How to Perform Accurate Mass Flow Meter Calibration

Introduction:

Accurate mass flow meter calibration is an essential aspect of flow measurement in many industries, including oil and gas, chemical processing, and manufacturing. Flow meters are used to measure the flow rate of liquids, gases, and vapors in a process, and calibration is necessary to ensure their accuracy and reliability. The process of calibration involves comparing the output of the flow meter under test to a reference standard. This article will provide an overview of the mass flow meter calibration process, including the types of flow meters and standards used, and the steps involved in performing an accurate calibration.

What is Mass Flow Meter Calibration?

Calibration is the process of determining the accuracy and reliability of a flow meter by comparing its output to a reference standard. Mass flow meters measure the mass flow rate of a fluid passing through a pipe, rather than the volume flow rate. The mass flow rate is a more accurate measure because it is not affected by changes in density, temperature, or pressure. Calibration ensures that the flow meter is providing accurate and repeatable measurements over its specified range, which is critical for process control and safety.

Types of Mass Flow Meters:

There are several types of mass flow meters, including Coriolis, thermal, and differential pressure. Each type has its own advantages and disadvantages, and the type chosen for a particular application depends on the specific requirements of that application.

Coriolis Flow Meters:

Coriolis flow meters measure flow by detecting the twisting or bending of a vibrating tube caused by the fluid passing through it. The magnitude of the twist is proportional to the mass flow rate. Coriolis flow meters are highly accurate and can measure flow rates from 0.1 to 10,000 kg/h with an accuracy of ±0.1%. Their main disadvantage is their cost and size, which limits their use in applications where space and budget are limited.

Thermal Flow Meters:

Thermal flow meters measure flow by heating up a sensing element and measuring the heat loss caused by the fluid passing over it. The rate of heat loss is proportional to the mass flow rate. Thermal flow meters are relatively inexpensive and can measure flow rates from a few grams per hour to several thousand kilograms per hour with an accuracy of ±1%. Their main disadvantage is their sensitivity to changes in fluid temperature and composition.

Differential Pressure Flow Meters:

Differential pressure flow meters measure flow by detecting the pressure drop across a constriction in the flow path, such as an orifice plate. The pressure drop is proportional to the mass flow rate. Differential pressure flow meters are widely used in many industries and are relatively inexpensive. They can measure flow rates from a few liters per minute to hundreds of cubic meters per minute with an accuracy of ±1%. Their main disadvantage is their sensitivity to changes in fluid density and viscosity, which can affect the accuracy of the measurement.

Calibration Standards:

Calibration standards are devices used to provide a reference for a flow meter under test. They are typically calibrated by a national standards laboratory or an accredited calibration lab, and their accuracy is traceable to national or international standards. The two main types of calibration standards used for mass flow meters are gravimetric and volumetric.

Gravimetric Standards:

Gravimetric standards use a weighing scale to measure the mass of fluid passing through a flow meter during a calibration. The mass flow rate is then calculated by dividing the measured mass by the time interval. Gravimetric standards are highly accurate and provide a traceable reference standard. Their main disadvantage is their complexity and cost, which limits their use to high-end calibration labs.

Volumetric Standards:

Volumetric standards use a calibrated container to measure the volume of fluid passing through a flow meter during a calibration. The volume flow rate is then calculated by dividing the measured volume by the time interval. Volumetric standards are less accurate than gravimetric standards but are simpler and less expensive to use. Their main disadvantage is their sensitivity to changes in fluid density and temperature.

The Calibration Process:

The calibration process for mass flow meters typically involves the following steps:

1. Verify the flow meter is clean and in good working order

2. Connect the flow meter to the calibration standard

3. Adjust the flow meter to match the calibration standard

4. Record the calibration data

5. Calculate the flow meter’s accuracy and uncertainty

Step 1: Verify the Flow Meter is Clean and in Good Working Order:

Before beginning the calibration process, it is essential to verify that the flow meter is clean, free of debris, and in good working order. Any build-up or deposits in the flow meter can affect its accuracy and reduce its lifespan. A thorough cleaning of the flow meter before calibration is critical to ensure the accuracy and repeatability of the calibration results.

Step 2: Connect the Flow Meter to the Calibration Standard:

The flow meter under test is connected to the calibration standard, either by plumbing it directly into the standard or using adapters or hoses. It is essential to ensure that the flow meter is connected correctly and securely to prevent any leaks or loss of pressure during the calibration process. The calibration standard should be appropriate for the type and range of the flow meter under test.

Step 3: Adjust the Flow Meter to Match the Calibration Standard:

The flow meter is then adjusted to match the calibration standard. The adjustments may involve changing the flow meter's zero and span settings, modifying the flow meter's internal electronics, or adjusting the flow meter's mechanical components. The adjustments aim to make the flow meter's output match the calibration standard's output as closely as possible.

Step 4: Record the Calibration Data:

During the calibration process, the flow meter's output is recorded at several flow rates, typically spanning the flow meter's specified range. The calibration data should include the calibrated flow rates, the flow meter's output, and the calibration standard's output. The data should also include any adjustments made to the flow meter and the calibration standard.

Step 5: Calculate the Flow Meter’s Accuracy and Uncertainty:

The calibration data is used to calculate the flow meter's accuracy and uncertainty. The accuracy is the difference between the flow meter's output and the calibration standard's output, expressed as a percentage of the calibration standard's output. The uncertainty is the degree of doubt or error associated with the accuracy measurement. The uncertainty typically includes contributions from the calibration standard, the flow meter, and the measurement procedure.

Conclusion:

Accurate mass flow meter calibration is essential for ensuring process control and safety in many industries. The calibration process involves comparing the output of the flow meter under test to a reference standard to determine its accuracy and reliability. There are several types of mass flow meters, including Coriolis, thermal, and differential pressure, each with its own advantages and disadvantages. Calibration standards, including gravimetric and volumetric, provide a traceable reference for a flow meter under test. The calibration process typically involves verifying the flow meter's cleanliness and working order, connecting it to the calibration standard, adjusting it to match the calibration standard, recording the calibration data, and calculating the flow meter's accuracy and uncertainty. Accurate mass flow meter calibration ensures that flow meters are providing accurate and repeatable measurements over their specified range, which is critical for process control and safety.