Vibration fork switches are only suitable for point level measurement. They consist of an oscillating or tuning fork, which resonate in air with a known natural frequency. The resonance frequency will be reduced when the fork is brought into contact with a medium. The diagram below shows tuning fork type level switches used for point level measurement:
Principle of Operation
A tuning fork switch comprises a two prong fork which is driven to oscillation at its natural frequency, usually by a piezo-crystal assembly. The switch is usually mounted on the side or top of a tank using a flange or threaded process connection such that the forks protrude into the tank.
When in air, the forks vibrate at their natural frequency which is monitored by a detector circuit. When liquid covers the forks the frequency of oscillation drops and is detected by the switch electronics, which in turn changes the output state
of the level switch to operate an alarm, pump or valve. The frequency of operation of the switch is chosen to avoid interference from normal plant vibration which may cause false switching.
The material of construction is usually stainless steel, allowing use in high pressure and temperature applications, with options of coated wet side or exotic materials for corrosive applications.
Advantages of Vibrating Fork Level Switches
1. Vibrating fork level switches are virtually unaffected by flow, bubbles, foam, vibration, solids content, coating, properties of the liquid and product variation.
2. Advance checks and diagnostics help to ensure high reliability in both high and low level applications.
Application Limitations of Vibrating Fork Level Switches
1. Vibrating fork level switches are not suitable for very viscous media
2. Material build up between the forks cause the forks to be connected thereby disturbing level detection