So far, we have been concerned in this model only with numbers. We have not considered units, except in a small way in the final matrix. This has allowed us to skate over several problems where, in fact, our model has been wrong. This does not, however, invalidate in any way what we have learned about matrices and optimum solutions. We have said that our qualities blend linearly. Let us consider specific gravity. Let v
This last line demonstrates that specific gravity blends linearly by volume. Now consider Sulphur content. This is expressed as a percentage by weight. So the weight of Sulphur in any component i is w Now clearly the total quantity of Sulphur in a blend is the sum of the quantities in the components, so we have
We can multiply throughout by 100 to get
which is a statement that Sulphur content blends linearly by weight. But what did we do in our matrices? We simply said that all the qualities blended linearly. By ignoring the units, we allowed ourselves to miss the essential difference between blending of specific gravity and of Sulphur content. It so happens that VBI, like Sulphur content, blends by weight, although there are some which blend by more exotic quantities such as mol or pseudo-volume. In a real model, we would correct this mistake. But here it suffices to observe that we must be careful and precise. We must define the units of all the elements in our model - data, decision variables, constraints - and ensure dimensional consistency throughout. Apparently trivial alterations to an MP model can have dramatic effects on the optimal solutions. |