When engineers design supply systems, a few assumptions simplify the process of calculating pressure losses. One of the main assumptions is that the air density remains constant throughout the system. For this to be true, the process must be adiabatic (no heat loss), incompressible, isothermal (constant temperature), and isobaric (constant pressure). From the perfect gas law, even first-year engineers know that density increases as temperature decreases or as pressure increases. Likewise, density decreases as temperature rises or pressure falls. Since pressure drop due to friction is actually a function of density, density must be accounted for if it varies much from standard.