The Importance of Wood Density (Specific Gravity) and Its Component Parts

Abstract

A general definition of specific gravity and wood density was given in Chapter 1, but it will be of value to repeat a summary of it here. The two terms will be used interchangeably, since they measure the same thing although results are expressed using different units. Wood density and wood specific gravity both indicate the amount of actual wood substance present in a unit volume of wood (see Chap. 1.3.1). Although there are several different methods used to measure wood density, the standard way is to calculate the ratio between the dry weight of wood divided by the green volume of the same wood. This is often referred to as basic density (Tsoumis 1991). Thus, grams/cubic centimeter (specific gravity), pounds/cubic foot (wood density, English units) or kilograms/cubic meter (wood density, metric system) are all related. Because one ml of water equals 1 g, specific gravity is a unitless measurement and is expressed only, for example, as 0.45, not as 0.45 g/ml. The above classifications will be used throughout this book but the reader must observe carefully all data on wood density referred to in publications. Occasionally, one will find specific gravity or wood density expressed as the dry weight of wood divided by dry volume, or green weight or air-dry weight of wood over green volume or, very rarely, green weight of wood over dry volume. The values for wood density or for specific gravity will be quite different when such different methods of measurement are used.