meters used for trade settlement must undergo periodic verification and calibration
3. Stabilize production processes, precisely control material ratios, and prevent production anomalies
4. Avoid economic losses in material and energy settlements caused by measurement deviations
5. Detect hidden faults in sensors and piping to enable proactive equipment maintenance
6. Calibrate new meters and equipment after installation or replacement to confirm they are qualified and operational
7. Establish measurement traceability so that measurement data can be used for quality inspection and audit verification
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.
Standard Apparatus Method
This method uses a standard flow device—which has been verified by a national metrology authority and offers higher measurement accuracy—as the reference. The flowmeter to be calibrated is connected in series with the standard apparatus to collect flow data simultaneously.
By comparing the readings from both devices, the flowmeter’s error, repeatability, and linearity are calculated. The entire system is equipped with temperature control and pressure-stabilized piping to eliminate measurement errors caused by changes in medium pressure and temperature. This is a common and fundamental calibration method in the field of flow measurement.
Applicable Calibration Objects
This method covers virtually all liquid and gas flow meters, including vortex, electromagnetic, thermal, turbine, ultrasonic, rotor, and differential pressure flow meters. It is commonly used for factory batch calibration and laboratory legal metrological verification.
Static Volumetric Method
This is an offline volumetric calibration method. The calibration process involves delivering the medium at a constant flow rate into a standard calibration vessel. Once the liquid level in the vessel stabilizes, the flow is shut off, and the precise volume of the standard vessel is recorded.
This volume is then converted to a standard volume based on the medium’s temperature and pressure, and compared with the flowmeter’s cumulative flow to complete the calibration. Since readings are taken while the fluid is at rest, the process is simple to operate and involves low equipment costs.
Suitable Calibration Targets
Small- to medium-diameter liquid flowmeters, including common types such as turbine flowmeters, electromagnetic flowmeters, oval gear positive displacement flowmeters, and metal-tube float flowmeters.
This method is only suitable for liquid media at ambient temperature, such as water and oils, and is not suitable for gases or highly viscous, volatile fluids.
Dynamic Volume Method
An online, continuous volumetric calibration method. The standard volume tube contains a replaceable calibration section. As the fluid flows continuously, it drives a piston or displacer to move at a constant speed through the standard volume section.
By precisely capturing the start and stop signals of the displacer, the standard instantaneous volumetric flow rate is obtained. The medium flows continuously throughout the process, simulating actual field conditions and eliminating measurement errors caused by flow start-up and shutdown.
Applicable Calibration Objects
Large-diameter liquid pipeline flowmeters, such as in-line electromagnetic flowmeters, large-diameter turbine flowmeters, and ultrasonic liquid flowmeters. Widely used for online in-flow calibration in water supply and chemical processing pipelines; not suitable for gaseous media.
Mass Method (Weighing Method)
Using high-precision electronic scales as the measurement standard, the fluid continuously flows into a sealed weighing vessel. The flowmeter’s flow rate and the mass of the medium in the weighing system are recorded simultaneously.
The standard volumetric flow rate is calculated based on the medium’s density. Weighing is traceable to a mass standard, offering the highest measurement accuracy class and strong resistance to temperature and viscosity interference.
Applicable Calibration Objects
Liquid flow meters used in high-precision measurement scenarios, including micro-flow thermal flow meters, Coriolis mass flow meters, and high-precision gear flow meters.
This method serves as the primary calibration method for flow meters measuring petroleum products and fine chemical media; it can also be used for micro-gas flows when paired with a collection device.
Differential Pressure Method
This method traces calibration back to the flow calculation formula based on a throttling device. By varying the flow rate in the pipeline, it simultaneously collects readings from a standard differential pressure sensor and the output signal of the flowmeter under calibration to establish a correlation between flow rate, differential pressure, and medium density.
This allows for the correction of the flowmeter’s flow coefficient without requiring extensive medium circulation, and the calibration equipment features a simple structure.
Applicable Calibration Objects
Various types of differential pressure flowmeters, orifice plates, nozzles, Venturi flowmeters, wedge flowmeters, and balanced flowmeters. Suitable for gases, liquids, and steam media, this method is primarily used for offline coefficient calibration of throttling-type flowmeters.
Calorimetric Method (Thermal Method)
Calibration is performed based on the principle of fluid heat exchange. A constant, controllable amount of heat is introduced into the fluid in the pipeline, and the temperature difference between the fluid inlet and outlet is precisely measured.
The standard mass flow rate is calculated using the law of conservation of energy, and the instrument’s K-factor is corrected by comparing it with the flow rate output from a thermal flowmeter. The calibration process does not require large-volume or weighing equipment.
Applicable Calibration Objects
Various types of thermal mass flow meters for gases and micro-gas flow sensors, including insertion-type thermal gas flow meters and micro-range mass flow controllers. Primarily used for low-flow calibration of gases such as air, nitrogen, and biogas; not suitable for liquid media.
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.
Pre-Calibration Preparation and Operating Condition Inspection
Before calibration, complete the necessary preparations and inspect the operating conditions to confirm that the flowmeter to be calibrated is in good physical condition, with proper wiring and correct parameters, and that the standard apparatus has passed verification and remains within its validity period.
Check that the piping is free of blockages and leaks, and ensure that pressure-stabilizing and temperature-control equipment are operating stably.
Prepare the calibration medium appropriate for the instrument type, and record environmental and medium parameters for subsequent data correction.
Equipment Installation and System Commissioning
Install the flowmeter to be calibrated in series with the standard apparatus, ensuring the piping is straight and free of unnecessary throttling.
Start the equipment to circulate the medium and purge the piping and instrument of air bubbles and impurities to prevent any impact on measurement accuracy.
Commission the data acquisition system to ensure synchronized data collection, verify the instrument’s zero point and initial readings, and complete system commissioning.
Setting Calibration Flow Points
Set up at least five gradient flow points in accordance with metrology standards, covering the full measurement range of the flowmeter. For high-precision instruments, the number of measurement points may be appropriately increased.
Adjust the flow rate point by point; data collection may only begin once the medium’s pressure, temperature, and velocity have stabilized to ensure the validity and reliability of the data.
Synchronized Multi-Point Data Collection
Repeat testing at each flow point at least three times to ensure data repeatability.
Simultaneously record the flowmeter readings, reference data from the standard apparatus, and key parameters such as the medium’s temperature, pressure, and density.
Maintain stable operating conditions throughout the calibration process to avoid external interference and parameter changes, ensuring accurate and traceable data.
Data Calculation and Error Analysis
Organize the raw data and correct the flow values based on temperature and pressure parameters to establish a unified measurement reference.
Calculate indicators such as indication error, repeatability, and linearity error for each measurement point, and compare them against standards to determine compliance.
Exclude outlier data and analyze various sources of error, including equipment, operating conditions, and operational factors.
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.
FAQ
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.
