A prototype osmotic tensiometer with polymeric gel grains

An accurate field method that directly measures the entire range of relevan t soil water potentials for periods of months to years is not yet available . The osmotic tensiometer (OT) has the potential to fulfil these demands bu t is still in an early stage of development. We have designed a novel OT th at should overcome several drawbacks of former designs by the use of cross- linked polymeric gel grains instead of an aqueous solution of linear chain polymers inside the porous ceramic cup. For this reason, the cup can have l arge pores without polymer leakage. This ensures that water exchanges rapid ly with the soil, resulting in a negligible pressure lag with changing temp eratures, which is the most important source of a systematic measurement er ror of an OT. A second reason for better performance might be the inherent elasticity of the grains which should inhibit formation of a polymer cake a t the filter. To study several of the key variables of the new sensor, tran sient and stationary experiments were performed in an aqueous solution the temperature, salt type and salt concentration of which were varied. Under s imilar conditions the swelling of the polymer grains was studied in more de tail. The OT that we developed responds rapidly to changes in water potenti al, but is very susceptible to the influence of salts. To study and minimiz e salt influence, swelling tests of unconfined grains are proposed in combi nation with tests of the OT itself.