Suitability of auger drill sampling for the measurement of moisture content gradient in kiln-dried South African grown Eucalyptus grandis poles

Abstract

Moisture content (MC) gradient at the time of preservation plays a pivotal role in determining the success of the preservation process. The South African pole drying industry relies on auger drill sampling to determine whether the MC of dried poles is acceptable for preservative treatment. The objectives of this study were to determine: (i) the reliability and (ii) potential sources of errors of the auger drill MC sampling method on kiln dried E. grandis poles. Destructive oven-dry MC determination on sample blocks of 12.5 mm thickness and 25 mm width as the benchmark method and auger drill samples in increments of 25 mm were used to determine the MC gradient at the theoretical ground line (TGL) of poles. Results showed that the MC of 25 mm deep increment auger drill samples did not differ from those measured using the block method. However, there were differences between MC values measured using the auger drill and block methods when samples of 50 mm or 75 mm in length were considered. Differences in MC values measured after drying also occurred in parts of the poles where moisture was still high, i.e., above fibre saturation point. It was concluded that SANS 5983 (South African Bureau of Standards. Standard method for preparation of test specimens for moisture content determination in timber and timber products, 2008), which specifies taking single radial auger drill samples of depth 70±5 mm at mid length, is a reliable method for measuring MC in a kiln dried E. grandis pole. It is recommended that MC is determined in auger increments of 25 mm at the most critical zone of a pole, such as the TGL rather than mid length.