Supercritical fluids, such as supercritical carbon dioxide (SC-CO ₂), are increasingly widely used in industrial and scientific fields such as extraction, dyeing, power generation, and carbon capture due to their unique physical properties. However, when it is near the critical point, even small temperature or pressure fluctuations can cause drastic changes in physical properties such as density and viscosity, posing a challenge to accurate flow measurement. This article aims to review the mainstream measurement techniques under such harsh working conditions.

1、 Core Measurement Challenge

The core difficulty of measuring the flow rate of supercritical fluids lies in the nonlinearity and instability of their physical properties. Traditional flow meters are usually calibrated based on fixed fluid property parameters, while the density of supercritical fluids is not a constant value, which leads to a significant decrease in accuracy for instruments that rely on volumetric flow measurement, such as turbine flowmeters. In addition, high-pressure environments require mechanical strength and sealing of instruments, and there is a risk of measurement failure due to improper control causing fluid phase transition.

2、 Mainstream technology routes and analysis of their advantages and disadvantages

At present, there are two main technologies that can effectively address this challenge:

Coriolis mass flowmeter: This is currently the most mainstream and reliable solution. It directly measures the mass flow rate of the fluid, fundamentally avoiding errors caused by density changes. It has significant advantages of high precision and no need for additional temperature and pressure compensation. But its disadvantage is that the thick pipe wall under high pressure leads to higher costs, and there is a certain pressure loss, and it is more sensitive to pipeline vibration.

Ultrasonic flowmeter: This technology uses the propagation speed of ultrasonic waves in fluids for measurement, which belongs to non-contact measurement, has no flow resistance, and has low pressure loss. The Doppler method is applicable to fluids containing particles or bubbles, while the propagation time rule is more suitable for clean media. However, the speed of ultrasound itself is affected by fluid density, so when measuring supercritical fluids, high-precision temperature and pressure sensors must be used for real-time compensation, resulting in high system integration complexity.

3、 Conclusion and Prospect

In summary, in the flow measurement of supercritical fluids, the Coriolis mass flowmeter is considered a technology due to its direct measurement of mass flow rate, especially in processes that require high precision. Ultrasonic flow meters are competitive in applications with large pipe diameters and low pressure losses. In the future, with the development of multi-sensor data fusion and advanced compensation algorithms, intelligent flow meters combined with real-time temperature and pressure measurement will be able to better "handle" this special medium hovering at the critical point, providing solid guarantees for the improvement and safety control of related industries.