How to Calibrate a Flow Meter

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 Flow Meter Calibration?

Calibrating a flow meter is essential to ensure accurate and reliable measurements. The calibration process involves comparing the flow meter's readings to a known standard or reference and adjusting the meter if necessary. Here’s a general step-by-step guide to calibrating a flow meter:

• Understand the Flow Meter Type: Different flow meters (e.g., electromagnetic flow meter, vortex flowmeter, ultrasonic flow meter, turbine flowmeter) have unique calibration requirements. Refer to the manufacturer’s manual for specific instructions.

• Gather Equipment: You’ll need a calibration standard (e.g., a master flow meter, weigh scale, or prover), a flow loop, and any necessary tools.

• Ensure Proper Installation: Verify that the flow meter is correctly installed according to the manufacturer’s guidelines (e.g., straight pipe runs, proper orientation).

• Check Environmental Conditions: Ensure the fluid, temperature, pressure, and other conditions match the operating conditions for which the meter is being calibrated.

Flow Meter Calibration Method

Flow meter calibration is performed to ensure measurement accuracy and reliability. Below are the detailed steps and relevant information for flow meter calibration:

1. Preparation

Select a reference instrument: Choose a high-accuracy reference instrument capable of precisely measuring the flow rate of the meter under calibration. The accuracy of the reference instrument should be significantly higher than that of the flow meter being calibrated.

Select an appropriate calibration fluid: The calibration fluid should have properties similar to the actual process fluid. Ensure that the fluid’s temperature, pressure, and stability meet the required standards.

2. Zero Adjustment

With the flow meter in a no-flow condition, check whether the zero point is correct.

If the zero point is incorrect, adjust it by using the zero adjustment mechanism or by repositioning the sensor until the correct zero point is achieved.

3. Linearity Calibration

Measure the flow rate at multiple flow points using the reference instrument and record the results.

Compare the flow meter readings with those of the reference instrument and calculate the errors.

Based on the error values, adjust parameters such as the range settings or sensor position to bring the flow meter readings as close as possible to the reference values.

4. Repeated Calibration and Verification

Perform multiple calibration runs as needed to improve accuracy and reliability.

After calibration, verify the flow meter again using the reference instrument. If the results meet the expected accuracy requirements (for example, system accuracy up to 0.2%), the calibration is considered successful. If not, recalibration is required.

5. Precautions

Calibration must be carried out strictly in accordance with relevant operating procedures and standards.

Ensure that the calibration equipment, environment, and flow meter are all in stable and normal operating conditions to guarantee accurate and reliable results.

The calibration interval depends on factors such as operating environment, fluid properties, and flow meter type. In general, flow meters should be calibrated at least once a year.

How to Calibrate a Flow meter

Electrical Parameter Method

The electrical parameter method is mainly used to determine the accuracy of the flowmeter by measuring the electrical parameters of the electromagnetic flowmeter, such as resistance between electrodes, conductivity and capacitance. The advantage of this method is that it can be calibrated without downtime and has less impact on the production process.

The steps are:

1. Turn off the power of the electromagnetic flowmeter and make sure 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 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 measurement accuracy of an electromagnetic flowmeter is affected by the conductivity of the fluid and therefore needs to be calibrated at the time of measurement.

4. Measurement of capacitance using a capacitance meter. Capacitance is an important parameter in an electromagnetic flowmeter, which affects the sensitivity and measuring range of the flowmeter. 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.

It should be noted that the electrical parameter method can only make a preliminary judgment on the accuracy of the electromagnetic flowmeter, and if the electromagnetic flowmeter is found to have problems, it needs to be further repaired or replaced. At the same time, the calibration cycle of the electromagnetic flowmeter should be determined according to the actual use of the situation, in general, the calibration cycle is half a year or one year.

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 specific steps of the weighing method are 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 needs to be higher than the flowmeter to be calibrated.

2.  Install both flowmeters on the same pipe at the same time and keep the same installation position and direction.

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

4.  Record the readings of the two flowmeters and calculate the difference between them. This difference is the error of the flowmeter 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.

The advantage of the weighing method is that it is simple to operate, does not require special equipment, and is suitable for all types of electromagnetic flowmeter. However, the accuracy of this method is affected by many factors, such as the nature of the fluid, the conditions of the pipeline, the installation position of the flowmeter and so on. Therefore, the weighing method is only suitable for preliminary calibration, and for high-precision calibration, it is necessary to use more advanced calibration methods.

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 specific operation steps are: 

1.  First, you need 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 to ensure that its measurement range matches the flow range of the pipe to be calibrated.

3. Measure the prepared liquid through the electromagnetic flowmeter. During the measurement process, it is necessary to record the parameters such as the flow rate, flow velocity and temperature 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, flow velocity and temperature of the liquid, the calibration coefficient of the electromagnetic flowmeter can be calculated. The calibration coefficient indicates the proportional relationship between the electromagnetic flowmeter output signal and the actual flow rate.

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

Volume method is a simple and practical online calibration method for electromagnetic flow meters. By measuring the volume of the liquid, the calibration coefficient of the electromagnetic flowmeter can be calculated, thus realizing the calibration of the electromagnetic flowmeter. However, it should be noted that the accuracy of the volumetric method is affected by the nature of the liquid, the installation conditions of the flowmeter and other factors, so a variety of factors need to be considered comprehensively in the actual application to ensure the accuracy of the calibration results.