| Electrical moisture meters are a critical tool used by the timber industry to monitor kiln drying, and to assess the final moisture content (MC) of dried timber products. High quality drying with timber at a uniform MC, appropriate for the in-service conditions, is vital if expensive in service ‘movement’ problems are to be avoided.
The standard procedure for measuring MC is to cut a small sample of timber and dry it in an oven with accurate temperature control. This process is both destructive to the timber and can take a considerable amount of time. Moisture meters provide a means of instantly estimating the MC% that would otherwise be obtained using the oven-dry method. They can be broadly categorized into two different types: resistance (or conductance) and dielectric (or capacitance). While the fundamental relationships between the electrical properties measured and the MC of the timber are strong, there are many other factors that can affect the measured electrical property. Some of these factors are known and easily accounted for, while others are less well understood or poorly accounted for.
This website provides a compilation of two research projects undertaken with the support of the Forestry and Wood Products Research and Development Corporation (FWPRDC). The projects generated new species corrections and appropriate protocols for a range of commercially important species in the Australian hardwood and softwood industries. The projects were primarily aimed at quantifying the expected accuracy of the new corrections with the different species, so that industry could use the various meters with greater confidence. The original project reports are available directly from the FWPRDC website (hardwood project (FinalHardwoodRpt-Part1.pdf and FinalHardwoodRpt-Part2.pdf) and softwood project (PN04-2002.pdf)). The reports can also be directly downloaded from this site from the Downloads page. Also included in the Downloads page are a set of Microsoft® Excel spreadsheets for meter users to generate their own species corrections.
The table below provides a summary of three main methods for measuring MC in timber (Adapted from James, 1994)
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| OVEN DRY |
| RESISTANCE |
| DIELECTRIC | | DETAILS | Standard method for measuring wood moisture content. · Based on weight loss in sample placed in oven (preferably fan forced at 103°C ± 2°C) for 24-48 hours or until ‘constant’ weight is observed. · Expressed as a percentage of oven dry weight. | | Measures the resistance (or conductance) between 2 electrode pins. The contact resistance between pin and wood is predominantly what is measured. The meter measures the wettest wood that contacts both pins. | | Take two relatively similar forms: Either based on measurement of power loss factor (Power loss type) or in conjunction with the dielectric constant (Capacitive admittance type) of the specimen. Both use surface contact electrodes. | | ADVANTAGES | · Industry standard. |
| · Very strong effect of moisture content on conductance. · Overshadows other variables. | | · Surface electrodes make no holes in specimen. · Moisture content estimates are possible at moisture contents less than 6%. (Upper limit is about the same as for conductance type, but capacitive admittance type can give ballpark data at moisture content greater than fiber saturation). | | DISADVANTAGES | · Destructive. · Time consuming. · Needs care to ensure oven-dry weights are actually stable and samples fully oven-dry. | | · Extremely low conductance, at low moisture content, and upper limit is essentially fiber saturation point, because at higher moisture content, conductance is not well correlated with moisture content. · Requires electrodes that make holes in specimen. | | · Readings are influenced by variables other than moisture content to a greater degree than for resistance types. · Surface electrodes strongly influenced by material near the surface. Material below about 0.003m below the surface is not properly weighted in the reading. | | ADDITIONAL OBSERVATIONS | | | Electric moisture meters are affected by factors other than moisture content such as wood density, water soluble electrolytes in the wood, temperature, species and uneven moisture distribution. So even though the meters respond quite precisely to the electric properties that founds their calibration, these other factors can introduce variance into the data. | This website provides a compilation of two research projects undertaken with the support of the Forestry and Wood Products Research and Development Corporation |