C I N T R A F O R
Special Paper Abstract
Forest Products Measurements and Conversion Factors with Special Emphasis on the US Pacific Northwest
David Briggs. 1994. (161 pp) SP16 $20.00
Understanding forest products measurements is important to anyone in the forest products industry. Converting volume, area, lineal or weight measurements between the metric and Imperial systems is not as simple or straightforward as it would seem. Incorrect, obsolete, and ambiguous conversion factors can be very misleading and may result in large errors. Some reasons for this are:
Lack of standardization as to how measurements are taken and recorded. For example, logs may have diameter taken at one end, both ends, or at midlength; the measure may be inside or outside bark, and may be recorded to the nearest tenth, nearest inch or centimeter, or with the fraction dropped.
Even if the same measurements are recorded, they may be entered into different formulas that yield different results. Various cubic log volume formulas yield different results even when the same log measurements are used. Often, log scaling systems differ both in the way measurements are taken and in the formula used.
The units for expressing volume or weight are often different and may not be clearly defined. Log volume may be expressed in cubic feet cubic meters, board feet, koku, and so forth. A ton of chips may be wet or bone dry and may be a short ton, long ton, or metric tonne. Chips are also expressed as units and bone-dry units.
Even if the same unit of measurement is used, it may not represent the same quantity. A good example is the difference between a board foot in log scaling and a board foot in tallying lumber at a sawmill.
Correct conversion factors for some products, such as logs and lumber, vary with piece size.
Wood is variable. Wood density - the measure of mass of wood per unit volume - varies between species, between trees of the same species, and between parts of the same tree. Wood moisture content can vary from a few percent to more than half the weight of a piece. Since wood shrinks and swells as moisture content changes, these changes affect volume as well as weight. Many wood product measurements and conversion factors that are reported are ambiguous because no indication of such conditions is given.
While this book primarily focuses on forest products in the Pacific Northwest, several national and foreign systems are included because of the rapid globalization of markets. Because many readers may be relatively new to the industry and its terminology, a glossary is provided and descriptive background on the philosophy and assumptions that underlie formulas and measurement systems is presented. Procedures and illustrative examples are emphasized to encourage the reader to gather and use local information rather than rely on tables of averages. With the ready availability of computers and software packages, the focus on procedures is appropriate to enable readers to develop software to perform calculations they need. Tabulated species and regional averages are also presented. These values can be used as a rough check on calculations or as an approximation if local data are not available. The reader should be cautioned that tabled averages may be based on limited samples, so conversion factors calculated from these tables may differ substantially from the local reality. Although the examples were taken from actual situations, they should not be regarded as representative of the current forest resource or average industry practice. They were chosen to illustrate and reinforce the calculation methods, to provide insights on how systems attempting to measure the same product vary, and why some are less consistent or more biased than others. The appendixes provide tables of standard conversion between Imperial and metric measures, conversion factors used by the U.S. Forest Service in its latest assessment of the U.S. timber situation (Haynes 1990), some examples of board foot log rules, common and scientific names of U.S. tree species, and names of industry associations for products discussed in this book.
Information on the physical and mechanical properties of commercially important species, along with descriptions of products, processes, and grades, can be found in the Wood Handbook (USFS 1987), which is recommended as a companion to this volume. It is available through the U.S. Government Printing Office.
Unlike previous editions, this version. does not contain forestry yield tables, site curves, and so forth. This and other forestry information is well covered in Log Scaling and Timber Cruising, by J.F. Bell and J.R. Dilworth, available through the Oregon State University Press, Corvallis.
Readers are encouraged to send the author descriptions of measurement systems and conversion factors not covered in this edition. These inputs will be valuable in expanding the scope of future editions to include more foreign country systems as the forest products industry continues its globalizing trend.