In fluid measurement technology, two main types of instruments are commonly used to determine the mass flow rate of gases or liquids — the Coriolis mass flow meter and the Thermal mass flow meter.

Fundamental Difference

A Coriolis flow meter determines mass flow by measuring how much fluid passes through a vibrating tube at a given point in time. In contrast, a Thermal flow meter calculates mass flow by analyzing the cooling effect that the moving fluid has on heated temperature sensors.

It’s essential to note that these instruments measure mass flow, not volumetric flow. Volumetric flow varies depending on the density, temperature, and pressure of the fluid. To convert mass flow to volumetric flow, the mass flow rate is divided by the fluid’s density. Because density changes with pressure and temperature, the volumetric rate fluctuates even when mass flow remains constant.

Modern mass flow meters often use multivariable transmitters or flow software to automatically compensate for temperature and pressure variations, providing outputs for both mass and standardized volumetric flow rates.

Below is an overview of the working principles and common applications of both flow meter types.

Thermal Mass Flow Meter: Operating Principle

Thermal mass flow meters operate on the principle of heat dissipation. The meter includes two temperature sensors inserted into the fluid stream. One sensor is heated continuously (velocity sensor), while the other measures the actual fluid temperature. The amount of heat carried away by the fluid is directly related to its mass flow rate.

There are two primary operational methods:

1. Constant Temperature Differential Method

A fixed temperature difference is maintained between the two sensors. As the flow increases, more heat is carried away by the fluid, and additional power is required to maintain the temperature gap. The change in electrical current used to maintain this differential is proportional to the mass flow rate.

2. Constant Current Method

Here, a constant electrical current is applied to the heated sensor. The temperature difference between the two sensors then varies depending on the flow velocity — higher flow rates result in smaller temperature differences.

Both methods rely on measuring how effectively the fluid cools the heated sensor. Because gases have relatively low heat capacity, thermal mass flow meters are primarily used for gas flow measurement rather than liquids.

Typical Applications of Thermal Mass Flow Meters

Thermal mass flow meters are widely used in:

• Monitoring SO₂ and NOx levels in emission control systems
• Measuring air-to-fuel ratios in boilers and furnaces
• Tracking flare gas in natural gas and biogas operations
• Detecting gas leaks in valves and regulators
• Measuring compressed air in industrial pneumatic systems

Coriolis Mass Flow Meter: Operating Principle

Coriolis mass flow meters work based on Newton’s second law of motion, detecting the phase shift caused by the inertia of the moving fluid. The device typically includes one or more vibrating tubes — either straight or curved — through which the fluid flows.

In the bent-tube design, the measuring tube is vibrated at a fixed frequency using an electromagnetic driver. Sensors are placed at the inlet and outlet of the tube. When no fluid flows, both sensors vibrate in phase.

Once the fluid begins moving, its inertia resists the motion of the vibrating tube, causing a measurable phase shift between the inlet and outlet sensors. This phase difference is directly proportional to the mass flow rate of the fluid.

Coriolis meters can measure both liquids and gases, though they are particularly suited for liquids due to their higher density, which helps maintain steady oscillations.

Common Applications of Coriolis Mass Flow Meters

Coriolis meters are found in applications requiring high-accuracy liquid flow measurement, including:

• Industrial processes measuring adhesives, resins, or solvents
• Food and beverage bottling and filling operations
• Oil and gas industries for liquid flow detection and custody transfer
• Petrochemical applications involving downstream petroleum fluids
• Mining, paint, coating, and sealant production lines

Conclusion

When comparing the two technologies, Coriolis mass flow meters generally offer higher accuracy, making them ideal for custody transfer and critical process applications. However, their cost is considerably higher than that of thermal flow meters.

Meanwhile, thermal mass flow meters have evolved significantly, with new designs featuring better sensitivity, faster processors, and advanced algorithms that enhance measurement accuracy — all at a lower installation cost. As a result, they remain a practical choice for many industrial gas applications where ultra-high precision is not mandatory.

Need Expert Advice?

If you’re unsure which flow meter suits your process best, our technical team is here to help. Contact us anytime at [services@jwcontroltech.com] for professional guidance and tailored solutions for your application.