1.Theory of Operation
geokon Model 4500 Vibrating Wire Piezometers are intended primarily for long-term measurements of fluid depths and pore pressures in standpipes, boreholes, embankments, pipelines, and pressure vessels. Several different models are available to suit a variety of geotechnical applications. Calibration data is supplied with each piezometer.
All geokon vibrating wire piezometers utilize a sensitive stainless steel diaphragm (with the exception of model 4500C, which employs bellows) to which a vibrating wire element is connected. During use, changing pressures on the diaphragm cause it to deflect. This deflection is measured as a change in tension and frequency of vibration of the vibrating wire element. The square of the vibration frequency is directly proportional to the pressure applied to the diaphragm. A filter is used to keep out solid particles and prevent damage to the sensitive diaphragm. Standard filters are 50-micron stainless steel. High-air entry value filters are available upon request.
Two coils, one with a magnet insert, the other with a pole piece insert, are installed near the vibrating wire. In use, a pulse of varying frequency (swept frequency) is applied to these 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.
Portable readout units are available to provide the excitation, signal conditioning, and readout of the instrument. Datalogger systems, which allow remote, unattended data collection of multiple sensors, are also available. Contact geokon for additional information.
Figure 1: Model 4500S Vibrating Wire Piezometer
All exposed components are made of corrosion resistant stainless steel. If proper installation techniques are used, the device should have an unlimited life.
In salt water, it may be necessary to use special materials for the diaphragm and housing. The 4500INCO and 4500TI series piezometers are specifically designed to be used in this type of environment. For example, both models incorporate enhanced seals at the cable entry and filter connection; the 4500INCO uses a custom, dual O-ring seal, while the 4500TI is of an all-welded construction.