having units of volume/time, which are the same units as pumping speed for a vacuum pump. This definition proves useful in vacuum systems because under conditions of rarefied gas flow, the conductance of various structures is usually constant, and the overall conductance of a complex network of pipes, orifices and other conveyances can be found in direct analogy to a resistive electrical circuit. For example, the conductance of a simple orifice is liters/sec, where is measured in centimeters. Hydrology