Soil Moisture Tensiometer Materials and Construction

A porous cup when filled with water and connected to a suitable gage may be used for measuring the pressure in soil water (1, 5). This pressure may be either greater or less than atmospheric pressure, which is ordinarily taken as the pressure reference. At points below a water table the pressure is positive, but in unsaturated soil the water in a porous cup must be under negative or vacuum pressure in order to be in equilibrium with the water in the soil. The combina-tion of a porous cup and a vacuum gage for measuring the equivalent negative pressure3 or tension of water in unsaturated soil has been called a “tensi-ometer ” (1). Improvement in both the design and materials for tensiometer construction will undoubtedly continue, and no one design will be best for all types of use. Care must be exercised in the selection of materials and in the construction of any tensiometer if it is to give trouble-free operation, and sometimes difficul-ties develop which are not easy to anticipate. Numerous requests have come to the writer for information on the design and construction of tensiometers. Since these units are not commercially available at present, it is hoped that this paper will be of assistance to those who wish to construct tensiometers for their own use. Three units that have performed satisfactorily under field and laboratory conditions are described. TENSIOMETER CUPS Porous cups suitable for use with tensiometers and of almost any desired shape can be made up in the laboratory by the drain-casting process. Experi-mentation with different clays and firing processes may be necessary to obtain a suitable porous structure. The cup wall should be permeable to water, and yet when the wall is wet the “air-entry value ” must be greater than one atmos-phere. This value is simply the pressure difference required to produce an air leak through the cup wall when it is saturated with water. There is to a certain extent, a reciprocal relation between these two properties; when one is high the other is low. Experience indicates that the permeability should be as high as possible and still have the air-entry value greater than oneatmosphere.