Industrial systems are composed of multiple components whose security has not been addressed for a while. Even if recent propositions target to improve it, they are still often exposed to vulnerabilities, since their components are hard to update or replace. In parallel, they tend to be more and more exposed in the public Internet for convenience. Although awareness of such a problem has been raised, there is no precise evaluation of such a risk. In this paper, we define a methodology to measure the exposure of industrial systems through Internet. In particular, a carefully designed scanning approach, named WiScan, is proposed with a low footprint due to the high sensitivity and low resources of targeted systems. It has been applied on the entire IPv4 address space, by targeting specific SCADA ports.