Evaluation of the portable pressure transducer technique for measuring field tensiometers

Abstract

The use of a portable pressure transducer system which couples to tensiometers via a syringe needle and septum stopper was assessed. Changes that occurred to pressure within the tensiometer on insertion of the needle were significant and up to 120 min was sometimes required before complete re-equilibration to the original tensiometric pressure was achieved. Tensiometer readings with less than 5% error were attained by allowing 3-4 min re-equilibration time at soil matric potentials between -1 and -25 kPa, but up to 35 min was required at -40 kPa. As well as increasing with decreasing initial pressure in the tensiometer, re-equilibration time was dependent on soil properties. The re-equilibration times reported are specific to the volume of air left in the tensiometer below the septum stopper (1.2 cm3). A larger air bubble will decrease re-equilibration time but could cause undesirable thermal errors during field use. Transducer readings should be made after allowing time for complete re-equilibration of the tensiometer following insertion of the needle. However, for practical field measurement of low matric potentials, it will be necessary to read the portable transducer before full re-equilibration and correct the reading based on previously determined re-equilibration patterns. These re-equilibration patterns should be specific to each tensiometer and should cover a range of matric potentials. It is also important that the air bubble below the septum stopper be maintained constant for each reading, and that the septum stoppers be replaced regularly. With careful use, these systems are convenient for monitoring large numbers of tensiometers spaced apart in the field.