A flow meter is a test device used to measure the flow of process fluids and gases in industrial plants and facilities. Common flowmeters include electromagnetic flowmeters, mass flowmeters, turbine flowmeters, vortex flowmeters, orifice plate flowmeters, and ultrasonic flowmeters. Flow rate is the rate at which a process fluid passes through a pipe, orifice plate, or vessel in a given amount of time. Control and instrumentation engineers measure this value to monitor and adjust the speed and efficiency of industrial processes and equipment.

Ideally, test equipment must be “reset” from time to time to prevent inaccurate readings. However, due to aging electronics, coefficient deviations, etc., in industrial environments, flowmeters are calibrated periodically to ensure accurate measurements for safe and timely operation.

What is Flowmeter Calibration?

The verification and calibration of flowmeters are carried out in accordance with the verification regulations for various flowmeters issued by the State Bureau of Weights and Measures. Applied flowmeter, except for the standard throttle device does not have to carry out experimental calibration, the rest of the flowmeter flowmeter manufacturers usually after the production of their products for internal calibration, or send them to an independent calibration organization for adjustment.

Flow rate is an important parameter in automated production processes such as petroleum, metallurgy, power, chemical, pharmaceutical and food. Flow measurement calibration is indispensable from process monitoring to product quality inspection. The level of supervision of flow measurement and calibration is also directly related to the quality of production, production efficiency and the full utilization of energy.

Electromagnetic Flowmeter Online Calibration Method

At present, the common methods of electromagnetic flowmeter online calibration are mostly standard meter method and electrical parameter measurement method, weighing method, volume method.

Standard meter method 

The principle of the standard meter method is through the installation of electromagnetic flowmeter pipeline with a standard meter (readings are considered as standard values, the standard meter installation location should be in line with the requirements of the relevant standards and specifications), the measured value of the standard meter and the electromagnetic flowmeter measured value comparison, such as the error to meet the maximum permissible deviation will be assessed as compliant with the requirements.

The main steps of the method are:

1. Select a suitable standard meter. The standard meter should have high accuracy and stability to meet the calibration requirements of electromagnetic flowmeter.

2. When selecting the standard meter, it is necessary to consider its measurement range, accuracy, sensitivity and other factors to ensure the accuracy of the calibration results.

3. Prepare the calibration equipment. Before the implementation of calibration, you need to check the electromagnetic flowmeter and standard meter connection piping, valves and other equipment to ensure that it is in good working condition. At the same time, it is also necessary to prepare a certain amount of standard liquid for flow measurement in the calibration process.

4. Calibration operation. First, the standard liquid is measured through the electromagnetic flowmeter and standard meter, and the readings of the flowmeter and standard meter are recorded. Then, the calibration coefficient of the electromagnetic flowmeter is calculated according to the calibration formula of the electromagnetic flowmeter. Finally, the calibration coefficients are applied to the actual measurement to improve the measurement accuracy of the electromagnetic flowmeter.

5. Calibration results analysis. In the calibration process, the calibration results need to be analyzed to determine the calibration effect of the electromagnetic flowmeter. If the calibration results meet the requirements, you can continue to use the electromagnetic flowmeter for measurement; if the calibration results do not meet the requirements, you need to re-calibrate or replace the electromagnetic flowmeter.

Electrical Parameter Method

The principle of the electrical parameter measurement method is to measure the basic characteristics of the instrument electrical parameters, and the measured value is compared with the standard value, the standard value is generally obtained from the instructions of the electromagnetic flowmeter, such as the relative display value error in the maximum permissible error range and the repeatability to meet the standard requirements, the accuracy of the measurement is unaffected, and can be used normally or assessed as compliant with the requirements.

The specific operation steps are as follows.

 1. Turn off the power of the electromagnetic flowmeter and make sure that the electrodes are isolated from the fluid to prevent electrode contamination.

 2. Use a multimeter to measure the resistance between the electrodes. Normally the resistance between the electrodes should be within a certain range, if the resistance value is out of the range, it may indicate that there are problems with the electromagnetic flowmeter. 

3. Use a conductivity meter to measure the conductivity of the fluid. Conductivity is a property of the fluid and is related to the type and concentration of the fluid. The accuracy of the electromagnetic flowmeter measurement is affected by the conductivity of the fluid and therefore needs to be calibrated at the time of measurement.

4. Use a capacitance meter to measure capacitance. Capacitance is an important parameter in electromagnetic flow meters, which affects the sensitivity and measuring range of the meter. Therefore, the capacitance needs to be measured during the calibration process.

5. According to the measurement results, the electromagnetic flowmeter can be adjusted to ensure its measurement accuracy.

Weighing method

Weighing method is a more traditional calibration method, mainly applicable to smaller caliber electromagnetic flowmeter. The basic principle of this method is to be calibrated electromagnetic flowmeter and a standard flowmeter installed in the same pipe at the same time, by comparing the two flowmeter readings to determine the accuracy of the flowmeter to be calibrated.

The weighing method is performed as follows.

1. Prepare two electromagnetic flowmeters, one is the flowmeter to be calibrated and the other is the standard flowmeter. The accuracy of the standard flowmeter should be higher than that of the flowmeter to be calibrated.

2. Install both meters in the same pipeline in the same position and direction.

3. Turn on the meters and allow the fluid to pass through the pipe. At this point, both meters will measure the flow rate of the fluid and display it on their respective readouts. 

4. Record the readings from both meters and calculate the difference between them. This difference is the error of the meter to be calibrated. 

5. According to the size and direction of the error, adjust the zero point or sensitivity of the flowmeter to be calibrated to achieve the purpose of calibration.

Volumetric Method

The volumetric method is a commonly used online calibration method for electromagnetic flow meters. The basic principle is to measure the volume of the liquid flowing through the electromagnetic flowmeter, so as to calculate the flow rate, and then realize the calibration of the electromagnetic flowmeter.

The procedure is as follows.

1. First of all, it is necessary to prepare a certain amount of known volume of liquid, such as water or standard liquid. The volume of these liquids need to be measured accurately, and to ensure that its conductivity in line with the working requirements of the electromagnetic flowmeter.

2. Install the electromagnetic flowmeter on the pipe to be calibrated, and ensure that its measuring range matches the flow range of the pipe to be calibrated.

3. Pass the prepared liquid through the electromagnetic flowmeter for measurement. During the measurement process, it is necessary to record the flow rate, flow velocity, temperature and other parameters of the liquid, as well as the output signal of the electromagnetic flowmeter.

4. According to the output signal of the electromagnetic flowmeter and the parameters such as flow rate, velocity and temperature of the liquid, the calibration coefficient of the electromagnetic flowmeter can be calculated. The calibration coefficient indicates the proportionality between the output signal of the electromagnetic flowmeter and the actual flow rate.

5. According to the calibration coefficient, the electromagnetic flowmeter can be calibrated, so as to ensure its measurement accuracy in actual work.