Slide caption: "Paul Hennon using shigometer on Douglas-fir root for Phellinus weirii. Note: Didn't work for stain."

Additional information from www.fs.fed.us/t-d/pubs/html/97241202/97241202.html : "The Shigometer is a battery-operated, lightweight field ohmmeter developed over many years by Alex Shigo of the USDA Forest Service Northeastern Forest Experiment Station (see figure 4). This instrument generates a pulsed direct current and registers (in thousands of ohms) the resistance to the current as it passes through the wood or bark. Before wood begins to decay, it becomes wetter and contains more ions, which will directly affect the resistance to current. The resistance is also affected by temperature and the Shigometer will not work on frozen wood. Internal decay is detected with the use of a twisted-wire probe. A 0.1 inch (2.4mm) hole is drilled into the stem section to be measured and electrical resistance readings are monitored as the probe is inserted. In sound wood tissue, relatively high resistance readings are obtained and as wood becomes discolored or decayed, electrical resistance progressively decreases.

... It is agreed that the Shigometer is a valuable tool for determining the current vigor of trees and forests (cambium testing), but since resistance changes begin prior to damage caused by decay (xylem testing), the Shigometer technique for decay detection requires careful application and interpretation. Basic biological knowledge of the material being evaluated is essential. Skill in instrument use increases with greater awareness of local biological principles and practice with the instrument."

Photo by: Unknown
Date: c.1985

Credit: USDA Forest Service, Region 6, State and Private Forestry, Forest Health Protection.
Source: Region 6, Forest Health Protection slide collection; Regional Office, Portland, Oregon.

Image provided by USDA Forest Service, Region 6, State and Private Forestry, Forest Health Protection: www.fs.usda.gov/main/r6/forest-grasslandhealth

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