Vibrating Fork Level Sensor Principle and Comparison

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.

History

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?

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

1. 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).



2. 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:



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.

Performance vis-a-vis Vibrating Fork Level Sensor Tines Length

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
44mm	Liquids	1600	Foam	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.

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

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