Tuning forks are used in various fields from acoustics to medical diagnostics. In their miniature form tuning forks made of Quartz crystal are used in watches for measuring time.

Tuning Fork level sensor
Tuning Fork Level Sensor

40 years in the Industry

Tuning forks level switches have been commonly used in the industry since the 1970s. They were introduced as alternatives to Rotating Paddle Level Switches.

Vibrating Fork Level Sensors have stood the test of time and continue to dominate the point level switch market.

Sapcon Instruments has been manufacturing vibrating fork level sensors since its inception in the year 1983. Since then, Vibrating fork has evolved into a compact and fast level switch. The success and popularity of these level sensors can largely be attributed to:

  • Calibration Free Operation
  • Relatively Long Operational Life Expectancy

How do the tuning fork tines vibrate?

Pressure applied by the piezoelectric stack on the diaphram causes it to shear, which in turn drives the tines apart from each other. See the figure below for a simulation of the process. When the pressure is removed the tines return to their orginal position.

Tuning Fork
Amplitude Change in Vibrating Fork for Solids

The application of force when applied at the fork's Natural Frequency makes the it oscillate at its maximum amplitude for a given power.

What is the principle behind tuning fork level sensor's switching?

A tuning fork sensor vibrating continuously at its natural frequency, and is monitored for two parameters:

  • Frequency of operation

  • Amplitude of Vibration

Level Sensing for Solids like powders or granuels

As the material level increases and comes in contact with the Vibrating Fork tines, the amplitude of osciallation of tuning fork level switch dampens. When the amplitude goes below a set threshold, the onboard electronics causes a change to the outputs(Relays/ PNP).

Vibrating Fork for Soild
Vibrating Fork Level Sensor for Solids

Level Sensing for Liquids

The natural frequency of oscillation for the tuning fork decreases as it covered with the liquid application media. The threshold frequency is set to match the natural frequency of tuning fork sensor under water as shown here:

Vibrating Fork for Liquids
Vibrating Fork Level Sensor for Liquids: Factory Calibration

Switching Point Hysteresis for Point Level Switching

Turbulence can be caused by flow of material or due to an agitator or stirrer inside the silos. This can cause fluctuations in both the amplitude and frequency of operation for the tuning fork. For reliable level sensing a switching hysteresis is maintained around the swiching point. This prevents frequent toggling around the swithching point.

In vibrating fork for solids, changing the sensitivty may affect the hysterisis for the sensor. In general hysterisis margin decreases with increase in sensitivity.

The natural frequency of tuning fork increases with increase in the length of tines. in two rows, one for solid and another for liquid

A comparison of the tuning forks vis-a-vis their performance and end-use has been shown below for a better understanding.

Performance vis-a-vis Vibrating Fork Level Sensor Tines Length
Tines Length 220 mm 155 mm Vital- 100 mm Elixir -100 mm 44 mm
SLM-205 Vital-185 Vital-100 Elixir-110 Elixir-44
Application Media Solids Solids Solids Liquids/Solids Liquids
Natural Frequency in air (Hz) approx. 80 150 300 300 1600
Buildup immunity
Response time(fastest) 3 sec 2 sec 0.5 sec 1 sec 1 sec
Typical Application General Purpose General Purpose Fast Packaging Machines and compact silos Non-foaming liquids or free-flowing Solids Hygienic Design.all freely flowing liquids,liquids with foam, compact silos

On site settings and configuration

A simple slide switch interface makes on-site adjustments possible.

Top-View of Vital - Vibrating Fork Level Sensor
Configurable Performance Parameters
Performance Parameter Effect Application Side Effect
Sensitivity Ampltitude of vibration and threshold ampltiude used for switching. Detecting Low Density Media Switching Position and buildup immunity
Time Delay Additional Time Delays for process automation Preventing malfunction by ignoring turbulence in silos. none

Comparison of Vibrating Fork Level Sensors vs other Point Level Switches

Comparing Vibronic Level Sensors with others
Product Principle Vibrating Fork Capacitance RF-Admittance Rotating Paddle Remarks
Type of Application Media Free flowing solids and liquids Free flowing solids and liquids Solids, Semi-solids with tendancy to buildup Granular Solids RF-Admittance covers most materials
Calibration Free No calibration requirement leads to quick installation and commissioning
Dependance on Physical Property
  • Density > 0.7 g/cm3
  • Particle Size < 10 mm
  • Freely flowing solids
Dielectric Constant
Freely flowing solids
Dielectric Constant Density > 0.5 g/cm3 Dielectric constant variantion is difficult to track, variation in density can be measured easily. Performance of Vibrating Fork and Rotation Paddle is more predictable for a given application.
Budget ₹₹₹ ₹₹ Vibrating Forks are generally priced the lowest
Expected Life(depends on application parameters and site conditions) 5-10 years 5-15 years 5-15 years 2-3 years Application parameters that affect useful life(all parameters do not affect each instrument):
  • Operating Temperature
  • Ambient Temprature
  • Operating Pressure
  • Mounting Position
  • Output Frequency Change
  • Installation Conditions
  • Environmetal Conditions

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