Amplitude vs. Frequency Shift Tuning Fork -When to Use Which

Classically used to tune musical instruments and to assess hearing loss, tuning forks have found widespread use in a variety of surprising applications. A tuning fork’s ability to reliably and accurately output a specific frequency has made it a component of choice in devices ranging from watches to radar guns to gyroscopes.

Tuning Forks have also found use in level-sensing switches for tanks and industrial containers. Conventionally, a level-sensing switch uses a float or pressure sensor to detect the level of the fluid that’s being contained. While these are time-tested designs, they do have some flaws: they cannot work with all types of fluids; they are not solid-state; and they usually require downtime for maintenance. A tuning fork level switch, which is entirely solid-state, overcomes those problems and more by using vibration to detect material level.

FTLWhen you have determined a need for high or low level point detection you’ll need to decide which type of switch is most appropriate for your particular industrial application. There are two distinct types of tuning fork level switches and they operate on different principles; amplitude-shift and frequency-shift. One is designed to work on liquid products and one for solids or granular products, but they operate on different principles.

Amplitude-shift tuning forks are most typically used with granulated or bulk solids, such as polystyrene granules, powdered milk, plastic granules, sawdust,sand, shavings and flour. These tuning forks are powered by a piezoelectric crystal driver that causes the fork to oscillate at approximately 120 Hz, enough to cause the fork to vibrate without reducing sensitivity. The vibration is monitored by an electronic control module, which looks at the amplitude of vibration. As the process material contacts the forks of the unit, the amplitude of vibration will change. The electronic module will notice the significant amplitude shift and send out a signal to the control system or monitoring device. In other words the product stops the vibration of the forks which is detected by the electronics.

A frequency-shift tuning fork is designed and engineered for liquid applications such as syrups, sauces, water, slurries, and oils. This system is powered and vibrated by a patented  piezoelectric drive, running at approximately 1000 Hz. Like amplitude-shift configurations, the electronic module monitors the frequency of the fork assembly.

The resonant frequency in air is determined for each switch unit. This frequency shifts when the fork is immersed in a liquid which the electronics sense and change the status of the output.

As the unit only looks at the frequency it means it is not affected by buildup, gas bubbles and turbulence. Materials with varying density, viscosity, suspended particles and composition changes do not affect the reliable operation of the tuning fork. The standard unit will not see foam but with special versions the unit can be set-up at the factory to detect dense foams.

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Have a question? We can help.

Topics: E-direct products, Endress+Hauser Products, Automation, tuning fork, level switches, pressure sensors, Liquiphant


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