Response of the osmotic tensiometer to varying temperatures: Modeling and experimental validation

In an osmotic tensiometer, the hydrostatic pressure of a confined polymer s olution is measured to infer the soil water potential, Essential in the ope ration is the osmotic potential of the polymer solution. Because the osmoti c potential depends on the temperature, a significant and undesired pressur e lag may develop when the osmotic tensiometer is used under conditions of varying temperature. To understand and eliminate the pressure lag, a dynami c transport model was derived that describes the time response of the osmot ic tensiometer while assuming complete temperature equilibration and a resi stance to water transfer that is fully located within the filter. Essential elements of the model are the filter conductance and the sensitivity of th e pressure transducer, for which submodels are set up, The transport model is validated with measurements on an osmotic tensiometer that uses an inorg anic membrane as filter. The experiments were done for instrument-limited c onditions by placing the osmotic tensiometer in free water. For these condi tions, the model describes measurements with reasonable accuracy. The new t ransport model clarifies the behavior of the osmotic tensiometer for changi ng temperatures and can be used to design an osmotic tensiometer with a low er pressure lag.