Magnetic Flow Meter Calibration Procedure – User Guide

After a period of use, electromagnetic flow meters may experience a decrease in accuracy. Magnetic flow meter calibration eliminates systematic errors inherent in the instrument, ensuring the reliability of its measurement results. This meets the needs of process control, cost accounting, and trade settlement.

Electromagnetic flow meters are suitable for measuring the flow rate of conductive liquids in closed pipelines and are widely used in various industries such as chemical engineering, wastewater treatment, agricultural irrigation, and urban water management. Therefore, electromagnetic flow meter calibration is a crucial issue for users. In fact, the calibration cycle and implementation process should be considered before purchasing.

RFQ

Calibration vs. Verification:

Calibration is the process of comparing and correcting the instrument’s output value to bring it closer to the “true value,” focusing on improving accuracy. Typically, we calibrate the flow meter in-house before shipment and provide a calibration report. Users can then perform periodic calibrations according to their needs.

Verification is a conformity assessment conducted by a legally authorized body according to regulations, focusing on legal compliance. For example, if you purchase an electromagnetic flow meter with an accuracy of 0.5%FS, you can submit it to a third-party organization for testing and obtain a flow rate test report to verify whether the 0.5% accuracy is accurate.

The Importance of Electromagnetic Flowmeter Calibration

1. Ensure the measurement accuracy of electromagnetic flowmeters. Electromagnetic flowmeters are crucial in water treatment, chemical engineering, and various manufacturing industries. Ensuring their accuracy guarantees the accurate measurement of liquid consumption and output.
2. Save costs and improve efficiency. Measurement errors are not directly visible to the naked eye, but after calculation, they can represent significant economic costs. Accurate flow monitoring provides accurate analytical data, preventing raw material waste, optimizing production processes, and saving costs.
3. Comply with industry standards and regulations. Various industries have their own industry standards and quality management systems (ISO 9001), all of which require accuracy in flow measurement and regular calibration.
4. Extend the lifespan of electromagnetic flowmeters. Regularly calibrating flowmeters stabilizes measurement accuracy and prevents malfunctions.

Electromagnetic Flowmeter Calibration Cycle

Calibration Cycle: The verification procedure JJG 198-94, “Verification Procedure for Velocity Flowmeters,” stipulates that the calibration cycle for flowmeters with an accuracy class of 0.1, 0.2, and 0.5 is six months. For electromagnetic flowmeters with an accuracy class lower than 0.5, the calibration cycle is generally specified as two years, although longer cycles are also possible.

Furthermore, in some practical applications, strictly adhering to the procedures is very difficult. For example, large-diameter electromagnetic flowmeters are difficult to install and disassemble, making actual flow calibration difficult to achieve during periodic calibration. Online periodic verification and inspection are often used instead.

Magnetic Flow Meter Calibration Procedure

1. Select the appropriate water pump based on the pipe diameter and flow rate for the calibration test.
2. If the system uses compressed air power, turn on the air compressor to reach the required air source pressure to ensure rapid switching of the commutator and normal operation of the clamp meter.
3. After the electromagnetic flowmeter is correctly installed and connected, it should be preheated for approximately 30 minutes.
4. If using a high-level water tank, check if the overflow signal of the pressure stabilizing tower appears. Before the formal test, circulate the calibration medium in the pipeline system for a certain period of time, while checking for leaks at all sealing points in the pipeline.
5. Before starting the formal calibration, fill the sensor of the flowmeter under test with the calibration medium, then close the downstream valve for zero-point adjustment.
6. At the start of the calibration, first open the valve at the front end of the pipeline, then slowly open the valve after the flowmeter under test to adjust the flow rate at the calibration point.
7. During the calibration process, the flow rate stability at each flow point should be within 1% to 2%—for the flow rate method, while for the total flow rate method, it can be within 5%.
8. The temperature change of the test medium should not exceed 1°C during the calibration process at a single flow point, and should not exceed 5°C during the entire calibration process.
9. The downstream pressure of the flowmeter under test should be sufficiently high to prevent flashing and cavitation within the flow path (especially in narrow-bore sections).
10. Each measurement should last at least the minimum allowable measurement time of the device, generally not less than 30 seconds. For Class A instruments (referring to electromagnetic flowmeters with frequency output, and insertion-type electromagnetic flowmeters with frequency output), the absolute relative error of the number of pulses output by the flowmeter in a single calibration should not exceed 1/3 of the repeatability of the flowmeter under test.
11. Each calibration point should be calibrated at least three times. For Class 0.1 and 0.2 flowmeters, each calibration point should be calibrated at least six times.
12. After each test, the valve at the front end of the test pipeline should be closed first, then the pump should be stopped to prevent the pressure stabilizing equipment from being emptied. Simultaneously, all remaining test medium in the test pipeline must be vented, and finally, the control system and air compressor should be shut down.

The calibration points for evaluating instrument performance are generally specified as follows:

For Class A instruments, referring to electromagnetic flowmeters with frequency output, and insertion-type electromagnetic flowmeters with frequency output, calibration points should include qmin, 0.07qmax, 0.15qmax, 0.25qmax, 0.4qmax, 0.7qmax, and qmax. When the flow rate at the latter few calibration points is less than qmin, this calibration point can be disregarded.

For Class B instruments, referring to electromagnetic flowmeters that output analog signals or can directly display instantaneous flow, calibration points should include at least 5 verification points, including qmin and qmax, and should be evenly distributed.

For calibrations not used to evaluate instrument performance (such as factory calibration), fewer calibration points may be specified.

Calibration of Electromagnetic Flowmeters with Clamp-on Ultrasonic Flowmeters

The accuracy of electromagnetic flowmeters is generally 1.0%, 0.5%, or 0.2%, while that of ultrasonic flowmeters is 1.0% or 0.5%. According to the calibration procedures for velocity flowmeters, the error of the testing device must be smaller than that of the electromagnetic flowmeter, which obviously does not meet the requirements for online calibration of electromagnetic flowmeters.

However, this is quite common in practical applications. This is because the disassembly, transportation, delivery, and installation costs of large-diameter electromagnetic flowmeters are significant. Furthermore, some process pipelines require continuous production and cannot be shut down. Therefore, using clamp-on ultrasonic flowmeters for inspection and calibration remains feasible and meaningful. This is because errors caused by metering disputes, electrode scaling, or converter malfunctions are generally much larger than 0.5% or 1.0%.

For example, one of our clients, a water supply company, previously installed a DN1200 in-line electromagnetic flowmeter, with most of the measured pipeline underground. In this case, only clamp-on ultrasonic flowmeters could be used, purchased using a “V”-type installation method, with two probes installed at the top of the pipeline for measurement.

Our instantaneous measurement was 3190 m³/h, while the electromagnetic flowmeter’s measurement was 3183 m³/h. Over a period of time, the two measurements consistently differed by approximately 6-7 m³/h. The cumulative values ​​over that period were: ultrasonic flowmeter 532 m³/h, electromagnetic flowmeter 530 m³/h, with an error of 0.375%.

The larger the pipe diameter, the greater the time difference Δt between the forward and reverse flow, allowing for more precise measurement of the time difference and thus more accurate flow rate measurement. Therefore, using a high-precision ultrasonic flowmeter for on-site calibration of the electromagnetic flowmeter is feasible.

Technical Support

Magnetic flow meter calibration is crucial for ensuring the accuracy and consistency of measurement results. Therefore, all users should perform magnetic flow meter calibration regularly to guarantee the normal operation of the flow meter.

Our Sino-Inst electromagnetic flow meters undergo factory calibration before shipment, and a calibration report is provided. We also provide technical support for users encountering calibration problems during use. Please feel free to contact us at any time.

Zhang Wei

Zhang Wei, possesses 20 years of experience as an automation instrumentation engineer, specializing in the research, design, installation, commissioning, and maintenance of automation instruments.

Face to various instrument communication protocols (such as Modbus, Profibus, etc.), with solid hardware circuit design and software programming skills (proficient in C language and PLC programming). Has extensive project experience; projects he has led and participated in have all achieved outstanding results, improving product accuracy, reducing costs, and increasing production efficiency.

Possesses excellent communication and coordination skills and a strong team spirit, enabling him to quickly respond to customer needs and provide high-quality automation instrumentation solutions.