Volume 2, Issue 1 (March 1979)
Use of the Osmotic Tensiometer to Measure Negative Pore Water Pressure
The osmotic tensiometer was developed to provide a method of direct measurement of tension in the pore water of soils. A study was undertaken at the University of Saskatchewan to investigate the instrument's operational characteristics and its applicability to practical engineering problems. The study covered long-term stability, response time to changes in pore water pressure, and response to changes in ambient temperature. It was found that the internal prestress pressure tended to diminish with time, probably due to leakage of the confined solute through the semipermeable membrane. However, it was possible to individually calibrate each instrument to correct for this effect. Second, the pressure response of the osmotic tensiometers indicated that a transient flow process was occurring; the equilibration time of the device was primarily a function of the configuration and materials of the unit and the compressibility of the solution in the chamber. Third, it was found that changes in ambient temperature considerably affected the internal reference pressures. These results indicate that osmotic tensiometers may be useful to measure negative pore water pressures, providing that the temperature is controlled and the pore water pressure is constant. This may restrict their usefulness to research applications.