A tensiometer is a sealed, water-filled tube equipped with a vacuum gauge on the upper end and a porous ceramic tip on the lower end. Reservoir and cap. The reservoir acts as a water supply for the body tube. The cap on the reservoir must provide an airtight seal for the tensiometer or the device will not work. The body tube provides support and a liquid connection between the porous tip and the vacuum gauge.
Tensiometers come in various lengths.
The ceramic tip is porous, but the openings are so small that when saturated with water, air cannot pass through within the range of soil water tensions to be measured. Water moving out through the porous tip causes the vacuum dial gauge reading to change indicating the suction, or tension, at which the water is being pulled by the surrounding soil. A bar is the unit of pressure, either positive or negative, that has been adopted for the expression of soil suction. This text was extracted from the publication, L-796.