Vibrating Fork
Measuring principle
TI-149-High Integrity Water Limiters-2017-2
The vibrating fork type level switch working principle is based upon detecting the change in harmonic vibration
frequency of the sensing element as a resul
t of the presence of the water between the forks. Typically there are two
Piezoelectric elements or crystals. When these crystals are compressed or pulled under tension they generate an
electric signal. Conversely when electricity is applied to these crystals they produce motion. This motion is attached
to the fork sensing element and vibration at the natural resonant frequency of the mechanical fork is created. The
second piezoelectric crystal is used to transform the motion received from the first crystal back into an electric
signal. When the fork sensing element vibrates free in air (or steam) the measuring system sees this as one state
(water not present). When the water covers the forks, the frequency of the vibration is changed and the second
crystals electric signal changes, thereby detecting the material or fluid presence.
The vibrating fork type level switch working principle is simple, effective, reliable and a cost-effective m
eans to
measure or detect the presence and absence of water level.
Transmitter
Tank Wall
Process Fluid
Figure. 2 - Vi
br
ating Fork
Advantages
Vibrating fork sensors operate independently of pressure or temperature fluctuations and are also completely
unaffected by changes i
n
density, foaming or steam. Build-up on the probe or on the container wall does not affect
the measurement either. Vibrating Fork probes detect the physical presence of the water rather than the
conductivity of the fluid. This allows the device to operate in boiler water with very low or varying conductivity such
as RO or De-mineralised water.
Integrity
The Vibrating Fork unit that Fulton supplies is certified to provide SIL2 level of protection against low water level.
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