The integration of Ad-Hoc networks into real environments is now becoming more and more common and supervision and control systems are no exception. The efficiency of the communication in these networks as well as various other factors, are governed by the working area, the number of nodes, mobility, transmission power, etc. In this paper, the mobility and reachability of mobile nodes appearing spontaneously in a large installation such as can be found in a water purification system are studied. These nodes form an Ad-hoc network and communicate between each other in order to finally reach a fixed node which can offer them information from the rest of the system in real time or act as a gateway to other remote networks. The study was done analytically using stochastic activity models, and using the minimum number of nodes necessary to successfully serve a water purification system with a high percentage of communication cover.