Discussion
Porosity of a bulk grain mass is given by
Answer: C
Correct answer: One minus bulk over true density
Porosity is the fraction of the bulk volume that is air:
\[ \varepsilon = \frac{V_{\text{void}}}{V_{\text{bulk}}} = 1 - \frac{\rho_b}{\rho_t} \]
The identity follows because the same mass appears in both densities, so the ratio \( \rho_b/\rho_t \) is the solid fraction and its complement is the void fraction. Expressing it this way is convenient because bulk and true density are both measurable - the first by filling a known container, the second by liquid or gas displacement - whereas void volume is not measured directly. Typical cereal grain lots sit near 40 per cent porosity, which is what makes aeration through a bin possible at all.
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