Laboratory and industrial testing was conducted on a commercial, dielectric moisture meter (Wagner model L612) and an accompanying stack sampling probe (Wagner model L712). The purpose of the testing was to identify and quantify the various factors affecting the accuracy of moisture content (MC) estimates obtained when using this equipment. Laboratory tests were conducted to determine which wood-related properties and environmental factors had an influence on meter readings. Testing of black spruce over a range of MC’s from 9 to 18% identified the need for a new correction factor for this species. Moisture gradient, wood temperature, surface roughness, and board width were found to have an effect on meter reading. Effect of wood temperature is significant but is roughly half the effect previously documented on DC-resistance. The presence of knots or compression wood in the sampling area also influenced meter reading. Temperature that the wood was dried at was not found to have an effect.
Simulated and actual industrial tests were conducted to test the applicability of various corrections identified in the laboratory testing phase. For black spruce, improved estimates of MC were obtainable by applying the newly developed species correction and, depending on the specific tests, other corrections for wood temperature, surface quality, etc. Tests were also conducted using the stack sampling probe. Even under well controlled laboratory conditions, there was not a good correlation between readings taken with the stack sampling probe and either readings with the handheld meter or oven-dry moisture content. Readings taken with the stack sampling probe typically underestimate the actual MC. More accurate and consistent readings were obtained when sampling wood stacks at a uniform wood temperature.
Readings from both dielectric and DC-resistance meters were compared against oven-dry MC for laboratory and industrially dried lumber. The DC-resistance meter performed marginally better when evaluated on the basis of proportion of readings within given error limits. MC estimates from the dielectric meter can be correlated with DC-resistance MC estimates regardless of which correction factors are applied. Dielectric and DC-resistance meters are not used totally interchangeably in the wood industry as each has specific applications where they are advantageous to use. As a result, any small difference in accuracy is not always the determining factor.