Introduction; Theory Of Operation - Geokon 4430 Instruction Manual

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
gage can vary from a minimum of 1 meter to over 25 meters.
The basic sensing element is a vibrating wire strain gage in series with a precision music
wire spring which is coupled to a movable shaft. As the shaft moves in or out of the sensor
body, the tension in the spring, and also in the vibrating wire element, change. 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 gage 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 (gage 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 gage length and sensor range provide for optimum sensitivity. For maximum strain
resolution, a long base gage 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.
End Flange
Extension Rod Transducer Shaft Alignment Pin Transducer Shaft Housing
End Flange PVC Outer Tube Alignment Slot Electromagnetic Coils Thermistor Instrument Cable
(4 conductor, 22 AWG)
Nominal Gage Length
Swagelok Fitting
(1m, 39")
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 so as to display the reading.