Due to the great risk of contamination by leaking in underground fuel storage tanks (UST) of gas stations all over the world, the establishment of effective monitoring methods in this environment is extremely necessary. Among UST monitoring methods the tightness test is one of the most effective ones in identifying leaks, it can be done in two different ways, either wet part test or dry part test. But while both of the tests are permitted, they show a great difference in rigorousness, when it comes to approving or not a tank. This study envisions to deeply explore the causes of the difference of rigorousness between both tests, and discover ways in which simulations can approach the real situation. The research allowed us to identify not only the cause of such difference in rigor, but also to establish a constant that approximates the theory to the real situation.