The free space in each cargo hold is measured, and this measurement is used to determine the volume occupied by the cargo. This volume, when divided by an assumed stowage factor, gives the approximate weight of the cargo. This method provides no more than an approximation. Furthermore, the exact stowage factor is seldom known, and the assumed figure may be quite inaccurate. The stowage factor can be ascertained correctly only by a laboratory analysis of samples from the cargo. Proper sampling is essential since the analysis must take into account the nature of the cargo, the moisture content, the percentage of foreign matter present, and the age of the commodity.
The figure may also vary considerably for other reasons. For example, in grain cargoes, so-called ‘spout lines’ may develop since grain in a cargo hold tends to separate into heavier and lighter components. Also, almost all bulk grain cargoes settle during transport as the kernels and shells collapse. The result is an increase in weight per unit volume and a lower stowage factor. Accordingly, in such cases, the weight of cargo calculated on the basis of free space measurement after loading will indicate a greater quantity of cargo than that calculated before discharge, if the same assumed stowage factor is used.