Calibration Of Stress Wave Timer for Amazonian Species

Abstract

Non-destructive testing is increasingly being used on wooden structures. One of the preferred instruments for this task is the Stress Wave Timer, because it is portable and easy to use. This instrument needs to be calibrated to minimize possible reading errors. Tests were performed on samples of different sizes from four Amazonian timber tree species, with differing distances between the start/stop accelerometers and varying gain settings. The aim was to identify errors and find the best ratio between the Metricard SWT’s start and stop accelerometers. The best ratio was observed with settings of 40 for stop and 1 for start. The calculation of the confidence interval for the mean of the stress wave velocities expresses a degree of uncertainty ranging from 2% to 8%, regardless of the species and according to the start/stop ratio. A more in-depth evaluation led to the conclusion that the greatest degree of uncertainty occurs with short pieces. This confirms the need to calibrate the equipment, especially when using the instrument with pieces approximately equal to or less than 70 cm in length. The value for offset time (intersection of the line with the axes) was determined by a simple regression analysis, and indicated that the data correction factor varied from 1.9 to 2.2 depending on the start/stop ratio, at least in the samples studied.