Vibrating Fork Level Sensor Principle and Comparison

Beginner

By Dhananjay Palshikar on April 18, 2019

Introduction

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. The two-pointed tines of the fork sensor start vibrating at a constant frequency when striking against any surface. The fork sensors are tuned at a particular frequency according to the type of industry and are generally factory calibrated. The pitch of the tuning fork may vary as per the length of the fork tines.

Tines of Tuning Fork — Tuning Fork's Tines

History: Tuning Fork Level Sensor

Tuning fork 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. Sapcon contributed over 35+ years as one of the best vibrating fork type level switch manufacturer in India. 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 tuning fork's tines apart from each other. See the figure given aside for a simulation of the process. When the pressure is removed the fork tines return to their original position.

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

Vibrating Fork type Level Sensor for Solids — Amplitude Change in Vibrating Fork for Solids

Fork Sensor Working Principle

The working principle of vibrating fork level sensor is based on vibrating a tuning fork sensor continuously at its natural frequency and detecting the change of frequency and amplitude in the presence of application media. The monitoring process of given parameters depends upon type of application media.

Amplitude of vibration
Frequency of operation

Amplitude of vibration: Vibrating Level Switch for Solids

As the material level increases and comes in contact with the Vibrating Fork tines, the amplitude of oscillation 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

Frequency of operation: Liquid Level Sensor

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

Vibrating Fork for Liquids — Vibrating Fork Level Sensor: 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 switching point. In vibrating fork for solids, changing the sensitivity may affect the hysteresis for the sensor. In general hysteresis 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.

Performance vis-a-vis Vibrating Fork Level Sensor Tines Length
Tines Length	220mm	155mm	Vital-100mm	Elixir-100mm	44mm
Tines Length	Application Media	Solids	Solids	Solids	Liquids/Solids	Liquids
Natural Frequency in air (Hz) approx.	80	150	300	300	1600
Buildup Immunity					Foam
Response Time (Fastest)	3 sec	2 sec	0.5 sec	1 sec	1 sec
Typical Application	General Purpose	General Purpose	Fast packaging machines & compact silos	Non-foaming liquids or free-flowing Solids	Hygienic Design. free-flowing liquids, liquids with foam, compact silos

On-site Settings and Configuration

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

Configurable Performance Parameters
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

Fork Type Level Switch vs other Point Level Switches

Comparing Vibronic Level Sensors with Point Level Switches
Product Principle	Vibrating Fork	Capacitance	RF-Admittance	Rotating Paddle	Remarks
Type of Application Media	Free-flowing solids & liquids	Free-flowing solids & 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 Temperature Operating Pressure Mounting Position Output Frequency Change Installation Conditions Environmetal Conditions