A piezometer is either a device used to measure liquid pressure in a system by measuring the height to which a column of the liquid rises against gravity, or a device which measures the pressure (more precisely, the piezometric head) of groundwater at a specific point. A piezometer is designed to measure static pressures, and thus differs from a pitot tube by not being pointed into the fluid flow.
Observation wells give some information on the water level in a formation, but must be read manually. Electrical pressure transducers of several types can be read automatically, making data acquisition more convenient.
The first piezometers in geotechnical engineering were open wells or standpipes (sometimes called Casagrande piezometers) installed into an aquifer. A Casagrande piezometer will typically have a solid casing down to the depth of interest, and a slotted or screened casing within the zone where water pressure is being measured. The casing is sealed into the drillhole with clay, bentonite or concrete to prevent surface water from contaminating the groundwater supply. In an unconfined aquifer, the water level in the piezometer would not be exactly coincident with the water table, especially when the vertical component of flow velocity is significant. In a confined aquifer under artesian conditions, the water level in the piezometer indicates the pressure in the aquifer, but not necessarily the water table. Piezometer wells can be much smaller in diameter than production wells, and a 5 cm diameter standpipe is common.
Piezometers in durable casings can be buried or pushed into the ground to measure the groundwater pressure at the point of installation. The pressure gauges (transducer) can be vibrating-wire, pneumatic, or strain-gauge in operation, converting pressure into an electrical signal. These piezometers are cabled to the surface where they can be read by data loggers or portable readout units, allowing faster or more frequent reading than is possible with open standpipe piezometers.
The simplest instrument for measuring pore water pressures in ground is an open standpipe. This is a small (usually 19mm) diameter plastic pipe with a porous section at the bottom. The pipe is installed inside a borehole and the porous section is positioned at the depth where the pore water pressure is to be measured. The annulus between the porous filter and the borehole is filled with sand, the top and bottom surfaces of the sand are sealed with bentonite and the rest of the borehole is filled with a cement/bentonite grout.
The pressure of the ground water pushes water into and up the standpipe until the level of water inside the standpipe (h) is equivalent to the pore water pressure in the ground at the elevation of the porous filter.
Electric piezometers consist of a deflecting diaphragm and a porous filter separated by a small reservoir of water. Deflections of the diaphragm are detected using a vibrating wire or a strain gauge and are converted to an equivalent pressure using a suitable calibration.
The piezometer is inserted into a borehole and the annulus between the porous filter and the borehole is filled with either sand or cement/bentonite grout.
Water from the ground forces its way into the reservoir and causes the diaphragm to deflect until the pressure inside the reservoir is the same as the pore water pressure in the ground at the elevation of the porous filter.
If a piezometer is installed above the prevailing ground water table, the pore pressure in the soil could be negative and the water in the piezometer will tend to be drawn out of the reservoir. If this happens air can eventually form inside the piezometer and it will not function reliably.