Vacuum systems are composed of vacuum chambers, valves, traps, and pumps, which are connected by vacuum lines (ducts) in various serial and parallel configurations, making less or more complex vacuum circuits. To predict the behavior of assembled systems, parameters such as throughput, pumping time, conductance or resistance of individual vacuum components, and the entire vacuum assembly must be known or determined. 235 Gas flow is one of the most vital processes that takes place in vacuum systems. Analyses show that gas flow is not simple even at vacuum conditions because its characteristics change with reducing pressure, and it depends on the dimensions of the vacuum components, vacuum ducts, and their geometrical configurations. How the characteristic features of a particular flow are attained depends not only on the cross section and shape, but also on the length of a duct.