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1. THEORY OF OPERATION
Geokon Model 4500 Vibrating Wire Piezometers are intended primarily for long-term
measurements of fluid, and/or pore pressures in standpipes, boreholes, embankments, pipelines
and pressure vessels. The Model 4500MLP is designed to permit the easy installation of several
piezometers inside a single borehole. It eliminates the need for alternating sand and bentonite
zones by allowing the entire hole to be backfilled with bentonite grout by tremie piping it in
through a grout pipe.
The basic Model 4500S piezometer (Figure 1) utilizes a sensitive stainless steel diaphragm to
which a vibrating wire element is connected. In use, changing pressures on the diaphragm cause
it to deflect. This deflection is measured as a change in tension and frequency of vibration in the
vibrating wire element. The square of the vibration frequency is directly proportional to the
pressure applied to the diaphragm.
Two coils, one with a magnet insert, the other with a pole piece insert, are located close to the
vibrating wire. In use, a pulse of varying frequency (swept frequency) is applied to the coils
causing the wire to vibrate primarily at its resonant frequency. When the excitation ends, the
wire continues to vibrate. During vibration, a sinusoidal signal is induced in the coils and
transmitted to the readout box where it is conditioned and displayed.
Figure 1 - Model 4500S Vibrating Wire Piezometer
Care needs to be exercised when choosing the pressure range of the piezometer. During
installation, the full pressure of the wet bentonite grout will be felt by the piezometer. Geokon
piezometers can withstand overranging up to 100 percent of the calibrated range without
affecting the calibration, shifting the zero reading, or damaging the unit. At higher overrange
pressures, the piezometer may temporarily cease reading until the grout sets up and the pressure
returns to normal.
