A piezometer is either a device used to measure static 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.
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 groundwater leaking to the surface. In an unconfined aquifer, the water level in the piezometer would be coincident with the water table. 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 5cm diameter standpipe is common.
In recent decades, piezometers have been developed which are pressure gauges of various types in durable casings which can be buried or pushed into the ground and which will measure the groundwater pressure at the point of installation. The pressure gauges can be vibrating-wire, pneumatic, or strain-gauge in operation. These piezometers are cabled to the surface where they can be read bydataloggers or portable readout units, allowing faster and/or more frequent reading than is possible with open standpipe piezometers.