Cosmic Rays (CR) are particles which come to the earth from Universe. Their origin and production mechanisms are still unknown. The Pierre Auger Observatory is located in Mendoza, Argentina. It is dedicated to the study of CR. When CR arrive to the earth's atmosphere they produce a shower of secondary particles called air shower. The surface detector (SD) of the Pierre Auger Observatory consists of tanks full of pure water, where CR produce Cherenkov radiation, when going through them. This light is detected by three photomultiplier tubes (PMT) located on the top of each tank. Depending of the angle of arrival direction of the primary CR, each PMT is able to register different signal than the other. The goal of this study is to look at these effects of direct light on the PMT's to explore if they change in time. The obtained results may give information about the physical status of the tanks in order to monitor the work of the SD, and to estimate possible systematic effects on the measurements. The current results of this study are shown.