Moisture Meter Manuals & Species Adjustment Tables
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Technical and Species Adjustment Commentary
Specific Gravity Summary
The specific gravity (density) values for a species* are based on the best, current world data, and are used to determine the correct species settings factor within the meter. The values are average density values for the species. A coefficient of variation (COV) of about 10% describes the variability inherent in many common domestic (US) species.
* The specific gravity based on oven dry weight and 12% moisture content.
Commentary on Species Adjustment
In 1992, a study was conducted at the Forest Research Laboratory of Oregon State University on species correction for the hand-held moisture meters from Wagner Meters. The species tested were Douglas Fir, Lodgepole Pine, Western Red Cedar, Western Hemlock, White Fir, Western Larch, Engelmann Spruce, and White Oak. Three to four 40-piece samples of each species were tested. Specific gravity was found to be the primary factor in species adjustment. A species equation as a function of specific gravity and the meter reading was obtained using the multiple-regression technique (R-square = 0.95) as follows:
AF = 8.77 + (0.25 * MM) – (15.86 * SG) – (0.62 * SG * MM)
AF = Species Adjustment
MM = Meter Reading
SG = Specific Gravity in oven dry weight and 12% moisture-content volume basis.
The species adjustments provide the adjusted moisture measurements that are based on the species adjustment determined using the species adjustment equation, with rounding to the nearest 0.5.
Wood is not a uniform material. The specific gravity of solid-sawn lumber varies within the piece and among pieces. In the OSU study, the average specific gravity for each species differed from the individual sample by plus or minus 1% to plus or minus 8%. For general applications, average specific gravity values can be found in the Wood Handbook (USDA Agriculture Handbook No. 72, 1999). Except for one species for which the experimental value is 7% higher, the species’ overall average specific gravity values obtained in the OSU study are comparable with those in the Wood Handbook. The exception may be caused by unknown biases in the sampling scheme. The Wood Handbook values are used in the tables, except for the imported species, unless otherwise noted.
Species adjustment can be determined for lumber sorted, or otherwise known, to have specific gravity different from the species’ average. One example is lumber graded under the Dense rules. If the specific gravity of a lumber sample is known, species adjustment can be determined by the species adjustment equation.
The species adjustment equation provides a way to expand the use of your hand-held moisture meter from Wagner Meters for lumber of any species groups having similar species-specific gravity values. One example is Hem-Fir. For a species group, one way to determine the species adjustment is by the use of a weighted average of the individual species’ average specific gravity values. The weighing procedure used in the ASTM D2555 by standing timber volume can be used. Species adjustment is not recommended for any species group having a broad range of species-specific gravity values. There are no recognized limits on species group species adjustment. Species adjustment for species groups should be used with knowledge on the variability of species involved and the effect of it on species adjustment. If the species mix in the lumber production of a species group is controlled or known to have specific gravity different from that used for the species group, a better estimation of species adjustment can be determined using the known specific gravity in the above species correction equation.
Determining the Species Setting for an Unknown Species
The Species Setting Tables based on specific gravity of solid wood, are provided in the manual. If you don’t know the species of the wood you are using, or the specific gravity differs from the handbook because of a different growing region, use the following procedure.
Determining the Specific Gravity
Following are the steps for calculating the Specific Gravity (SG) for a wood sample:
- Select a sample of wood that is approximately 12% moisture content, with all edges being true.
- Carefully measure the Length, Width and Thickness dimensions of the sample, preferably using a caliper. Convert the Length, Width and Thickness measurements to feet (ft.).
- Carefully measure the weight of the sample, to the tenth of an ounce. Convert the weight to pounds (lbs.).
- Using the volume and the weight of the sample, and the weight of a cubic foot of water, calculate the specific gravity.
|10 inches ÷ 12 inches = 0.833 ft.|
|Width:||7 ½ inches|
|7 ½ inches ÷ 12 inches = 0.625 ft.|
|Thickness:||1 ½ inches|
|1 ½ inches ÷ 12 inches = 0.125 ft.|
|From the above dimensions, calculate the sample’s volume:|
|0.833 x 0.625 x 0.125 = 0.065 cu. ft.|
|Weight of the Sample = 20.0 oz.|
|20 oz. ÷ 16 oz. = 1.25 lbs.|
|Calculate the Specific Gravity (SG) of the sample:|
|(Weight ÷ Volume) ÷ (The weight in lbs. of a cubic foot of water*)|
|(1.25 ÷ 0.065) ÷ 62.34 lbs. = 0.31 SG|
*A cubic foot of water weighs 62.34 lbs.
In order to ensure that the value obtained for the specific gravity is statistically significant, the specific gravity of a number of pieces must be calculated and the average specific gravity determined. Use this value of specific gravity for that species setting for your Wagner moisture meter.