Response of the Osmotic Tensiometer to Varying Temperatures Modeling and Experimental Validation

In an osmotic tensiometer, the hydrostatic pressure of a confined polymer solution is measured to infer the soil water potential. Essential in the operation is the osmotic potential of the polymer solution. Because the osmotic potential depends on the temperature, a significant and undesired pressure lag may develop when the osmotic tensiometer is used under conditions of varying temperature. To understand and eliminate the pressure lag, a dynamic transport model was derived that describes the time response of the osmotic tensiometer while assuming complete temperature equilibration and a resistance to water transfer that is fully located within the filter. Essential elements of the model are the filter conductance and the sensitivity of the pressure transducer, for which submodels are set up. The transport model is validated with measurements on an osmotic tensiometer that uses an inorganic membrane as filter. The experiments were done for instrument‐limited conditions by placing the osmotic tensiometer in free water. For these conditions, the model describes measurements with reasonable accuracy. The new transport model clarifies the behavior of the osmotic tensiometer for changing temperatures and can be used to design an osmotic tensiometer with a lower pressure lag.