10.5.3 Vibrating Wire Piezometer
A vibrating wire piezometer is the most commonly deployed piezometer and is suitable for almost all applications. It consists of a vibrating wire pressure transducer and signal cable. It can be installed in a borehole, embedded in fill, or suspended in a standpipe. Readings are obtained with a portable readout or a data logger. It converts water pressure into a frequency signal via a diaphragm, a tensioned steel wire, and an electromagnetic coil. The piezometer is designed to cause a change in tension of the wire with change in pressure on the diaphragm. When excited by the electromagnetic coil, the wire vibrates at its natural frequency. The vibration of the wire in the proximity of the coil generates a frequency signal that is transmitted and recorded in the readout device or data logger. An increase in water pressure, reduces the tension in the wire by deforming the diaphragm inward thereby reducing the vibrating wire signal frequency. The readout device processes the signal, applies calibration factors, and displays a reading in the required engineering unit. One should read atmospheric pressure and the other downhole pressure. By subtracting the atmospheric from the downhole pressure, the true water level can be obtained. Key advantages of Vibrating Wire Piezometers include high resolution of 0.025% of full scale, high accuracy, rapid response and reliable signal transmission.
Figure 11: Schematic diagram Vibrating Wire Piezometer installed in a hole