Vibrating Fork Level Sensor Principle and Comparison
By Dhananjay Palshikar on April 18, 2019
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 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 diaphragm causes it to shear, which in turn drives the tines apart from each other. See the figure given aside for a simulation of the process. When the pressure is removed the tines return to their original position. 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
- Level Sensing for Liquids
- Density > 0.7 g/cm3
- Particle Size < 10 mm
- Freely flowing solids
- Operating Temperature
- Ambient Temprature
- Operating Pressure
- Mounting Position
- Output Frequency Change
- Installation Conditions
- Environmetal Conditions
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).
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:
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 switching point. In vibrating fork for solids, changing the sensitivity 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.
A comparison of the tuning forks vis-a-vis their performance and end-use has been shown below for a better understanding.
|Tines Length||Application Media||Natural Frequency in air (Hz) approx.||Buildup Immunity||Response Time(fastest)||Typical Application|
|220 mm||Solids||80||3 sec||General Purpose|
|155mm||Solids||150||2 sec||General Purpose|
|Vital-100mm||Solids||300||0.5 sec||Fast packaging machines & compact silos|
|Elixir-100mm||Liquids/Solids||300||1 sec||Non-foaming liquids or free-flowing Solids|
|1 sec||Hygienic Design. freely flowing liquids, liquids with foam, compact silos|
On-site settings and configuration
A simple slide switch interface makes on-site adjustments possible.
|Performance Parameter||Effect||Application||Side Effect|
|Sensitivity||Amplitude of vibration and threshold amplitude 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
|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
|Calibration Free||✔||✖||✖||✔||No calibration requirement leads to quick installation and commissioning|
|Dependance on Physical Property||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):