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1. INTRODUCTION
1.1 Theory of Operation
The Geokon Model 4430 Vibrating Wire Deformation Meter is designed to measure axial
deformations in boreholes in rock, concrete or soil. It can also be embedded in soils in
embankments such as earth dams and highway fills. The units can be installed in series providing
incremental deformation measurements over any length. Base lengths of the gauge can vary from
a minimum of one meter to over 25 meters.
The basic sensing element is a vibrating wire strain gauge in series with a precision music wire
spring that is coupled to a movable shaft. As the shaft moves in or out of the sensor body, the
tension changes in the spring as well as the vibrating wire element. This change in tension is
directly proportional to the amount of extension and, through calibration, a calibration factor that
relates the frequency of vibration to the amount of extension is determined. The unit is stress
relieved after manufacture providing for excellent stability over long periods of time.
The gauge sensor is attached to a flange at one end and by a connecting rod of some length to a
flange at the other end. The sensor and the rod are covered by a plastic (PVC) tube which holds
the end flanges apart at a predetermined distance (gauge length), and insures that the rod is free
to move. As the flanges move apart, the movement is conveyed by the connecting rod to the
sensor and measured by the readout system. Different combinations of gauge length and sensor
range provide for optimum sensitivity. For maximum strain resolution, a long base gauge with a
short range transducer will give best results. For maximum deformation: short base length,
longer transducer range. The flexibility of the system allows the user to choose the most useful
combination of range and sensitivity according to predicted movements.
Nominal Gage Length
(1m, 39")
End Flange
Swagelok Fitting
Instrument Cable
End Flange
Electromagnetic Coils Thermistor
(4 conductor, 22 AWG)
PVC Outer Tube
Transducer Shaft Housing
Alignment Pin
Alignment Slot
Extension Rod
Transducer Shaft
Figure 1 - Model 4430 Deformation Meter
Readouts available from Geokon, used in conjunction with the Vibrating Wire Deformation
Meter, will provide the necessary voltage pulses to pluck the wire and convert the measured
frequencies to display the reading.