Flow Research

Could a magnetic or vortex flowmeter ever have the accuracy of a Coriolis meter? In an article published in the January edition of Processing magazine, I argue that a key feature that underlies the accuracy of a flowmeter is the relation between the physical principle underlying its operation and the output value that represents flow. If there is a tight connection between the two, with few intervening variables, then the connection is said to be tightly coupled. Such is the case for many Coriolis flowmeters. If there are many intervening variables between the operating principle and the flowmeter output, and/or the intervening variables are imprecisely determined, then the connection is loosely coupled. In that case, flowmeter accuracy is not likely to be very high.

This could mean that, as of now, magnetic and vortex meters can’t be as accurate as Coriolis meters, at least when Coriolis meters are measuring liquids. (It is well known that Coriolis meters are less accurate when measuring gas rather than liquids due to the low density of gas.) The connection between the operating principle of a flowmeter and its output should be of interest to product managers and designers, as they try to design more accurate and reliable flowmeters. But it should also be of interest to end-users as they try to balance accuracy, reliability, repeatability, and overall performance with purchase price and lifecycle costs.

This question is not unique to magnetic and vortex meters; it can be raised for any kind of meter. That means the question is worth thinking through, no matter what type or types of flowmeters you are dealing with. Here is the article link: https://www.processingmagazine.com/process-control-automation/instrumentation/flow-measurement/article/55342221/the-key-features-that-underlie-flowmeter-accuracy. I welcome any comments.

In case you haven’t had a chance to review our latest study on Coriolis meters, which just came out in Q3 2025, here’s a link to that page: https://www.flowresearch.com/coriolis/.

To review our magnetic flowmeter study, which is about to be released, go to https://www.flowresearch.com/mag/.

Yours in flow,

Jesse Yoder

What is mass? Mass may be defined as the amount of matter in an object. I define matter as a material object—anything that exists independently of perception and can be observed simultaneously by more than one person, such as desks, cars, or stars. These objects are made up of molecules, which in turn are made up of atoms. Atoms consist of protons and neutrons in the nucleus, with electrons bound to them. Protons in turn are made up of quarks, gluons, and other subatomic particles. The mass of a proton is almost entirely derived from the near light speed motion of its quarks and other subatomic particles. That is the ultimate origin of mass, with energy.

Volumetric flowmeters measure how much space the fluid occupies per unit time. Mass flowmeters measure how much matter passes per unit time. Equivalently, mass flow quantifies how many molecules, and of what type, pass a given point per unit time. Mass flow is measured by the following flowmeters: Coriolis, thermal, mass flow controllers, and multivariable. Of multivariable, the main flowmeters are vortex, differential pressure (DP) and ultrasonic.

In Coriolis flowmeters, the measured signal arises from the inertial resistance of flowing mass to oscillatory acceleration imposed by the measuring structure. The inertial resistance appears as a time (phase) difference between inlet and outlet motion, which is directly proportional to mass flowrate. While this phenomenon is usually described in terms of the Coriolis force, this is purely for mathematical convenience. The same phenomenon can be described in terms of inertial mass.

Thermal flowmeters require heat transfer from the sensor to the flowing fluid. There are two methods: constant current and temperature differential. Both methods make use of the principle that higher velocity flows result in greater cooling. Thermal flowmeters rely on specific heat capacity, thermal conductivity, density, gas composition, and flow regime to accurately convert heat loss into accurate mass flow.

In differential pressure, vortex, and ultrasonic flowmeters, mass flow is obtained by first determining volumetric flow or velocity and then calculating density from pressure, temperature, and fluid-specific models. These models relate pressure and temperature to density based on assumptions about molecular behavior; multivariable flowmeters must therefore be configured for the particular gas or steam being measured in order to infer mass flow.

These distinctions highlight why different flowmeter technologies interact with mass in fundamentally different ways, depending on whether mass is sensed directly through inertia or inferred through models of fluid behavior. You can find out more about all these topics at https://flowresearch.com.

Why are some flowmeters more accurate than others? Flowmeters come in many types — e.g., Coriolis, ultrasonic, vortex, magnetic, differential pressure, turbine, and variable area. Each one measures flow in a different way and each one comes with its own accuracy specification. But the deeper question that underlies all this is why some flowmeters are more accurate than others.

I believe the single most important factor is this:
Some flowmeters are more accurate than others because their operating principle is physically coupled to true mass or volumetric flow. Some technologies measure flow directly, with almost no assumptions. Others require inference, modeling, or secondary variables that introduce uncertainty because they cannot be measured with precision. What is “tight coupling” and why is it the key to understanding flowmeter accuracy?

A New Way to Think About Flowmeter Accuracy
Many flowmeter discussions focus on electrode materials, bluff body geometry, signal processing, Reynolds numbers, transducer signals, or installation effects. But beneath all of that lies a simpler, more fundamental truth:
Flowmeters differ in accuracy because they differ in how closely coupled the measurable signal is to actual flow.
Think of it as a spectrum:
• Tight coupling → higher accuracy
• Medium coupling → medium accuracy
• Loose coupling → lower accuracy
The concept of tight vs. loose coupling can be most easily seen by looking at examples.

Coriolis flowmeters have tight coupling. Coriolis meters measure mass directly via the deflection of a vibrating tube caused by inertial mass. ΔT is directly proportional to mass flow. There are few intervening variables.

Positive displacement meters have tight coupling. Each “fill and sweep” cycle displaces a known volume. There are almost no assumptions. They measure actual volume, although as mechanical meters they are subject to wear. Their accuracy can also be affected by variations in temperature and pressure.

Magnetic flowmeters have relatively tight coupling. Magnetic flowmeters measure velocity via Faraday’s Law; volumetric flow = velocity × pipe area. They require conductivity but have few secondary factors. They are very stable if the pipe is full and the diameter is known. Particulate matter such as sand can damage or erode the electrodes, and can cause uneven flow. Air bubbles can disrupt the conductivity of the meter.

Vortex meters are a medium in terms of coupling. Vortex shedding frequency is proportional to velocity; however, the velocity reading depends on Reynolds number, bluff-body shape, installation, vibration, and velocity profile. Temperature and pressure readings are required for mass flow measurement, introducing two more variables. Vortex meters just count the vortices without regard to their size, strength, and coherence. They have looser coupling than Coriolis meters because the accuracy of vortex meters depends on a variety of imprecisely determined conditions.

Thermal flowmeters have medium coupling. Heat transfer is proportional to mass flow, but the reading depends on fluid properties (specific heat, thermal conductivity). They are good for clean gases; less so for liquids or varied gas mixtures.

Variable area flowmeters have loose coupling. Float position is affected by viscosity, density, friction, and user interpretation. Manual reading introduces additional looseness.

This analysis can be performed for any flowmeter. I have picked a representative sample. In general, a tight physical coupling between a flowmeter occurs when the reading depends on few variables and these variables can be determined with a high degree of certainty. The coupling becomes looser as the flow reading depends on more variables and these variables cannot be measured precisely. Values such as temperature and pressure that are read “live” and that reflect current conditions are preferable to ones read off a table.

Calibration, a favorable flow profile, removing impurities from the fluid, and proper installation can all improve the performance of any meter. However, the principle of operation of certain meters such as vortex and thermal make it unlikely that these meters will achieve the accuracy of Coriolis and positive displacement meters.