When we specify process instrumentation for level monitoring applications, we’re always wary of variances in the materials being measured that can skew the sensor’s accuracy. This is because most sensors are calibrated to measure within a tight band of expected material properties such as viscosity, bulk density, or specific gravity, and any swings in these properties can lead to operational failures.
What if there were a sensor technology that didn’t rely on consistent material properties, but instead measured the difference between air and whatever media the sensor was subjected to? In this fashion, such a sensor wouldn’t care about ‘detecting the right material’ – it would only care about detecting ‘any material that is not air’. This type of sensor certainly exists and is known as a vibrating fork level sensor.
Vibrating fork instruments consist of two extruding paddles or “forks” that are connected to a piezoelectric crystal. When energized, this piezoelectric crystal produces a natural harmonic frequency through the forks. A second piezoelectric crystal measures this frequency, and now has a reference point for the forks operating in free air.
Once the forks are immersed into a fluid or bulk material, that material presents restriction and dampens the vibrating frequency of the forks, which the second piezoelectric crystal senses immediately. In this way, vibrating fork sensors are calibrated to detect the condition when materials contact the forks and reduce their vibrational frequency, which then switches a relay or dry contact circuit.
Application Examples of Vibrating Fork Level Detection
Vibrating fork sensors serve many unique applications in level and material detection, including:
- Bulk Solids Storage – any bulk solid material that can present even a minor amount of physical resistance to the forks is a great candidate for monitoring with vibrating fork level sensors. Since the forks are simply measuring a change in frequency compared to free air, vibrating forks work great with products such as powders, grains, seeds, nuts, and sand.
- High / Low Level – vibrating fork sensors provide discrete relay switching that is perfect for high- and low-level detection in bulk solids as described above, as well as with fluids and even gases. Vibrating forks are commonly found installed in storage tanks, process vessels, and retention bunkers.
- Density Changes – vibrating fork sensors do not need to measure the absolute frequency of their vibrations, but instead just the change in frequencies when exposed to varying materials. In this way, vibrating forks can also be used to detect changes in fluid and gas densities, which is very useful in setting density alarm points.
- Phase Changes – as an extreme example of density changes, materials that change phase (such as from a fluid to a gas) can be directly monitored using vibrating fork sensors. High pressure hot water and volatile chemical circuits can flash from fluid to gas which may not be caught by a standard pressure sensor, but will certainly be caught by a vibrating fork sensor.
- Pump Run-Dry Protection – vibrating forks are ideal solutions for detecting empty circuits feeding pumps, screw conveyors, and similar transfer equipment. Compared to common flow switches that measure material velocity, vibrating forks are more ideal as they detect the presence or absence of materials against free air.
- Hazardous Processes – vibrating fork sensors are highly desirable for hazardous material processes, as they offer fewer risk points than sensors with moving parts or whose electronics operate in close contact with those materials. A vibrating fork sensor’s electronics are safely distanced from the process media, measuring the transferred vibrational frequencies from the safety of the out-of-process transmitter head, not from the wetted sensor housing as with other sensor designs.
- Safety Systems – further to the above point, vibrating forks are so safe that they are often the default selection for level switching in certified SIS (safety instrumented system) applications. Even better, vibrating fork level switches can be safely tested without removing the sensor from the process by using a simple magnetic tester.
Key Considerations for Specifying Vibrating Fork Sensors
When specifying new vibrating fork level sensors, buyers should consider the following nuances against the needs of their applications.
- Material Compatibility – while vibrating forks can accurately detect material presence across a slew of fluid, gas, and solid types, the sensor’s wetted materials must still be checked for compatibility so that failure due to abrasion, corrosion, or chemical reactions does not occur. Stainless steel 316L, exotic Alloy C-276, and coatings such as PTFE, ETFE, PFA) are the most commonly selected materials.
- Bridging and Cleaning – vibrating forks do run the risk of material bridging (packing the space between each fork). Powders and solids can cake onto the forks, as can high viscosity fluids and even vapor-laden gases. This buildup can distort the fork’s frequency over time, requiring good access for inspections and regular cleaning.
- Maintenance – vibrating fork sensors have no mechanically moving parts nor any sensitive sensing diaphragms or membranes, making them one of the easiest sensor types to maintain. Vibrating forks should be inspected routinely for damage and tested for proper operation, but otherwise have no special maintenance requirements.
- Natural Harmonics and Interference – vibrating sensors are virtually unaffected by common process upsets such as varying flow rates, bubbles, entrained air, suspended solids, foam, mechanical vibration, turbulence, pressure, or fluid property changes. This makes vibrating fork sensors great for challenging, unruly applications, but engineers do need to confirm that the overall system will not generate natural frequencies that will present interference.
- Environmental Rating – vibrating fork sensors are available in industrial, general commercial, hygienic, pharmaceutical, explosion proof, intrinsically safe, marine, and many other environmental ratings. In addition, these instruments commonly include combinations of ATEX, CE, UL, and NFPA Class/Div certifications.
- Temperature Limits – vibrating fork sensors work across wide temperature ranges, with some sensor options extending between -90°F and 500°F. Process temperature swings do not affect vibrating forks nearly as much as they do other sensor types, making these sensors extremely reliable in aggressive conditions.
- Testing and Calibration – testing vibrating fork sensors typically can be performed without pulling the sensor out of the process, using a magnet to pull down the forks’ vibrational frequency enough to confirm that switching occurs as desired. Calibration is equally simple and does not easily drift over time – annual proactive calibration is typically all that is needed.
As a veteran-owned small business, Whitman Controls is dedicated to supplying premium quality, reliable, technologically advanced instrumentation for use in nearly any application. Our Bristol, CT manufacturing facility embodies over 40 years of engineering, fabrication, and customer service expertise, serving both end-user and manufacturing customers nationwide through direct and distribution channels. Our values drive us to provide the highest level of servant partnership that you can find. To discuss your applications or to learn more about our capabilities, please contact us at (800) 233-4401, via email at [email protected], or online at www.whitmancontrols.com.
