Piezometer

Electric piezometers consist of a deflecting diaphragm and a porous filter separated by a small reservoir of water. Diaphragm deflections are detected with a vibrating wire or strain gauge and converted to pressure using a suitable calibration.

The piezometer is installed in a borehole, with the annulus between the porous filter and the borehole backfilled with sand or cement/bentonite grout. Groundwater enters the reservoir and deflects the diaphragm until the internal pressure equals the pore water pressure at the filter elevation.

If installed above the prevailing water table, pore pressure can be negative and water may be drawn out of the reservoir. Air entering the instrument can then affect reliable operation, so installation and commissioning need care.