The Soil Moisture Suction Sand Unit "PAHS" has been designed to study and understand the water retentivity principles in terms of soil suction, the relationship between water retentivity and soil and the derivation of soil moisture characteristic curves.

The unit is large enough to permit the simultaneous evaluation of several samples and consists of a suction system and a soils container:

• Suction system: It consists of a water circuit that includes a water tank, a pump, a water jet pump and a pressure gauge (vacuometer). The pump propels water from the water tank and circulates through the water jet pump, which sucks the water of the soils container.
• Soils container: It consists of a cylindrical tank filled with saturated sand (not supplied) that contains three soil sample retaining rings. Inside of the sand the soil samples are placed as well as the humidity meters. A cover is provided to prevent evaporative water loss during the practical exercises and several filters avoid sand transport to the water tank. The soils container includes a pressure gauge (manometer) and an air inlet (pressure inlet) to study of the effect of the atmospheric pressure.

Soil samples placed on the sand are subjected to suction. This suction is applied by means of the water jet pump and the water pump. In order to maintain the suction, a volume water supply is required.