WPS 264 Density

Density & specific gravity

• ρ = m/V, g/cm³ or kg/m³

• Specific gravity is ratio of density of a material to density of water. Dimensionless.

• Influenced by moisture, structure, extractives & chemical composition.

• Can vary by more than twofold within species.

Density

• Changes with MC

• Moisture adsorption increases weight & volume

• Moisture loss reduces weight & volume

• Only 2 conditions where weight & volume constant;

  • ovendry state (no moisture, V_dry)

  • fully swollen state (MC_max / V_max)

Ovendry density

• r₀ = M₀ / V₀

• r₀ higher than wet density because dry volume smaller than swollen volume

• r₀ is specific gravity

Basic density

• R = m₀ / V_w

• R lower than r₀ because swollen volume larger

Air dry density

• Density at constant MC in air (~12-15%)

• American standard; ovendry weight / air-dry volume

• European standard; air-dry weight / air-dry volume

Apparent density

• If MC content not exactly known, then density is called apparent density. This value not reproducible & depends on MC.

• Has value for handling of wood eg. shipping.

• r_u = m_u / V_u

Density-MC chart

• From r₀ position, go up curve to specified MC to get r_u (horizontal line)

• From r_u & MC, go all the way down the curve to y-axis to get r₀

Comparison of densitites

Influence of MC

• Increasing MC increases density

  • first volume increases up to max swelling capacity

  • thereafter volume stays constant

  • weight increases with MC until MC max

• Moisture can only be adsorbed until MC_max reached

Max MC

• MC_max = 100 [(1/r₀) - 0.67] + 30

• r₀ specific per species; cherry = 0.5, walnut = 0.69, palissander = 0.87

• given

Apparent density above & below FSP

• Above FSP: r_u = [r₀ (1 + u)] / (1 + 0.84 * r₀ * u)

• Below FSP: r_u = [r₀ (1 + u)] / (1 + 0.3 * r₀)

• given