VW2100 Vibrating Wire Piezometer
RST Instruments Ltd.
Page 6
The Vibrating Wire is set to a pre-determined tension during the manufacture. The
instrument housing is evacuated and sealed using electron beam welding to ensure
a perfect seal and a long working life. An O-ring placed behind the diaphragm seals
the back of the assembly within the housing. A coil/magnet assembly is built into
every VW transducer which is used in conjunction with the RST readout box, to pluck
the Vibrating Wire and measure the VW’s vibration period.
1.1
M
ODEL
VW2100
The RST VW2100 Vibrating Wire Piezometer is designed to be embedded in earth
fills and concrete or inserted into boreholes and pipes as small as 19 mm (3/4 inch)
in diameter. The VW2100 piezometer consists of a small diameter cylindrical housing
containing a pressure transducer and thermistor. One end is fitted with an insert that
holds a micrometric high air or low air entry filter. The opposite end contains the
cable entry sealed with an epoxy compound. All parts are made of stainless steel.
The entry filter is set in the front end of the housing and sealed with an O-ring. With
the filter in place, the diaphragm is protected from solid particles, and senses only
the fluid pressure to be measured. The filter housing is easily removable for
calibration of the transducer. The filter assembly can also be replaced with a pipe
thread adapter to use the gauge as a pressure transducer (Model VW2100-PT).
1.2
M
ODEL
VW2100-DP
The RST VW2100-DP Vibrating Wire Piezometer is designed to be driven into
unconsolidated fine grain material such as sand, silt, or clay. The external housing is
a thick-walled cylinder fitted with a pointed shoe at one end. The opposite end is
fitted with a male thread adapter at the cable en
try, which fits standard “EX” drill
rods. Three port holes above the point are equipped with micrometric filters. The
data cable passes through the threaded end and can be fed up through the drill rods
to the surface. The cable entry is sealed with an epoxy compound. Both high and
low air entry filters are available.
2
V
IBRATING
W
IRE
P
RINCIPLE
The sensing element of the Vibrating Wire piezometer is a high strength steel wire
attached to the diaphragm. The vibrating wire is excited by two coil/magnets set
around the connecting over tube. In operation, external pressure on the diaphragm
will move the diaphragm a very small amount, which changes the tension on the
vibrating wire. This tension change is directly proportional to the resonant, or natural,
frequency at which the vibrating wire will vibrate.
The VW2106 Readout Unit generates plucking voltages to the coil/magnet in a
spectrum of frequencies, spanning the natural frequency of the vibrating wire. This
plucking allows the vibrating wire to find its current natural frequency related to the
pressure it is currently experiencing. In turn, the oscillation of the vibrating wire
generates AC voltage in the coil. This output signal is amplified by the VW2106
Readout Unit, which also discriminates against harmonic frequencies, to determine
the resonant frequency of the wire.