A widely used technique described in handbooks, tensiometry involves the measurement of water pressure in a chamber separated from the soil (or other medium) by a thin porous membrane. The system is designed and constructed in a way that allows equilibration of water in the chamber with water in the pores of the soil, so that the measured pressure may correspond to the matric pressure of the soil water.
We use fast-responding solid state pressure transducers with small (6 mm diameter) ceramic disk membranes to measure matric pressure at points of a core surface, especially during brief intervals between SSC runs. From the 1980s until about 1997 for this purpose we used a transducer modified by Dave Stonestrom of the NRP Infiltration and Drainage Project to accelerate equilibration by small adjustments to the volume of the measurement chamber. We have built disk tensiometers of unusually large diameter (see large-core methods) to obtain rapid matric pressure measurements on the faces of the samples.
We also use tensiometers for temporary contact or insertion into fresh, large cores to obtain indications of in situ field matric pressures. For this purpose we use relatively small cup tensiometers, which have more soil-contact area than disks of comparable size.
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