brand name of a device consisting of a specialized micro-drill bit that drills into trees and graphs density differences that are used to detect decay. Invented and trademarked by Frank Rinn of Heidelberg. Germany.
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Resistograph is an electronic high-resolution needle drill resistance measurement device, developed by Frank Rinn (since 1986 in Heidelberg, Germany).
A thin, long needle is driven into the wood. The electric power consumption of the drilling device is measured, recorded and printed. Resistograph devices are different from other resistance drills because they provide a high linear correlation between the measured values and the density of the penetrated wood. They are used for inspecting trees and timber in order to find internal defects, and to determine wood density and growth rates.