This paper deals with the improvement of process instrumentation design. We suggest a flow sensor arrangement method based on an accurate analysis of the graph representing the process. Om algorithm leads to a sensor arrangement (with a minimum sensor number) which satisfies either a total or a partial observability of the process and takes into account the unmeasurability of some streams. A Branch and Bound type optimization method which enables to find an optimal arrangement of those sensors has been adapted The optimality of the solution is considered from a cost point of view. These technics are applied to opened literature examples and their efficiency is enlighted.