Piezometers should ideally have a very short screen and filter zone, so that they can represent the hydraulic head at a point in the aquifer. If the filter zone is located at a specific isolated depth, the piezometer is defined punctual, or, if the piezometer has a filter on all its length, is defined windowed. The windowed piezometer is cheaper than the punctual one, but cannot give information on vertical flows. The main problem with the piezometers is the time-lag between the variation of piezometric level in the aquifer and the respective variation in the piezometer. This time-lag is related to the piezometer (type, shape, etc.) and the soil. Modern piezometers with little time-lag are the piezometric cells, where the pressure on a membrane is measured by the pressure of gas (pneumatic piezometric cells), by vibrating thread extensimeters or by electrical extensimeters (strain gauges piezometers).
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