Vibrating Wire Piezometer — Types and Operating Principle

A vibrating wire piezometer is used to measure pore water pressure in the soil, earth/rock-fill foundations and concrete structures.

Measurement of pore water pressure is used to detect the level and flow pattern of groundwater. It is also used to determine the flow pattern of water in earth/rockfill & concrete dams and the load-bearing capacity of soil or rock.

Vibrating wire piezometer finds its applications in the measurement of positive or negative pore pressure in the soil, concrete mass or rock which also includes:

• Construction control, stability investigation and monitoring of earth dams, embankments, foundations, shallow underground works and surface excavations.
• Uplift and pore pressure gradients in foundations, embankments, abutments and fills.
• The hydrological investigation, groundwater elevation study and water supply operations.
• Waste and environmental applications
• Soil improvement & stability and slope stability

Vibrating Wire Piezometer: Operating principle

The vibrating wire piezometer is made up of a magnetic, high tensile strength, stretched wire. One end of this wire is anchored or attached, while the other end is secured to a diaphragm that deflects in some proportion to the applied pressure. The diaphragm is deflected proportionally, with the change in the pressure which, in turn, affects the tension in the wire.

Therefore, as soon as there is any change in the pore water pressure, it is reflected in the tension of the stretched wire.

The high tensile strength wire is plucked by a coil magnet and resonates at a frequency proportional to the tension in the wire. The frequency can be measured as:

f = [σg/ρ] 1/2/ 2l Hz where;

σ = tension of the wire

g = gravitational constant

ρ = density of the wire

l = length of wire.

The frequency resonated by the wire leads to an alternating current in the coil magnet. The pore pressure is measured as being proportional to the square of the frequency and the Encardio-rite model EDI-51V readout logger displays this directly in engineering units.

What is a Vibrating Wire Piezometer?

Piezometers are available in various capacities that are used in specific conditions. A thermistor is present in all kinds of piezometer which monitors temperature and may even make a temperature correction in the zero reading. (Discussed Later)

There is also a tri-polar plasma surge arrestor present inside the transducer housing that protects the vibrating wire pluck and read coils from electrical transients that may be induced by direct or indirect lightning strikes.

Types of Encardio Rite’s Vibrating Wire Piezometers

Model EPP-30V Electron Beam Welded Heavy Duty Piezometer

Encardio Rite’s Model EPP-30V Piezometer has the vibrating wire and coil magnet protected in a stainless steel body which, in turn, is electron beam welded to the diaphragm. This creates a vacuum of 1/1000 Torr inside the sensor, making it immune to the effect of water which may enter or any other corrosive materials. The stainless steel construction protects the piezometer from chemical corrosion which may happen at any of the locations where the piezometer is used.

The Model EPP-30V electron beam welded heavy duty piezometer is provided with a low air entry value ceramic flat filter which has a thickness of 3 mm and a grain size of 40–60 microns. The water that leaks through internal pores or seams in rock formations of dam foundations, mass concrete of structures, foundation soil of structures, reclaimed land soil etc, finds its way to pressurise the diaphragm through the filter. Through the entire process, the filter is held in its place by a locking nut.

The locking nut can be reassembled by a bent nose plier which is provided with the product. The locking nut is usually removed to fill up the cavity behind the filter with de-aired water or for saturating the filter with water.

The wires from the coil magnet end on the glass to metal seal which is integrally electron beam welded to the stainless steel body of the piezometer.

The coil magnet is connected to the pins marked as red and black whereas, the other two pins are connected to a thermistor. The cable connection is maintained by [Continue Reading…]