Most types of flow meters, especially mechanical types that rely on rotating gears, tend to lose accuracy over time. However, Coriolis flow meters deliver extremely accurate measurements for years or even decades of continuous usage, as they don’t rely on mechanical parts. But even this technology can in some cases produce noisy measurements due to vibration. This presents concerns, as vibration is common within the industrial and commercial environments where Coriolis flow meters are found.
Below is a brief overview of contexts where accurate operation of a mass flow meter can be impacted by vibration, and how to remedy the issue.
Problems arise when the frequency of process vibrations match the resonant frequency of the Coriolis flow meter—a rare issue, as their resonant frequency changes continuously.
Most objects have a resonant frequency, also known as a natural frequency, which is determined by an object’s mass, size, shape, and other variables. When an object is exposed to a vibration at or close to its resonant frequency, it will store that physical energy and then release it against in the form of a vibration at its resonant frequency. This phenomenon is seen in the well-known ‘singing wine glass’ party trick. It often happens by accident, such as when a passing car causes a table to vibrate.
While any meter or other piece of hardware has a resonant frequency, this poses a special problem for Coriolis flow meters. These devices rely on vibration to measure mass flow, by measuring the amount of deflection in two vibrating tubes as the measured liquid or gas flows through them. If an outside vibration were to match the natural frequency of the meter, it would vibrate in turn, affecting the accuracy of the mass flow measurement.
Most flow meters take steps to account for outside interference. Rheonik and some other manufacturers incorporate a mass bar (or “mass block”) and torsion rod into their measuring mechanisms. The mass bar is used to increase the mass of the two tubes, while the torsion bars impart constant momentum on the tubes, back and forth. Increased mass and movement mean the tubes have greater inertia, and are less affected by outside vibration. (Consider how a daily peeking out of your lawn may tremble when a motorcycle drives by, while the tree next to it doesn’t show any visible movement.)
But increasing the inertia of a flow meter’s mechanism isn’t the only means of resistance it has.
The resonant frequency of a Coriolis flow meter is a product of its size and the scale of the mass flow being measured, and thus is always changing.
Larger meters designed to measure larger mass flows utilize lower frequencies, while small meters that measure very small mass flows utilize higher frequencies. Typical operational frequencies for long-path, bent-tube Coriolis flow meters, such as those made by Rheonik, utilize frequencies ranging from 300 Hz in the smallest meters to 80 Hz in the largest.
Short-path bent-tube and straight-tube meters made by other manufacturers typically use much higher frequencies, as they must generate greater sideways velocity to produce tube deflections of a magnitude great enough to measure. With higher frequency of operation, they also take more measurements per second—needed to ‘massage’ the measurement electronically due to greater inherent mechanical instability, which is why Rheonik opted for a long-path, bent-tube design. The documentation for your Coriolis flow meter will indicate the frequency used for generating measurements.
The mass of your Coriolis flow meter, plus the operating frequency used to generate tube deflection and derive measurement, plus the mass, temperature, and density of the fluid being measured all contribute to the resonant frequency of a Coriolis flow meter in a given moment. This means that the resonant frequency of a Coriolis flow meter constantly changes while in operation.
A key symptom of vibrational interference is that process outputs will change unexpectedly, or the meter itself will go into a fault state.
Thankfully, vibrational interference is rare nowadays, but it does still happen. When an external vibration causes a Coriolis flow meter to generate inaccurate measurements, the result is typically obvious changes in the output being generated. For instance, if you utilize a Coriolis flow meter to regulate the addition of hardener into a resin mix, discovering that the resin isn’t setting as expected can mean that the flow meter is registering inaccurate measurements, and thus over- or under-dosing.
However, rather than causing a meter to work inaccurately, vibrational interference will often simply cause a Coriolis flow meter to enter a fault state. This is because mass flow meters require a control feedback loop in order to establish and maintain the resonant frequency of the tubes at the heart of the meter. They constantly measure the frequency and amplitude of the tubes’ vibrations in order to know how much additional power is needed to maintain consistent oscillations in the range necessary to produce accurate measurements. Should the fluid passing through the tubes suddenly increase or decrease in density, the meter must compensate so that the oscillation of the tubes doesn’t correspondingly decrease or increase in frequency.
When vibrational interference occurs, it is rare that the effect will be so subtle as to generate a seemingly normal measurement. More often than not the result is that signals from the mechanism will be so outside the norm that the meter’s electronics can’t understand what’s going on, and will instead throw up a fault.
This is a good thing. Your meter will not be merrily generating inaccurate measurements until you discover the issue via your process output. It’s the equivalent of your car’s ‘check engine’ light coming on and calling your attention to the issue.
One of the most common causes of vibrational interference of a Coriolis flow meter is placing two flow meters in proximity to one another.
If two of the same model of Coriolis flow meter are installed in close proximity to one another, and they are measuring the same process fluid, then interference will likely arise. This is because two identical meters will have the same resonant frequency. The consequence is that they will each generate external vibrations that match the frequency of the other meter, superimposing in each mechanism and causing instability and interference. This phenomenon is known as “crosstalk.” Given the general rarity of interference, and the existence of the above phenomenon, our first suspicion whenever interference occurs is that two or more Coriolis flow meters are being operated in close proximity.
Usually, the only solution is to redesign the installation so that the two meters are not placed close to one another. But Rheonik is one of the few manufacturers that allows the measuring mechanisms in meters of the same model to be tuned to different frequencies, so that they can coexist near one another.
If the source of interference cannot be narrowed down to a nearby flow meter or other obvious source, selecting a more robust meter may be necessary.
Sometimes, an environment is so literally and figuratively noisy that isolating the source of interference is extremely difficult. Vibrations can be conducted through the air from nearby sources (hence the “crosstalk” between proximate meters), or across hundreds of feet through mechanical systems, pipes, floors, etc.
Some manufacturers advise getting a better sense of what’s going on by examining the raw, unfiltered signal produced by the meter. But it’s rare that scrutinizing this data yields a meaningful diagnosis of the issue. This is especially the case in most applications, since personnel are often not equipped to make such diagnoses, nor do they desire to do so.
More delicate Coriolis flow meter simply cannot be expected to deliver accurate measurements in those rare environments that generate problematic vibrations. This is why we work exclusively with Rheonik meters, which rely on mechanical solutions to mitigate outside interference, rather than the high degree of electronic signal filtering that many other measurement solutions rely on.
But what many end users find even more useful is that Rheonik meters feature internal diagnostic capabilities. They can indicate the degree of reliability of the measurements being generated, and show if operating conditions are out of range. For example, their meters have an option to change the backlight of the display to indicate the “Assurance Factor” of the measurement:
- White for normal operation
- Amber when measurements may be compromised by the presence of noise or changing conditions
- Blue when a sensor is experiencing disturbance and measurements are definitely compromised
- Red when disturbance is so severe that the meter has entered a fault state.
Selecting a Coriolis flow meter robust enough to stand up to your operational environment is the best means of mitigating noise and vibration, and ensuring accurate measurements in the long-term.