EVALUATION OF THE PORTABLE PRESSURE TRANSDUCER TECHNIQUE FOR MEASURING FIELD TENSIOMETERS

The use of a portable pressure transducer system which couples to tens iometers 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 befor e complete re-equilibration to the original tensiometric pressure was achieved. Tensiometer readings with less than 5% error were attained b y allowing 3-4 min re-equilibration time at soil matric potentials bet ween -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-equilibra tion times reported are specific to the volume of air left in the tens iometer below the septum stopper (1.2 cm3). A larger air bubble will d ecrease re-equilibration time but could cause undesirable thermal erro rs during field use. Transducer readings should be made after allowing time for complete re-equilibration of the tensiometer following inser tion of the needle. However, for practical field measurement of low ma tric potentials, it will be necessary to read the portable transducer before full re-equilibration and correct the reading based on previous ly determined re-equilibration patterns. These re-equilibration patter ns 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 s eptum stoppers be replaced regularly. With careful use, these systems are convenient for monitoring large numbers of tensiometers spaced apa rt in the field.