Resistance Recording Drills for Decay Assessment

A Technical Report from The Bartlett Tree Research Laboratories

Resistance recording drills or resistographs are tools used to evaluate decay and other defects in tree stems and branches as part of advanced tree risk assessments. These tools use a small diameter drilling needle that is driven into the stem or branch at a constant speed. The device measures the electric power consumption necessary to drive the needle into the wood, which is highly correlated with wood density. As the drilling needle passes through bark and wood, the resistance that is encountered is measured and recorded. Low resistance values help identify decayed wood, hollows, and cracks in the stem or trunk. Wood density varies considerably among species, so resistograph measurements must be interpreted within the context of the species being assessed.