500-62201-001 Rev. M
Page 26 of 33
Commentary On Specific Gravity
In 1992, a study was conducted at the Forest Research
Laboratory of Oregon State University on species correction
for the Wagner Hand-Held Moisture 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 on species correction. A species correction
equation as a function of specific gravity and the meter reading
was obtained using multiple regression technique (R-square =
0.95) as follows:
CF = 8.8772 + (0.2492 * MM) - (15.8649 * SG) - (0.6204 * SG
* MM)
CF = species correction; MM = meter reading; SG = species
average specific gravity in oven-dry weight and 12 %MC
volume basis
Wood is not a uniform material. Specific gravity of solid sawn
lumber varies within the piece and between pieces. In the
OSU study, the average sample specific gravity for each
species differed from the individual sample by plus and minus
1% to plus and minus 8%. For general applications, average
specific gravity values can be found in the Wood Handbook
(USDA Agriculture Handbook '72, 1987). Except for one
species whose 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 species
list.
Species correction 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 correction for this lumber sample can be
obtained by the species correction equation.
The species correction equation provides a way to expand the
use of the Wagner Hand-Held Moisture 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 correction 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
correction 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 correction.
Species correction for species groups should be used with
knowledge on the variability of specific gravity of all individual
species involved and the effect of it on species correction. 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
correction can be determined using the known specific gravity
in the above species correction equation.
